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Gerald_D Sat 29 April 2006 07:02

Calculate the Voltage and VA size of the DC power supply for stepper motors/geckos
The original version of this thread archived June '08.

You will typically hear that a power supply is 48 Volts, 500 VA. The VA is for Volts.times.Amps, therefore that aforementioned 500VA power supply can deliver about 10 Amps at 48V.

For driving a stepper CNC router, you need to get the voltage right so that it matches the stepper motors that you selected, but this is not critical. A 45V supply will work nearly as well as a 48V supply. If the voltage is too high, the motors could get too hot. If the voltage is too low, you can loose speed & torque

The VA part of the calculation is also not critical. An old surplus power supply of 800VA will drive a system for which you calculated 500VA. The physical size of the supply is determined by the VA rating - a 800VA supply would only be slightly bigger than a 500VA. If your power supply has too low a VA rating, it could get hot, or loose a little bit of Volts at high loads. If it has too high a VA rating, there is no downside on performance, but your mains could start tripping out when you switch it on.

Okay, that was a broad introduction of the 2 important numbers involved, now we get to the detailed calculation:

Decide on your motor wiring configuration and get the appropriate motor current and voltage specs...

Voltage: Believe it or not the power supply voltage to the drives should be about 20 times higher than the rated voltage given for stepper motor. Most agree, the higher the better, the limit is decided by overheating motors. But, the power supply voltage must not exceed the capability of the drive - the GeckoDrive 200 family is limited to 80V max. Also, the drive wants a certain minimum voltage, and the GeckoDrive wants 24V minimum.

So, where do we go in the range 24 to 80V? Find the inductance of the motor from its specsheet in milliHenry, take the square root of the mH value and multiply that by 32. For example, a motor that is rated 1.5mH needs: 32 x √1.5 = 39 Volt. Edited to add: That is a conservative formula by Maris Freimanis, intended for motors constantly running at full load. We know that our CNC application will never run one motor at full load all the time. I recommend to select the power supply voltage 20% higher than that formula. The motor heating can be controlled by a simple setting of the drive/motor current. (It is not so simple to change the voltage later). Member "Richards" has many posts on having low voltages, but that has shown to produce low cutting speeds and weakly performing CNC machines.

VA-size ("Watts"): Again, this comes from the motors' specs. Each stepper motor will have a max. Amp rating. Add all the Amps of all the motors together, and then you will know the peak Amps that your motors may draw. If you have 4 motors marked at 2 Amp, we expect the drivers to be able to supply 8 Amp. Surprisingly, a Gecko stepper driver does not need 8 Amps input to supply 8 Amps output, it mostly needs much less. There is agreement that your power supply need only be able to supply 67% (two thirds) of the driver's current requirement. Those 4 x 2 Amp motors will only need 5.3 Amps from the power supply. (Hard to believe, but we havn't proved it wrong yet.) Multiply the last mentioned Amps with the Voltage selected above and you have the VA value

Therefore, start looking for a packaged power supply that firstly meets the voltage spec and secondly exceeds the VA spec. Or consider to design & build your own.

reza forushani Sat 24 February 2007 21:42

I do have a question, the power supply has two black and two red leads. I guess for 110v like my situation I need to tie both blak together and both reds together running parallel. For 220v I guess would be serial? Do I need both blacks and reds? It seems I am getting the right output by just using one black and one red. see this thread for the relay

Gerald_D Sun 25 February 2007 09:58

Using one black and one red will give you the right voltage output, but you will only get 250VA out of the 500VA transformer.

Why did you ask for 24V and 9V DC?

reza forushani Sun 25 February 2007 14:14

9V for the PMDX board and Relays, etc
24V in case I want to do some other lights, etc
Just to CMA

Jay Waters Sat 03 March 2007 10:47

Being that I am electronically challenged, how else would you go about supplying power to other items that need different voltages, such as the lights mentioned above without buying a power supply like Reza's? Say, for example, that I built a power supply like Gerald did. How would I get the correct voltage for the other items?

Jay Waters Sat 03 March 2007 11:23

Found what I was looking for here

Mike Richards Sat 03 March 2007 11:33

You will need a voltage source for each item that requires a different voltage. A 24V lamp will need a 24V. A 5VDC device will require a 5VDC power supply.

(That's why a standard PC-type power supply generates so many different voltages. Components inside the computers require different voltages.)

One way to get multiple voltages is to buy a transformer that has multiple 'taps'. However, for a DC power supply, each 'tap' must have its own rectifier and its own capacitor. If the DC supplies are regulated, to give a precise voltage, the supply will also require a voltage regulator.

Depending on the amount of current required, a higher voltage can be regulated down to a lower voltage. I sometimes use a 24VDC power supply with several regulators to give me 24VDC, 15VDC, 12VDC and 5VDC. HOWEVER, and this is important, when you use simple regulators to reduce the voltage, heat will be produced - sometimes lots of heat. Unless you only need a few milliamps of current, it's best to either buy a switching power supply that can efficiently produce several different voltages, or buy a multi-tap transformer.

Gerald_D Sat 03 March 2007 11:36

Alternatively, avoid buying stuff that needs "odd" voltages. eg. If you have 115V in the system, use a 115V light.

Jay Waters Sat 03 March 2007 15:02

Thanks, Mike and Gerald, just trying to get things straighened out on paper and in my brain before I commit to buying anything. Everyday is just one step closer to building and the opportunity of learning something new from you guys.

Loren Gameros Tue 20 March 2007 13:33

Hi Greald,

I am having some technical difficulties in figuring out which power supply to use. I sent an email to John at Antek asking which power supply to use and this is what he said:


"The PS-8N70 will give you 800Watt 12A at 70Vdc. It is little more than you need". the price is $120 each plus $10 shipping.
"Or I can give you the 600W 8A at 70Vdc. It is same rating as you need". The price is $110 each plus $10 shipping.
I may not have those the our site now. I can build it in a few days.


I am using the Gecko 202 and the Oriental Motor Vexta-Step type PK299-01AA motors with the PMDX-133.

So which power supply do I use?

Thank You.

Gerald_D Tue 20 March 2007 13:50

Loren, we run 4 of the Gecko G202 and OM PK299-01AA motors on only a 300VA power supply with no problems. As I said right at the beginning of this thread: "The VA part of the calculation is also not critical. An old surplus power supply of 800VA will drive a system for which you calculated 500VA. The physical size of the supply is determined by the VA rating - a 800VA supply would only be slightly bigger than a 500VA."

As long as you can afford the price, and fit the supply inside your case, you can use a big supply to drive a small load without a heat penalty. I start asking heat questions when guys are forced to use big supplies because their motors want lots of amps.

Loren Gameros Tue 20 March 2007 14:38

Ok, Thank You.

Loren Gameros Wed 21 March 2007 13:45

Just to follow up.

John at AnTek says that I will need a 600W 8A at 70Vdc to drive the 4 (202) geckos and 4 (2 amp) oriental motors. Being that I am a novice at this I have ordered based on his recomendation. Wish me luck.


Gerald_D Wed 21 March 2007 23:45

Relax, you will be okay

Michael Cunningham Sun 08 April 2007 22:34

So do the geckos' actually increase/decrease the voltage to the steppers? I think they can handle up to 80v.

Using the example above of 49.4.. would it be okay to use a 70v 800va supply and the geckos reduce the voltage? Why?

Gerald_D Mon 09 April 2007 00:25

The gecko's function is actually quite complex and we cannot just simplistically say it increases/decreases this or that. The input to a gecko is simple - pure DC at a steady voltage. The output is a "digital" waveform which is "chopped" up. A normal multimeter is okay for measuring what goes in, but tread carefully to measure what comes out. (My multimeter shows about 200V coming out, which is plain impossible). Having said all that, now to answer your question.....

The maximum input voltage to a gecko is stated as 80V. That is a conservative rating and is one of the reasons why it has such a good reputation for long-life. Other producers, use the same components would rate at 100V, but they don't have the reputation for long life.

The gecko itself can do nothing about increasing/decreasing voltage - it is a current controlling device. The user can select a current output level (when running and when at standstill).

Richards Wed 08 October 2008 12:01

1 Attachment(s)
Here's an OpenOffice Calc spreadsheet that computes the maximum power supply voltage for a motor based on the inductance of the motor. It computes the capacitance that you'll need when using four motors based on the motor's current rating. It also computes the VAC and VA ratings for the required toroidal transformer to generate the maximum voltage. Of course the computations assume that the motors will be drawing full current from the power supply (which is rarely the case); however, it provides the maximum values that you would usually need.

Unless otherwise specified, I used the unipolar (half-coil) figures from Oriental Motor in my computations. Just substitute the bipolar-series current and inductance data if you prefer to wire your motors bipolar series.

The spreadsheet is zipped, so you'll need to unzip it before you can use it.

Substitute the model number, the current rating and the inductance rating for the motors that you plan on using for any of the motors listed to compute values for your motors.

As an example, the PK296A2A-SGxx motor, which is a geared motor that several of you are using, shows that a 27.71 VAC toroidal transformer would give the maximum voltage for that motor. However, looking at, the closest AVEL toroidal transformer is the Avel Y236750 330VA 25V+25V Toroidal Transformer that they sell for $60.33 plus shipping. That transformer would give you about 35VDC (25 X 1.41 = ~ 35VDC). That's the transformer that I would use.

(If you don't have OpenOffice, you can download it at no cost from I use the OpenOffice programs exclusively on my Windows and Linux computers.)

Gerald D Wed 08 October 2008 13:23

Thanks for the spreadsheet Mike. I am still entirely comfortable in de-rating the transformer VA size by a third. See this thread, and my measurement results there on 26 September 2008.

gmessler Wed 08 October 2008 16:36

Thanks for the spreadsheet Mike.:)

I like the idea of open office. Just wish I knew Linux better.

For those that can't open these docs or don't want to load the viewer I've found a website that will do conversions online.

Just find the file and upload. There a many different export options listed.


gmessler Wed 08 October 2008 16:52

Hey Mike,

Here are a few more numbers for your spreadsheet. I got curious what my motors would be so I pulled the info from the oriental site.:D

for the holding torque wired Bipolar (Series)/Unipolar

44 pound inch = 703.999 ounce inch

44 pound inch = 703.999 ounce inch

22 pound inch = 351.999 ounce inch

Not sure what they'd be wired half coil.

Richards Wed 08 October 2008 17:33

Thanks for posting the thread where you listed the 300 VA, 28VAC to 30VAC suggestion. I knew that I'd read it, but obviously I didn't look far enough.

Your suggestion on transformer size (both VAC and VA ratings) are almost exactly what I would suggest. My line voltage tends to run a little high most of the time (125VAC), so I'm more comfortable using a transformer that produces just a few volts less than maximum. The standard 25VAC AVEL toroidal transformer gives me 35VDC to 38VDC depending on what the power company is sending down the line. Also, your suggestion of 300VA would be an excellent choice, even though the spreadsheet calculates about 10% higher. As you've mentioned, the motors pull less current than their ratings would indicate. (I've run four PK296B2A-SG3.6 motors from a 250VA supply without any problems, but I would like a transformer that puts out a little more than 250VA. The over-the-counter AVEL transformers have ratings of 250VA, 330VA, 500VA, 625VA, and 800VA; so, the closest match from AVEL would be the 330VA unit. Other suppliers might have transformers with other ratings. Having one custom wound would let you specify almost any VA rating.)

The reason that I wrote the spreadsheet this morning was to get a quick and dirty cross-check for matching motors and power supplies to the Gecko G540 unit. I might have a non-CNC application coming up that only needs the 23 frame size motors so I used the spreadsheet to show me the figures.

OpenOffice is available for Windows. The programs included with OpenOffice do the same things as the programs included with Microsoft's Office. OpenOffice can usually read Microsoft Office files, at least the pre-Vista versions.

I just noticed that you posted the torque ratings for the PK296A1A-SG7.2, the PK296A2A-SG7.2 and the PK296A2A-SG3.6 motors. According to the footnote on the Oriental Motor site, the motors are rated the same for Bipolar Series or Unipolar (half-coil) due to torque restrictions of the gear-head, otherwise the torque rating would be a multiple of the PK296-0xA motors, depending on the gear reduction. For those users who need more than 22 in*lb or 44 in*lb (depending on 1:3.6 or 1:7.2 ratios), they could use the PK299 motor with a 1:3.6 belt-drive to get about 139 in*lb of torque. With a 25 tooth spur gear (1.25" pitch diameter), the 1:3.6 belt-drive would move an axis 0.000545" per step which is almost 6X better than the original steppers on my Shopbot (20 tooth, 1" pitch diameter, 1,000 steps per revolution, no gear reduction).

Richards Sat 25 October 2008 20:15

I've been running some heat tests on an Oriental Motor PK299-F4.5 motor wired Bipolar Parallel connected to a Gecko G203v stepper driver. The current limit resistor is 220K, which limits the current to about 5.75A, slighly less than the motor's limit of 6.3A.

The motor's data sheet shows that the motor has an inductance rating of 2.5mH when wired Bipolar Parallel. Mariss's formula to find the maximum power supply voltage for the motor showed that 50VDC was about maximum (32 X SQRT(2.5) = 50.6VDC). I had an AVEL 18 + 18 toroidal transformer on the bench, which when wired in series should give me about that voltage. It actually measured 52VDC, which was deemed close enough for the test.

I wrote a simple G-code file that spun the motor in one direction for six seconds, paused for 1/2 second, spun in the reverse direction for six seconds, paused for 1/2 second at feed speeds of 120-ipm, 180-ipm, 240-ipm, 300-ipm, 360-ipm, 420-ipm and 480-ipm. I configured Mach3's motor tuning setup at 1526.7 steps per inch and the acceleration at 25 (0.065 G's), as if the motor were driving a 3.6:1 belt-drive with a 1.5-inch pitch diameter spur gear (30-tooth).

At 52VDC, after about an hour of running the motor was 87 C.

I used my Variac to cut the voltage to 42VDC (30VAC X 1.41) and still got 80 C.

At 35VDC (25VAC X 1.41) the temperature dropped to 70 C.

The Gecko G203v stabilized at 43 C. It is mounted to a 5" X 5" X 0.25" piece of aluminum.

The 250VA Avel toroidal transformer basically stayed at room temperature. The room temperature is 22 C and the transformer is only 25 C.

The results surprised me a little. Basically, 85 C is a very safe temperature for a stepper motor. As Gerald has pointed out, the insulation in a stepper motor is designed for an 80 C rise in temperature. With the room temperature at 22 C, the motor's temperature could have been as high as 102 C before I'd reached the maximum temperature.

At all voltages, the motor was too hot to touch. So, bottom line, Mariss's formula seems to work with Oriental Motor stepper motors. The PK299-F4.5 is the largest motor that I have. It is rated at 6.3A Bipolar Parallel and 880 oz*in, so it is a larger motor than most of us would actually need, especially if we gear it to 3.6:1 with a belt-drive. That would give over 3,000 oz*in of holding torque (compared to the 1,300 oz*in I have with the 7.2:1 Oriental Motor Alpha steppers).

When I put the PK299-F4.5 motor to work, I'll probably use a 30VAC transformer to give about 42VDC. That would easily give me the speeds and torque that I would expect and the temperature would be just about right to produce all the hot water that my family could use (if only that were possible).

Edited: At 50VDC, the motor sounded smoother than it did at lower voltages. I could probably play with the motor tuning pot on the G203v to see if I could tune the motor a little for lower voltages.

shaper Wed 11 February 2009 05:51

Oversized Transformer
I've finally gone and committed to building, so I've bought a G540 and am planning on running it with Motionking 34HS9801, driver supply will be at 50V as per the max for the G540 and have gone tough and calculated the VA rating for the transformer to be 330VA with secondary voltage of 35V. So my question is, I'm looking for a suitable transformer locally. I can find a couple at 300VA with secondary voltages of 35V or several at 500VA with 35V sec, if I were to use the 500VA transformer would this work? I'm thinking that the G540 will only daw the required current to drive the motors so the oversized transformer will just be under utilised and cost a fraction more. Other reason for thinking this way is that if at some stage down the track I wanted to upgrade I could do this with G203's and run the motors at full rated current. Or am I off the mark here?

Richards Wed 11 February 2009 06:24

The 500VA will work fine. As you said, the G540 will only draw as much current as it needs, so the extra capacity will not be used. I'm doing almost the same thing. I have a 500VA 25+25 toroidal transformer that I use with my new G540. It works just fine with the PK296B2A-SG3.6 drives that I have.

The 34HS9801 has an inductance rating of 4.1mH, so 50VDC is about 78% of the maximum voltage that the stepper should be able to handle. That 78% figure is exactly where I started with the G540 and some size 23 motors (PK268-02AA). I used a 48V power supply when they were rated at 61V. The G540 got warm and the drives got hot, but everything was well within temperature specs.

Gerald D Wed 11 February 2009 06:27

Your thinking is correct. The 500VA will do the job. (I am also fairly confident that the 300VA will do okay on the G540). The only downside of bigger VA (aside from cost & size) is the inrush current when switching on, but you shouldn't see that at 500VA already.

javeria Wed 11 February 2009 06:31

Inrush current is a big problem to overcome - adds a lot more complexity and expenditure - look at my experience here


Gerald D Wed 11 February 2009 07:04

That was a 1 500 VA supply that caused Irfan his troubles.

shaper Wed 11 February 2009 15:57

Thanks Mike, Gerald and Irfan

It probably wasn't obvious but I had been through and checked all the voltages etc for my setup, when I get started proper (I'm about to go on Holidays for a few months) I'll start my own thread and post all this (spreadsheet) as part of it.

I agree Gerald, even with inrush I'd be surprised if the 500VA transformer would draw a starting current that exceeded the 10A that single phase circuits here are rated for, as you say though a 1000VA or 1500VA would.
I'll give the 300VA option some more consideration, what are likely to be the drawbacks of this, obviously the current on the secondary side will be lower so I'll get lower holding torque, correct? I would really like to get hold of the a 330 or 350VA transformer but for the shipping cost I might as well buy the 500VA that's available locally (plus i can have it tomorrow when my Gecko arrives if I'm happy with this option)


Richards Wed 11 February 2009 16:20


Remember that the 34HS9801 is rated 4A per motor. You will need to limit the current to 3.5A or lower by using a 3.5K or lower value resistor. So, 3.5A X 4 motors = 14A and 14A * 35VAC = 490VA. The 500VA transformer would be a perfect match. (Of course, you probably won't be driving those motors anywhere near their peak current levels, so a 300VA transformer would probably work fine, but for the small amount of difference in cost and size, I would use a 500VA unit.)

anton Mon 22 June 2009 10:33

I see that the PK296A2A-Sg7.2 is 1.5 mH indctance if wired unipolar, and 6 mH inductance if wired Bi-polar series. DO you guys all wire the motor in unipolar or Bi-polar? Am I right in saying either way is does not make much difference on the powersupply, as with bi-polar 78 VDC would be got by putting the output from the 2 x 24v 300-500VA torodial in series, and for uni-polar put the outputs in parralal? Will the diffence in amperage not make a diff?


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