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Richards Wed 21 October 2009 08:55

If you wire those motors half-coil you will get better performance and still have all the torque that the gearbox is rated to handle. Wiring a motor half-coil gives about 70% of the torque as the same motor wired bipolar series. (1 / SQRT(2) = 0.7071)

Those motors, when wired half-coil, can draw up to 3A, so 47 X 3 / (7 - 3) = 35K. A standard 33K resistor works fine.

When you look at the torque charts, you'll see that a motor wired bipolar series is meant to be used at low speeds. That motor drops to about 100 oz*in at 250 RPM when wired bipolar series, but doesn't drop to 100 oz*in until it is spinning at 900 RPM when wired half-coil.

Travish Thu 22 October 2009 07:24

Thanks Mike and Gerald. I appreciated the guidence. 20 kohm is what I got to, so good to see it confirmed. I just couldn't find out what the Silly "I" standed for in the equation in the gecko manual. I've decided to go full coil for now, as I'll be doing plenty of 3d and and advanced pocket routines, so I hope to see the motors perform nicely. I'm enjoying this learning experience.

Gerald D Thu 22 October 2009 07:41

The I stands for current Intensity. They couldn't use C for current because C is the speed of light. No, I don't know why C is for the speed of light! :)

hennie Thu 22 October 2009 07:44

Mike something that I noticed on my motors is that when I wired them half coil it ended up getting hot verry quickly.I then changed to full coil and it seems to be fine but the moment I pump up the speed it gets quite hot.Before I had the heat thing the motors used 202 steppers then 1 packed up and I stuffed the other one up.I got some 203 `s to replace them .These are the two ones for the x-axis.Yesterday I surface planed the table and halfway I actualy stoped it for the motors to cool down.I am running 202 on the z-axis and the y-axis.I run 3:1 gearboxes Maybe i am pushing them to hard.

domino11 Thu 22 October 2009 07:49

Hennie, What is your power supply voltage? If your voltage was picked for full coil, switching to half coil would be driving the motor with too high a voltage I would think. Give us the specs for the motor and power supply. Maybe your voltage is too high for full coil too?

hennie Thu 22 October 2009 09:06

Heath I think that it gives me 57 volts dc with the motors rated at 3.5 amps.I will check it tomorrow when I am back at the shop had hospital food since yesterday afternoon.

Richards Thu 22 October 2009 18:51


When wired half-coil the voltage should be 40VDC or less. (I use 35VDC). Also, be sure to change the current limit resistors to something around 33K so that the motors can pull 3A through the stepper drivers.

(The motors are rated at 6mH inductance when wired bipolar series, but only 1.5mH inductance when wired half-coil. So, the forumla for MAX VOLTAGE = 32 X (SQRT(Inductance)).

Gerald D Thu 22 October 2009 23:51

Originally Posted by Travish View Post
I'm using PK296A2A-SG7.2 motors, so my amps per phase are 2.1 amps wired Bipolar?
To which Mike replied:

Originally Posted by Richards View Post
Those motors, when wired half-coil, can draw up to 3A, so 47 X 3 / (7 - 3) = 35K. A standard 33K resistor works fine.
Mike, are you sure about that? Our half-coil wired PK296A2A-SG7.2 motors have a 42VDC supply and 20k resistors for current limiting. It sounds like you go for 35VDC & 33k for current limiting?

Richards Fri 23 October 2009 07:01


Page C231 of the older Oriental Motor catalog shows:


- Bipolar Series
2.1 A current
6 mH Inductance

- Unipolar (half-coil)
3 A current
1.5 mH Inductance

I'm using the formula 47 * A / ( 7 - A ), which gives 35K for the PK296A2A-SG7.2 motor when wired half-coil. That page also gives 1.5mH for the motor when wired half-coil, so 32 * SQRT( 1.5 ) = 39.19 V. I use 33K for the resistor and 35VDC for the power supply.

If you reduce the current going through the motor to 2.1A then 47 * 2.1 / (7 - 2.1) = ~ 20K. That would also allow you to increase the voltage a little before the motor runs too hot.

I just checked the watts that the motor would produce . At 2.1A and 42VDC, it would produce ~ 88.2W, using the formula ( Amps * Volts ) = Watts. At 3A and 35VDC, it would produce ~ 105W. Using the Gecko formula of 32 * SQRT( Inductance ), Mariss gives 117W as maximum before the motor gets too hot. With 35VDC, my motors run between 65C and 75C, when I run them at 39V, they reach 80C to 88C. Mariss believes that a cool motor is not being worked hard enough. I would rather run a little cooler in exchange for longer life.

Gerald D Fri 23 October 2009 11:06

I must be loosing my marbles? I was quite sure that I always spoke of 2.1 Amps when there was a question on the current setting for PK296A2A-SG7.2 motor when wired half-coil. Maybe that is why we have cool motors and transformers at 42VDC and 300VA? Right now I not sure what resistors are in our system anymore . . .

Mike is of course right; the current limit for a PK296A2A-SG7.2 motor when wired half-coil is 3 Amp (not the 2.1 Amp I have in my head today).

Richards Fri 23 October 2009 11:35

I decided to run a spreadsheet on some Oriental Motor steppers against the formulas that Mariss published to see what how many watts each motor would produce. I expected to see a close match for each motor size, instead, I found quite a large variation.

(You'll notice that the PK296A2A-SG7.2 motor produces less heat when wired half-coil than it would if it were wired series, but that assumes that you're using the maximum voltage with each type of wiring.)

Series, 112.0 watts
Half Coil, 60.0 watts

Series, 168.0 watts
Half Coil, 179.6 watts

Series, 250.8 watts
Half Coil, 176.4 watts

Series, 164.6 watts
Half Coil, 117.6 watts

Series, 168.0 watts
Half Coil, 235.2 watts

Series, 254.4 watts
Half Coil, 227.7 watts

Series, 254.4 watts
Half Coil, 227.7 watts
Parallel, 318.8 watts

jehayes Mon 26 October 2009 14:51

Gecko G540 or G250 as Alternatives?
Any thoughts on using a Gecko G540 or four G250s instead of four G203Vs? The price difference is substantial and to my (untrained) eye, they seem to be equivalent.


G540 Spec Page here:

G250 Spec Page Here:

bradm Mon 26 October 2009 15:21

Joe, this has been discussed quite a bit. If you search on "g540" you should find at least five threads. Hint: I use one on my build, successfully.

jehayes Mon 26 October 2009 15:43

Brad: Thanks, I guess I missed them. Are you happy with the G540? Joe

bradm Mon 26 October 2009 18:22

Yes, I'm quite happy with it. The only issue I had was that the PWM output for spindle speed control turned out to be bad. I discovered it a full year after I'd purchased the G540 when I upgraded, and one quick email to Gecko later, they replaced it. Good people there. Just make sure your motors match the G540 specs (the OM PK296A2A-SG7.2 does).

javeria Tue 27 October 2009 08:15

and have a good heat sink for it too.

Richards Tue 27 October 2009 08:28

The G540 does not require a heatsink. Mariss ran some heat tests showing that if you simply have a computer CPU fan or case fan blowing against the G540's case, that the fan will keep the G540 well within the recommended temperature range.

javeria Tue 27 October 2009 10:18

I think I have read somewhere on the CNCzone of the 540 behaving better with a heat sink - what ever the way - its not bad to have a heatsink afterall (along with the CPu fan)

Richards Tue 27 October 2009 13:13

Those posts on the CNCzone forum led to Mariss's running those heat tests. Inside the G540, each of the stepper driver cards is using the G540's case as a heat-sink. Adding an additional heat-sink was not necessary as long as a fan was used to circulate air past the G540's case.

A similar condition exists when you use an aluminum or steel controller case. Circulating the air around the inside of the case allows the entire case to help dissipate the heat. Blowing air onto the outside of the case helps with the heat transfer. That's the method that I use to cool my Shopbot's controller on those wonderfully hot days where the temperature is over 100 F (unlike today where it's snowing outside).

javeria Tue 27 October 2009 16:08

Ok Mike - I (we) take your word for it :)

timberlinemd Tue 16 March 2010 00:55

Mounting the G540
I searched the forum for a pic of a G540 mounted in the control box and couldn't find one. So after reading the info from gecko about heat sinks for the G540 I decided that I will mount the drive on stand-offs (like Gerald shows his drives on a alu plate) without the plate and in front of the fan. Does anyone see any problem with this?

Richards Tue 16 March 2010 05:33


That is the recommended procedure. The G540's case is the heat sink for the boards inside the case. Blowing air against the case will dissipate the heat. The newer model G250x runs cooler than the original G250 modules that I have, but in either situation, keeping air moving past the G540 will greatly reduce any 'hot spots'. Make sure that the fan has free space front and back so that it has unrestricted air flow.

jehayes Wed 17 March 2010 11:16

Originally Posted by timberlinemd View Post
...I will mount the drive on stand-offswithout the plate and in front of the fan. Does anyone see any problem with this?
Steve, I mounted my 540 on two delrin blocks (just scrap from around the shop, wood would work as well.) No fan, no heatsink. I have not had any heating problems whatsoever. (Of course I live in a place that doesn't get above 70 degrees - ever!) :D

domino11 Wed 17 March 2010 12:36

I think I would stay away from wood in an electronic enclosure. It is another source of fuel. Delrin is a better choice.

bradm Thu 18 March 2010 18:57

Steve, I mounted my G540 in the side of my box. Cut the rectangle, drill pilot holes, self-tapping sheet metal screw it in place; the entire box becomes an extension of the heatsink.

The screw strip that you need to wire to your BOB and power supply will be inside the box; the DB9 connectors for the motors will be outside where they are ready to attach cables.

If you decide things are running too warm, add a fan across the back. It seems to me that this is consistent with how the G540 is designed.

timberlinemd Fri 19 March 2010 22:50

Thanks for the info!

renraku Fri 09 July 2010 02:56

resistor type
Hi Mike i need of a your personal suggestion : i have nema 34 - 5.6A - 4.4 NM
gecko drive G203V what resistor power is correct? 180K ? Thanks

Richards Fri 09 July 2010 11:56

For the G201, G203, or the G203v stepper drivers, the formula is:

47 X Amps / ( 7 - Amps) = Resistor.

So, for your motor:

47 X 5.6A / ( 7 - 5.6A) = 188K resistor.

A 180K or a 200K 1/4 Watt resistor would work fine ( or any other resistor that is +/- 5% of 188K).

renraku Fri 09 July 2010 13:20

Thanks Richards

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