Capacitors

[Kevin]

I have a question about the size of the capacitor required for a power supply. I am using PK296A2A-SG7.2 motors and an Avel Y236801 500VA 25V+25V Toroidal Transformer. It is my belief that a 50 volt is a high enough rating but I am uncertain as to the mf size. I see that 10,000 mf is used by some. Is it a problem if I use a 22000mf 75V Computer Grade Capacitor? And can someone explain what is required to add a bleeder resistor to drain a capacitor if necessary...

Thanks, Kevin
(quietly working away in AZ)

[Richards]

Kevin,
A 22,000uF 75V capacitor will work fine. I use 15,000uF to 20,000uF on my power supplies. Also, I use a capacitor whose DC rating is at least rated as high as the working voltage, which in your case would be 70VDC, so you're perfectly safe with both the capacitance and the voltage.

The bleeder resistor (if you use one) can be screwed to the capacitor's + and - terminals. All that it does is to drain or "bleed" off the electricity when the power supply is turned off. Capacitors can hold a charge for a very long time, so it's a safety feature. As Gerald has pointed out, with four stepper motors that act as bleeder resistors, a separate bleeder resistor is not necessary when used with multiple stepper motors. If you decide to use one, you can use a 47K ohm 1-watt resistor with a 70VDC power supply. That size resistor will still take about a minute to drain off most of the voltage and it will get warm, but it is a good compromize size. If you want the bleeder resistor to drain the capacitor faster, you could use a 4.7K 5-watt resistor. That would still take several seconds and that resistor will probably get HOT - so, be careful.

[Gerald D]

A 25V+25V transformer will give you 50V Alternating Current (AC), when that is rectified to Direct Current (DC) it will be 1.41 times higher; ie. just over 70VDC. A 50V capacitor will pop (explode) if you put it with that transformer and rectifier. Don't forget the rectifier (that will make a big BANG). The 75V capacitor will just make it. 22000 uF is plenty.

If you have 4 drives connected to the capacitor, they act as a good bleed resistor. You don't need a separate bleed resistor unless you are just messing around on the bench - but then you can just use a loose gecko as the resistor.

[Gerald D]

Ah, Mike and I were typing at the same time - glad we more or less said the same thing! :)

Just a note on capacitors - they do get old and wear down. If it is just capable of 75V today, it will be capable of less in 20 year's time. This is one area where you avoid buying war-surplus of dubious age and origin.

[Doug_Ford]

Kevin,

Post some pictures of your work so other guys can learn and cheer you on as you progress.

[Robert M]

Any one knows some « Online » good sources for capacitors ?
Don’t laugh, I can’t seem to find any even after some Goggle searching :o
Thanks ;)

[domino11]

Robert
Try www.digikey.com , www.mouser.com, www.newark.com , www.e-sonic.com. But my favorite place in montreal is www.addison-electronique.com 514-376-1740 8018 20e ave Montreal. They are fantastic for electronic parts. :eek::)

[Robert M]

Thanks Health…
Tried Addison a fee weeks back… Believe it or not, it took over 4 clerks to understand (immigrants) & know what the hell I was requesting… only to find out they could not find or order most items I was looking / requesting for.
They’ve gone more into cheep Audio and low quality commercial overstock. Not the good old Addison electronics store I knew back in the 80’s
Robert ;)

[domino11]

Robert,
What size capacitor are you looking for, I might be able to fix you up. :)

[Richards]

Copied from another thread:

. . . . . . a suitable capacitor; however, finding adequately sized capacitors with screw terminals is becoming more and more difficult, since most designers have switched to the "snap-caps" that are generally soldered onto a circuit board. (I recall seeing some photos of snap-caps being hot glued to a thin piece of MDF. That would work although I would prefer drilling holes through the MDF and securing the snap-caps with nylon ties.)

[Gerald D]

For soldering, Snap-in capacitors ("snap-caps"):

2754

2755



Screw terminal capacitor:

2756

Terminals have female threads - screws not shown.

[Gerald D]

Here are some candidates for screw terminal capacitors:

http://www.mouser.com/catalog/636/720.pdf

http://www.newark.com/95F4457/passives/product.us0?sku=united-chemi-con-36da153f050al2a

http://www.e-sonic.com/acc/products.aspx?partID=CGS153U050V4C&partIDExt=280&command=detail

http://www.tedss.com/item.asp?id=2020002403
http://www.tedss.com/item.asp?id=2020002774

[Nikonauts]

in case nobody notice, thread starter said 22,000mF.

I made a mistake by buying 15000uF, only to later realise that gecko recommends (80000*I)/V minimum. So for my config (50V, 14 A) it's 22,400uF minimum. I have a spare caps 1600mF 400V, given by a friend. Worry it might be too big.

Is it too big?

[Richards]

You PROBABLY have a 1,600uF 400V capacitor. It was common practice several years ago to use either uF or mF to mean 0.000001 farad. The lowercase letter "m" is used for micro. The uppercase letter "M" is used for mega. An MF is 1,000,000 farads. An mF is 0.000001 farad.

It's my understanding that before the U.S.A. even admitted that there was a "metric system", that mF was used interchangeably with uF.

The 1,600 mF capacitor is too small, but the 15,000 uF capacitor will probably work just fine. I use 11,000uF capacitors on various portions of my test bench to drive four stepper motors. The motors run just fine. Remember that a stepper motor draws more current the heavier that it is used. Most of the time, most steppers are drawing far less than their maximum current, so MOST of the time, the drain on the capacitor is less than maximum. IF you are going to ALWAYS run your machine at its maximum load, THEN you might need to use a larger capacitor. Capacitors can be connected in PARALLEL to increase the capacitance.

It's good practice to use a capacitor that has a working voltage about 2X the output voltage of the AC transformer because the capacitor will be expected to handle the PEAK voltage, not the average (RMS) voltage. The peak voltage is 1.414 X the average voltage. So, using a capacitor with 2X the voltage rating of the transformer gives you a safety margin. Using a capacitor that has a higher voltage rating than 2X the output AC won't hurt, but unless you just happen to have that size capacitor on hand, it will be larger and more expensive than necessary. Also remember that capacitors have a limited life. The dielectric inside a capacitor will eventually dry out. Unless a capacitor is overheated, that process takes years. It's good practice to examine the capacitors occasionally to see if they're leaking or bulging. It's also good practice to read a power supply's AC ripple (with the power supply under load). Most volt meters can read the AC ripple on a DC power supply by simply using the AC setting. If the ripple starts to go higher (more volts) and if the load on the power supply hasn't changed, then it's time to start thinking about getting new capacitors.

[Gerald D]

Small m, or lower case m, is the metric abbreviation for milli, being 0.001, or 1/1000th. Whenever a metric based person sees m in front, that is 1/1000th. . . . . . undoubtedly, no exceptions.

If NikoN says he has a 1600mF 400V capacitor, that is 1 600 000 (1.6 million) uF, or 1.6 Farad, which sounds like it needs a forklift to move around. If that "1600mF" capacitor can be carried in one hand, it is probably a 1600 microFarad, particularly if it is 400V rated. :)

[Gerald D]

A snag with the metric system is that the correct symbol for micro is µ , a character not found on any common keyboard. The nearest character is the u.

Here are all the metric multiplier names and symbols: http://en.wikipedia.org/wiki/Micro-

[Richards]

Gerald is right. I should have said that the lower case letter "m" WAS commonly used by capacitor manufacturers in the USA to represent "micro" back when we rebels (USA metric holdouts) were largely ignorant of the metric system.

I had to dig deep to find a capacitor that had the old style marking, but I found four in the power supply of my first computer, a 1977 IMSAI. The caps are soldered in, so I couldn't see the complete value, but "mF" was clearly visible followed by "MFD".

(That computer was built sometime before Jerry Pournelle, a writer for the defunct BYTE magazine, wrote in amazement that Bill Godbout was offering power supplies in his CompuPro brand computers that featured a bank of capacitors totaling 0.8 FARARD. Bill Godbout was thee premier computer manufacturer pre-Apple and pre-IBM P.C. My first trip to California was around 1980 to visit the Godbout factory to get some help setting up some CompuPro computer cards. If I remember correctly, the theory behind having that much capacitance was due to the fact that the S-100 bus computers used an unregulated 8VDC to 12VDC power supply. Each card in the computer had one or more 7805 type voltage regulators to convert the unregulated voltage to the required 5VDC. Since dropping voltage across a regulator produces heat and heat was the largest cause of chip failure - at the time - Bill Godbout built power supplies that furnished 7VDC so that less voltage had to be dropped across the regulator; therefore the regulators would run cooler and he would have fewer boards returned because of heat-caused stress failures. The 7805 chip will not work if the AC ripple is too great, after all, the regulator works by blocking extra voltage, not by producing voltage when the supply is insufficient. Because Godbout could safely use 10V capacitors, the capacitors' mass was less than you would expect and the idea worked for a very short time until Apple and IBM introduced switching power supplies to the computer world. Shortly before Apple and IBM took over the market, one switching power supply manufacturer offered a multi-voltage power supply that was specifically designed to produce regulated voltages high enough so that the S-100 bus computers, with their on-board regulators could be used with that power supply.)

Most of that is totally off-subject, but it might explain how things worked back in the wild-woolly days when personal computers were first being introduced to the masses.

Edited: To those who would wonder why you shouldn't just use a regulated switching power supply: A switching power supply works best when its load is constant. A stepper motor creates a load that is constantly varying. The variation can be extreme. When a stepper is coasting along, it may draw 25% of the current that it requires when it has to really work. Some switching power supplies cannot cope with that type of load.

[Gerald D]

"If you car lights dim when your car audio system produces a deep bass note, then your amp will be greatly helped with a car audio capacitor." <---source (http://ezinearticles.com/?Understanding-Car-Audio-Capacitors&id=374955)

Those awful booms emitting out of cars are typically powered by huge capacitors at supposedly around 2 Farad (example (http://www.savinglots.com/lotprod.asp?item=CAP160DBL)). However, the voltage ratings are low, and it is doubtful whether the Farad rating values are genuine, or hyped up for the car audio industry.

[jhiggins7]

I've noticed the terms "computer rated" and "audio" used to describe capacitors. If I have two 10,000 microfarad, 100 volt capacitors, and one of them is described as "computer rated" and the other is described as "audio," is there any real difference?

Regards,
John

[Gerald D]

I suspect the major difference is quality?

http://www.tedss.com/electronic-parts/capacitors/computer-grade-capacitors/

[Nikonauts]

Small m, or lower case m, is the metric abbreviation for milli, being 0.001, or 1/1000th. Whenever a metric based person sees m in front, that is 1/1000th. . . . . . undoubtedly, no exceptions.

If NikoN says he has a 1600mF 400V capacitor, that is 1 600 000 (1.6 million) uF, or 1.6 Farad, which sounds like it needs a forklift to move around. If that "1600mF" capacitor can be carried in one hand, it is probably a 1600 microFarad, particularly if it is 400V rated. :)

thank you, Gerald. It's the size of a Red Bull can. So i think it's 1600uF mislabeled as 1600mF.

So if i later want to add one more capacitor to the 15000uF, i guess it'd be in parallel to the one now?

[Gerald D]

Yes, in parallel.

[kanankeban]

Hi...
Im starting to think in my power supply...Im kind of paranoid with the charge of the capacitors I dont remember in full, but I think the paranoid factor was aquired when I was a child, and messed around with a capacitor :eek:.
My questions are,
1. why we dont need a bleed resistor if the geckos are conected?
2. although question 1 can we add a bleed resistor for extra protection?
3. can we add a kind of LED or warning light to inidcate that the capacitors are charged and warn not to play with them?
4. What are the logics of the bleed resistor and how is it installed on to the capacitors?
Thanks...

[Richards]

1. The Gecko stepper drivers act like bleeder resistors, i.e, the coils in the motors are connected to both + and - on the power supply, just like a bleeder resistor.

2. Yes.

3. You could add an LED; however, an LED uses Current to work, about 10mA. To get 10mA at 70V would require a 700 ohm resistor at 7W! You would want to use at least a 25W resistor to keep the heat down and a 100W resistor would be much better.

4. Connect one end of a bleeder resistor to the + (plus) terminal of the capacitor and the other end of the bleeder resistor to the - (minus) terminal on the capacitor. A 4.7K 5W resistor would work for a 70VDC power supply. A 1.5K 5W resistor would work with a 35VDC power supply. Just keep in mind that the stepper drives will have already drained the capacitor before the bleeder resistor does its job. The only time that the bleeder resistor would be necessary is if you have fuses installed between the power supply and each stepper and if all the fuses blow.

The value (resistance) of the bleeder resistor will determine how long it takes to empty the capacitor. The lower the resistance, the faster the capacitor will be drained. The lower the resistance, the more heat generated by the resistor. I like to use a resistor that is at least 5X larger (wattage) than required, to keep the heat reasonable.

A manually operated power dump circuit might be an option. If you use multi-pole E-stop switches, one of the Normally Open poles could be connected to a large transistor or FET. Then, when you pushed the E-stop switch, the N/O pole would close and the transistor or FET would quickly drain the capacitor. When you released the E-stop switch, the capacitor would be allowed to charge.

Personally, I just use the stepper motors as bleeder resistors. If I have any worry that the capacitor is still charged, I take a reading with my meter. If there is a voltage present, I grab my largest flat screwdriver with the thickest plastic handle and place the blade of the screwdriver across the capacitor's terminals. THAT drains the capacitor in a hurry!

[Gerald D]

3. can we add a kind of LED or warning light to inidcate that the capacitors are charged and warn not to play with them?


The LED's on the geckos serve that purpose - do not play with anything while there is light from those LED's.

[kanankeban]

The LED's on the geckos serve that purpose - do not play with anything while there is light from those LED's.

Thats a good advise for those like me with some electric & electronic disabilities :D, Thanks...

[javeria]

, I grab my largest flat screwdriver with the thickest plastic handle and place the blade of the screwdriver across the capacitor's terminals. THAT drains the capacitor in a hurry!

Well - dosen't this shorten the life of the capacitors? good practice or bad or just to use when in hurry?

RGDS
Irfan

[Richards]

So far, I've never blown all four fuses going from the power supply to the Gecko stepper drivers, so I've never had to drain a capacitor manually.

Too many times new users keep asking the "what if" questions. After being told that the stepper motors would act as bleeder resistors, some ask, "yeah, but what if they don't"? That leads the explanation of adding a bleeder resistor. When they see how large and unwieldy that bleeder resistor is, then they ask, "but what if you don't have a bleeder resistor and the stepper motors don't drain the capacitor"? That leads to the screw-driver.

The answer to the question is that when more than one stepper motor is attached to the same power supply, you don't need to add a bleeder resistor. Period.

[KenC]

I build Audio amp in my other spare time... hope my 2 cent worth could be helpful.

select Cap voltage rating > or = 2 x Vac.

Calc Min. Farad.

Case.
Transformer secondary Voltage, Vsec= 40Vac,
Required DC current rating, Idc=12A
Max. final ripple voltage of DC, Vripple , (typically 5%)

Vpeak= sqrt(2) x Vsec= 56.56 V
so,
Vripple(pk-pk)= Vpeak x 5% = 2.828V(pk-pk)

For 50 hz mains,
Cmin. = 0.01 * Idc / Vripple(pk-pk) = 0.01 * 12 / 2.828 =0.042434F = 42,434 uF##

For 60hz mains,
Cmin. = 0.00833 * Idc / Vripple(pk-pk) = 0.035360 =35,360 uF

Vdc =Vpeak - (Vripple/2) - Voltage drop accross bridge = 56.56 - 2.828/2 -1.4V= 53.746 V

You can decide on what type of ripple voltage you want and size the minimum cap size accordingly.

Hope this help.

[KenC]

From the above, I'm sure you could see the advantage of using separate rectifier, using 2 bridge, the current involve in each filter circuit halved and so does the cap size.

One can determine if the coils output voltage will equal when the measured resistor of coils are identical...

Richard is right, forget about the bleeder resistor.

Inrush current upon cold start can happen when cap is large... reduce the cap size or connect a NTC (negative temperature coeficient) thermister in series with the primary coil. The NTC thermister has high resistance when cold and zero resistance when reach operating temp.

The safest way to "series" up coils is after it is rectified and filtered. then you don't have any risk of connecting the coils in opposite phase. The only way to determine the phase of the coil is by looking at the AC waveform in a expensive oscilloscope.... now who want to do that!

[Gerald D]

Something I wrote in another thread:



The capacitor . . . . has two functions:

1. Smooth out the ripple of the Alternating Current

2. Keep a reserve of energy when the motors need to make a sudden move. (Also provides a place for the motors to dump energy when they have to stop suddenly) Mariss of Geckodrive suggests this formula: C = (80,000 * I) / V which results in (a) fairly large capacitor/s. A lot of guys have gone smaller without any obvious ill effects.

[domino11]

The only way to determine the phase of the coil is by looking at the AC waveform in a expensive oscilloscope.... now who want to do that!

Actually, I just connect the outputs in series, leave them floating and measure the output with an AC meter. If the two secondaries are in the proper phase you will get double the voltage. If you dont get double the voltage, then just flip one secondary winding and try again. When you get the correct output then the secondaries are phased correctly. No oscilloscope needed for that. Although it will work. :)

[Gerald D]

This post first made at Rough Cut on Diagonal (http://www.mechmate.com/forums/showthread.php?t=2077) in the Troubleshooting area:

A random thought . . . . what would be the effect of having too little capacitance at the power supply to the geckos?

[domino11]

A filter capacitors job in a power supply is to hold up the output voltage during the ac valleys after the rectification process. It will supply power to the load in this time frame. If the capacitance is too low, then the output voltage will droop during this time instead of being held steady. This droop is usually called ripple, as that is what it will look like when viewed on an oscilloscope. I think this ripple would have to be pretty bad to have any effect in the cut quality. (Please note this is from theoretical background and not actual experience on my part) Maybe Mike has an opinion on this one?

[bradm]

Hmm. I'm thinking there could be two effects, one ripple, but the other is a function of demand.

Ripple would be at 2 x 60Hz (or 50Hz) so 120Hz (100Hz). For that to be directly visible at the about 4 ripples per inch shown at the top of this thread (http://www.mechmate.com/forums/showthread.php?p=29318&postcount=1) would require that you be covering the inch in 4/120th of a second, so 30 inches per sec = 1800 IPM. Not likely. If you were moving that fast, you'd probably see ripples, but based on way outrunning the cutter's material removal capability. And Nils has already told us his speeds, which aren't anywhere near that fast.

Moving on to demand, then. Suppose the capacitance is low enough that when two Geckos try to draw full power simultaneously, the capacitors are completely drained and you have to rely solely on the power from the rectifier. That power would have a high degree of ripple, and could easily interact with the Geckos in a manner that caused underpowering of a motor. So the demand is toggling the power supply back and forth between well regulated, and full of ripples, cyclicly. When the power supply is ripply, the motors underperform. Seems plausible.

[domino11]

Brad,
I dont believe that the caps are getting fully drained. They will have a new charge every 8.3 uS. Also the geckos have a local filter capacitor as well for local energy storage (though smaller than the main capacitor that Nils has). They only way this would happen is if his filter capacitor is dead I believe.

One easy test would be to use a multimeter (in DC voltage mode) and see if there is any voltage drop from the non loaded state to the fast cutting (loaded) state. If there is not a real difference there, then it should not be a problem. If there is significant drop or a fluctuating type event happening, then this could be part of the problem.

Nils,
What value of capacitor are you using in your powersupply?

[smreish]

Heath,
If I am correct it is this unit (http://www.antekinc.com/details.php?p=243) from Antek. Noted is a 56V, 600w power supply with (2) 10,000 mf caps.

[sailfl]

Sean is correct, the Antek Power Supply that I am using is the PS-6N56R5R12 unit.

[Gerald D]

I am a little bit nervous of Antek quality and wouldn't be surprised if one of those caps were faulty.

But, I wouldn't jump to that conclusion and start stripping the power supply until we have really established how low capacitance is likely to affect our gecko/motor performance.

Brad, I hear you on the ripple frequency being high. But what is the chance of the ripple frequency, combined with the step frequency, producing "beats" at a low freqency?

[smreish]

Gerald,
Question?
- i would assume the ripple question would go to marius on the geckoforum?
- without a scope, how would a someone even begin to test this theory?

Thank you

[Gerald D]

I will put the question to Geckodrive.

For a practical test of the theory, the quick thought is to purposely reduce the capacitance in a power supply, but then I wonder if it could damage a gecko. . .

[domino11]

Sean, Nils
See my quick and dirty test on post #36 to try. If the voltage does not fluctuate during cutting and there is no large drop in dc voltage from loaded to unloaded states, the capacitor is probably ok. Sean is right though, a scope would be the best thing to use.

Also since Nils power supply has two caps in it, both would have to have gone open to have no capacitor in the circuit.

Gerald,
Don't forget that eack gecko has some capacitance built in and therefore the circuit would probably still work. How well is another issue.

[Richards]

When we look at the components in a power supply, the transformer changes the amplitude of the voltage, i.e. 220VAC in becomes 24VAC out, the bridge rectifier blocks the negative portion of the AC sine wave and adds it to the positive portion, i.e., 50 cycles per second becomes 100 cycles per second, and the filter capacitor stores the AC cycles so that the power appears to be DC.

Without a capacitor, the output would be just a series of sine waves going from about 0V to (SQRT(2) X output voltage). In other words, the output would basically be a positive AC signal. With a capacitor, the voltage is smoothed and filtered. The filtering refers to the fact that a properly sized capacitor removes all of the AC. That's why the standard formula is (80,000 X Amps) / Volts = Capacitance in microfarads. With a properly sized capacitor, there will be very little ripple. With an undersized capacitor, the current being drawn through the capacitor will empty the capacitor faster than the transformer and bridge can fill it, and the stepper motor will get less voltage than desired.

Stepper motors rarely all pull the maximum current at the same time, so using a capacitor that is at least 75% of the computed capacitance works well for me.

Be sure to select capacitors that have a VOLTAGE rating higher than the voltage of the transformer. The AC voltage is measure using RMS but the capacitor has to deal with PEAK voltage, which is SQRT(2) X RMS voltage or 1.414 X higher than the RMS voltage. When I use a 25VAC transformer to make a 35VDC power supply (remember 25VAC X 1.414 = about 35VDC), I use 50V capacitors or even 63V capacitors if they are readily available.

Now the real question is whether low capacitance can cause rough cuts. The answer is a definite "Maybe". When two axes are in motion at the same time and the stepper drivers are micro-stepping, one axis could push harder than the other axis which could cause a problem; however, I get that exact same pattern on my Shopbot PRT-Alpha which has Alpha motors and Alpha stepper drivers. Those drivers are supposed to be perfectly matched to the motors, meaning that the motors would never be starved for current or voltage.

My guess is that the rough cut is primarily due to factors other than low capacitance.

[smreish]

...and Mike, I certainly would agree with your summary comment :

"My guess is that the rough cut is primarily due to factors other than low capacitance"

[Gerald D]

The reply from Geckodrive:

" The capacitor is there to reduce ripple voltage and keep it nice and consistent. If the capacitor is inadequate you will run the risk of multiple things happening, including but not limited to:

1.) Unpredictable motor and drive heating due to fluctuating voltages that are out of the range for the motor (based on inductance).
2.) Stalling of the motor because it is being asked to do too much with too little voltage on the low end of the ripple.

You will get some pretty miserable performance but it will be immediately noticeable. It is not something that will sneak up on you and occasionally throw in a missed cut but will simply not work depending on the voltage required from the motor. "

[gooberdog]

A little Capacitor sizing question
I am going to run 4 PK299-F4.5A (parallel) motors with a 50V Antek power supply PS-10N50R12 - 50VDC 1000W Power Supply. It comes with 2 10000uF capacitor where the calc from Mike's spreadsheet [80000*(4*A)/V] is 39845uF. Which one should I observe?

[Richards]

Because you are running the motors with a parallel wiring connection, I'm assuming that you're going to drive the motors as hard as you can. You can START with the stock configuration, then, with an oscilloscope, watch the voltage as you run the motors. When ripple gets to be much above 5%, add another capacitor in parallel. Keep adding capacitors until the ripple stays within about 5%.

I've run four high-current motors on the test-bench with 11,000 uF with good results.

Speed and load determine how much current the motors are actually going to draw. The higher the speed and the greater the load, the greater the current draw. Because a CNC router is expected to handle a wide variety of speeds and loads, it's really hard to tell you exactly how much capacitance you'll need.

So, start with what you have and then add more as necessary.

[gooberdog]

Thanks Mike, I am a fish out of water with the electrical stuff, That is why I took circuit theory in my final semester as a senior ME student, sat next to all of the freshman EE's. I just don't want to make a stupid mistake.

[southernduckie]

Hi Mike,

Sorry yoou seen to have made a error in your calculations

3. You could add an LED; however, an LED uses Current to work, about 10mA. To get 10mA at 70V would require a 700 ohm resistor at 7W! You would want to use at least a 25W resistor to keep the heat down and a 100W resistor would be much better.

in fact it would require a 7000 ohm resistor(R=V/I , R= 70 / 0.010A) at 0.7 W (W=V x I , W= 70 x 0.010) which becomes realistic, a 1 watt resistor would be fine.

But i agree there is no need for a LED, bleed resistors should be looked at with sceptisum they are in use all the time the power supply is operating and will eventually fail DON'T trust them,(they also reduce the output of the power supply and reduce the effectiveness of the capacitor) use a resistor to bleed the capacitor and multi meter to check then short them out (most capacitor failures can be traced to excessive current flow).

Hope this clears things up, Andrew

[Richards]

Andrew,

Thanks for catching that error. I had the decimal point in the wrong place.

A 7000 ohm 1W resistor would work and it would be within specs, but it would still be too hot to touch. A 5W resistor would be the minimum size that I would use. (I don't like heat in electronic circuits because I never want to repair anything that I design.)

When I've built that type of indicator circuit on other projects, I've used a zener diode to clamp the voltage at 5V and then used that output to drive a transistor. The transistor drives the LED. Even an inefficient transistor would have a beta of 10:1, so you would only need 1mA of current flowing through the zener. A 47K to 70K 1/2W resistor would work along with a 1W zener (I tend to over spec wattage to keep the heat down).

[PEU]

Mike, sorry for resurfacing a two year old comment, but it fits almost my question, just wanted to be sure:

Im building a 34VAC*18A power supply, to get 48VDC, and I already have 2x 10000uF 100v caps, is 20000uF enough for this supply or I need to add more capacitance?

It also has 2 extra secondaries (9 and 4, to get 12 and 5vdc) but these are simple.

Thanks!

Kevin,
A 22,000uF 75V capacitor will work fine. I use 15,000uF to 20,000uF on my power supplies. Also, I use a capacitor whose DC rating is at least rated as high as the working voltage, which in your case would be 70VDC, so you're perfectly safe with both the capacitance and the voltage.

The bleeder resistor (if you use one) can be screwed to the capacitor's + and - terminals. All that it does is to drain or "bleed" off the electricity when the power supply is turned off. Capacitors can hold a charge for a very long time, so it's a safety feature. As Gerald has pointed out, with four stepper motors that act as bleeder resistors, a separate bleeder resistor is not necessary when used with multiple stepper motors. If you decide to use one, you can use a 47K ohm 1-watt resistor with a 70VDC power supply. That size resistor will still take about a minute to drain off most of the voltage and it will get warm, but it is a good compromize size. If you want the bleeder resistor to drain the capacitor faster, you could use a 4.7K 5-watt resistor. That would still take several seconds and that resistor will probably get HOT - so, be careful.

[Richards]

Pablo,
80000 X 18A / 48VDC = 30,000 uF. I doubt that you'll be pulling 18A, so 20,000uF might be adequate, if not, just add another 10,000uF capacitor in parallel with the other two.

Usually only the 48VDC is unregulated. The 5VDC supply would normally be regulated and perhaps the 12VDC, depending on what you're driving. The most simple way to regulate a 5V or 12V is to use a 7805 or 7812 regulator. Depending on the case style, and the heat sink used, they handle about 1A of current; however, they need about 3V extra. So, a 5VDC supply would start with an 8V or 9V unregulated DC source. The 7805 would regulate that higher source to 5VDC. A 12VDC supply would start with 15V or so.

TTL type circuits (chips) like a well regulated voltage source. They don't work too well if the voltage source is not regulated. The specs on the 78xx regulators say that they can have a source voltage up to about 35VDC. That is true, but remember that the wasted volts are turned into heat. Even dropping 3 or 4 volts across a regulator will cause significant heat.

You could use the 12V unregulated as the source for the 5V regulated line. I've done that many times. If you're just driving proximity sensors and relays, the 12V doesn't have to be regulated. If you're using the 12VDC as the voltage source for a PMDX-122, it doesn't have to be regulated. The PMDX-122 has a built-in 7805 regulator to change that 12V down to 5VDC.

[PEU]

I had a gut feeling that I should had ordered one more 10000ufx100v cap when I ordered them together with the proximity sensors, I think I can find 10000ufx63v locally, that should work too because I'm using just 48vdc.

For the 5 and 12v I have separate windings for each so I will just put a rectifier bridge, a capacitor on each side of the 78xx regulators and have both supplies regulated. I have a lot of LM350 lying around, with a couple of resistors they do the same job as specific regulators with the bonus that they can handle 3A

Thanks!!

[Richards]

Pablo,

Remember that a diode bridge will waste about 1.4V (two diode "drops") and that many regulators require a 3V drop (minimum), so that means that the 5V line must start out with 5V + 3V + 1.4V = 9.4V. To get 9.4VDC, your AC winding would need to be at least 6.6VAC before rectification. (I tried using 6.3VAC in one project to get 5VDC, but I had to replace those transformers with 8VAC units.)

The 12V circuit would require 12V + 3V + 1.4V = 16.4V minimum. That would require at least 11.6VAC before rectification.

[PEU]

Hi again Mike, is it normal for the transformer (EI not toroidal) to reach 60C (140F) measured with a digital thermometer?

what can cause such a high temperature? Keep in mind its only driving 2 geckos set at 1A each (10k resistor) and nothing else. BTW, its summer here, around 30C (86F) room temperature.

Thanks!

[KenC]

My EI transformer runs less than 50C at any weather condition under full operation condition. BTW, my EI transformer is rewind from a 450VA iron.
As you only run 2A max. I suspect poor winding or too thin secondary wires.

[Richards]

The 'standard' transformers that I've used with stepper motors run hot. I never measured the temperature, but they were too hot to touch. All of those transformers were the Triad brand, which is well known and reputable. The toroidal transformers that I now use exclusively all run at room temperature or just a few degrees hotter. None of them ever get hot to the touch.

Today, I ran a test on some steppers to see how hot they would get when I used a 47VDC power supply to power some PK296B2A-SG3.6 motors (which are rated at 1.5 mH inductance and 3A). I knew that I could use more than 39VDC with those motors because their current carrying capacity was underrated by 33% by the factory. They got toasty warm - about 45-degrees C - well under the 80-degree C rise that is permissible. However, the transformer never got above 72F in a room that was 70F. Even the Gecko G202 stepper drivers that I was using stayed under 80F.

I don't know why standard transformers run so much hotter than toroidal transformers. Many of the transformer web sites claim that toroidal transformers are much more efficient than standard transformers, but they don't explain why a standard transformer rated at 6A runs at a temperature that will blister your fingertips while a toroidal transformer also rated at 6A runs at room temperature under the same load.

[KenC]

Higher iron losses & hysteresis.
Even with these factors, 72C is abnormal... Maybe the factory did not underrate the tranny by 33%... Copper price had since sky-rocketeng in the past years & still rocketting... Anything can happen...

[andrewuk]

Hi would it be ok to use 68000uF capacitors as I might be able to get some verry cheap ? Thanks

[parrulho]

Yes you can, it only takes more time to discharge after you powers off. Just make sure the rated voltage of the capacitor is 10V or 20V more than the voltage of your power supply, just to be safe.