Building own 35V supply, based on Parts Express toroid transformer
 

I received my parts for the PS on Friday and wired them up to test the output, it wasn't difficult at all and I'm glad I followed the advice to make a simpler PS. I am sure that the KISS aspect of this basic PS will pay off in the long run.

Thanks for the good advice, and all the threads and posts that led to the understanding of them.

I was just re-reading this thread and thinking about my setup and now I have a question. I ordered the 25+25 transformer and was thinking that for my setup I'd wire it in series so I get about 50v, but then I noticed all the talk about the 1.4 times multiplication of the voltage when I send it through the rectifier and it maid me wonder if I'm better going with parallel wiring on the output to give me 25*1.4 or 35v. I'm using 4 OM PK296A2A-SG7.2 motors and I can't seem to find their spec sheet anywhere to do the calculations but I've seen the 35-40 figure a few times.

Any suggestions

Definitely 25+25 parallel to give 25VAC = 35VDC output. (That is for a "half-coil" wired motor)

Thank you for the clarification. The bummer is that when I did the test wire on Friday I crimped the wires into the connector parallel and thought that would only give me 25v so I undid it and re-crimped for 50, I had forgotten about the jump when it goes from AC to DC so now on Monday I get to cut the crimps off and redo it like I had it originally. It's only like 3 minutes of work, but having it right once and then changing it makes me feel kinda dumb sometimes.

Brian,
Parallel secondaries is the way to go as Gerald mentioned. Also I would be concerned with the G540 as it is rated at only 50V. I would not want to run that driver right at the 50V limit. But series seconndary windings with your transformer would give you about 70V.

Yeah, I'll definitely be redoing it in parallel on Tuesday. I knew I was shooting for the 35vdc I just forgot about the bump it gets from 25 through the rectifier so I was wiring the way it would give me more voltage before I realized that way was going to be too much more voltage.

There is a lot of misunderstanding about AC voltage and DC voltage. AC voltage is normally measured as an 'average' voltage (RMS or Root Mean Square). DC voltage is measured as the peak voltage. The difference between AC RMS voltage and DC peak voltage is AC RMS X SQRT(2). That's where the 1.414 multiplier comes in and that's how AC voltage from 25V transformer turns into 35VDC after being rectified and smoothed with a capacitor.

Mike, thanks for the info, that's an interesting and probably useful bit of knowledge I didn't previously know. I was familiar with the idea of peak and RMS, I just didn't know that DC was measured Peak and AC RMS.

Mike

On this then I have a question. Like Brian I too am using the G540, unlike Brian I'm using a motor that is less than ideal for this driver (MK34HS9801) and am happy to accept the trade off that this pairing will give me (largely as the OM motors are rediculously expensive here and I plan to build a second smaller machine with smaller motors once I've got a bit of experience) however I'd like to run these motors at 50V (max for G540) the thing that comes to mind is that often there is variation between published mains voltage (240V here) and what is actually delivered (+/- 10% comes to mind) the problem I have is that if I size a transformer at 35V to get the 50V, what happens when they are +10%? When you apply the stepdown ratio from primary to secondary and then convert to peak you are over the voltage for the drive, I can step down to 30V and trade off further or am I being overly concerned?

Jed

I don't have the experience of the other folks who helped me out (obviously since I asked them for help) but I would think that with such swings in your supply voltage maybe you'd be better of with a regulated power supply, I think they are designed to stay more consistent with less reliable input power. The price difference (at least here in the states) was moderate when I was looking into it and the only real drawback was the potential for dust damaging the power supply, which if you put it in your control box would be reduced.

Jed,
In an unregulated power supply, if the mains input rises, so does the output of the supply. The percent rise is the same, 10% rise on input, 10% rise on output. I dont think I would want to run a G540 much more than the rated 50V. Mariss, the designer of the G540 had this to say on the geckodrive yahoo group

Re: G540 max voltage?

The G540 is rated at 50VDC, it is designed for and tested at 60VDC
and it comes apart at 68 to 70VDC.

I don't encourage anyone to run it much above 50VDC because that 18 to 20VDC is
a safety margin. A derivation of Murphy's Law yields "If it can happen, it will
happen." Take these 3 "can happen" events:

1) Line voltage goes to the +10% tolerance. 50VDC becomes 55VDC.

2) Rapidly decelerate all 4 axis simultaneously from a high speed to zero. The
supply voltage bumps up 10% for 1/8th of a second.

3) A large AC load dump (compressor, air-conitioner, etc.) occurs. The voltage
surge brightens the shop lights for 1/2 second. The supply voltage bumps up
another 10%.

Now take the "will happen" part and make (1), (2) and (3) simultaneous; add them
up. I get 55V, + 5.5V and +5.5V for a total of 66VDC on the G540. It shrugs it
off like it never even happened.

Run the G540 at 60VDC and you get 66V, +6.6V and 6.6V. I get 69.2VDC and
probably a very dead G540 afterward.

The odds of (1), (2) and (3) happening simultaneously are fairly long, but given
enough time, it will happen. Weeks? Months? As Dirty Harry said "Do you feel
lucky?" :-)

Mariss

the original post Here

Heath

Thanks for the Post from Mariss, I don't know how Gerald (and the rest of you for that matter) does it as I have trouble keeping up with this forum let alone, the gecko group, cnczone and no doubt several others. In any case I'm not proposing to run the G540 over the 50V rather at it, was just concerned about the variation in supply voltage resulting in it being run over the 50V and what the result would be, Mariss's post answers this thanks I'll check my numbers and see how they compare to those included in your post.

Thanks

Based on the "The G540 is rated at 50VDC, it is designed for and tested at 60VDC
and it comes apart at 68 to 70VDC." I would check my average local mains voltage and pick an unregulated supply that gives 50VDC at that average mains voltage.

If your mains does go high by 20%, you are at the 60VDC design point - an unlikely (in Australia) and safe event. The G540 will come apart when your mains goes 40% over the norm - imagine what else in your country will make smoke if the mains goes over 300VAC? Your little G540 will be the least of your (and your insurance company's) concerns if that happens! :)

PS. I don't subscribe to the:

"2) Rapidly decelerate all 4 axis simultaneously from a high speed to zero. The
supply voltage bumps up 10% for 1/8th of a second.

3) A large AC load dump (compressor, air-conitioner, etc.) occurs. The voltage
surge brightens the shop lights for 1/2 second. The supply voltage bumps up
another 10%."

factors.

2.) Have measured the rise in voltage when the gantry decelerates and found nothing sigificant.

3.) Voltage surges on load dumps are typical for people running off small generators where the diesel/gas engine has to slow down. We are connected to a massive grid of generators and we just don't see voltage surges as envisaged there.

Gerald,
Sorry if I complicated matters here. Maybe I should have only posted the first half of Mariss' post. I just thought it should be quoted fully.

No, I am glad you posted the whole thing. This all just needs to go into a more topical thread though.

OK, I haven't hooked up the capacitor yet and that may be what's confusing me, but I am confused. I rewired the coils so that they're parallel on the input side from my 120vac and parallel on the output side and run it into the rectifier, when I measure it there I get ~25vdc. Is it after the cap that I will see it measure 35vdc? I may have tested it before I read any replies, but it may not be a bad idea to have it here too. I am trying to avoid a charged capacitor as long as possible since I won't have it hooked up to the gecko and I don't want to wait for it to discharge on it's own.

When you measure after the rectifier, it is still basically AC (huge ripple). The capacitor is needed for true DC.

Brian,
You might be waiting a long time for the capacitor to discharge on its own. Maybe weeks or a month. Even then it might not be discharged fully. You could always use a 100 watt incandescent light bulb in a pig tail with clip leads to run it down after. :)

Gerald,
Thanks for the conformation.

Heath,
Good idea.