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  #1  
Old Sat 29 April 2006, 07:02
Gerald_D
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Calculate the Voltage and VA size of the DC power supply needed to build or buy

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. 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.

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 should idealy be 3 to 23> 20 times the rated voltage of the 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 driver - the GeckoDrive 200 family is limited to 80V max. Also, the drive wants a certain minimum voltage, and the GeckoDrive wants 24V minimum. To summarise, the power supply voltage for a Gecko-driven stepper motor must be as high as possible within 3-20 times motor voltage, above 24V, below 80V. (Typical range for steppers is 50 to 75 Volt). (Edit 10 September '07: In a discussion with Mariss yesterday he said: "The "20 times rated voltage" business is a convenient stand-in for what really counts which is V / (SQRT L) being less than 1000. This 20X stand-in works so long as there is a fixed ratio between resistance and the turns of wire on the motor. Vexta is a good quality manufacturer and the use a better fill ratio on their windings which yeilds a lower than expected winding resistance.")

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 router 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 2/3 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.
  #2  
Old Fri 26 May 2006, 10:20
Dirk
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According to the build your own thread, when the AC current is converted to DC through the recitifier and the voltage increases by a factor of 1.414, then does the amperage remain the same, or decrease by this same factor? If so do we have to compensate?
Dirk
  #3  
Old Fri 26 May 2006, 10:48
Gerald_D
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Hi Dirk,

The amperage does decrease by the same factor. Do we have to compensate? I tried to get an answer to that question by posting the following example on the Mach Yahoo group:

"4 stepper motors @ 2 Amp, 3V each
Conventional wisdom says power supply voltage of 75V would be good.
Power supply current of two thirds x 4 x 2 = 5.333Amp.
If using a packaged supply then 75 x 5.33 = 400VA

If building own unregulated supply:
Voltage 75 / 1.414 = 53V AC
Conventional wisdom says transformer VA rating 53V x 5.33A = 282VA,

Therefore a 50V 300VA transformer will be good alternative to a 75V
400VA packaged supply.

What am I reading wrong, or is this just one of the mysteries of the AC
versus DC world?"


From the responses it was clear that the simple truth is that we don't have to compensate as long as the transformer doesn't get hot....

Not a satisfactory clinical answer, but it is valid nonetheless. Our routers very seldom draw peak amps on all motors, and one cannot put a mathematical formula on that. It appears that the two thirds rule of thumb is actually even 41% lower, and most people's transformers are not burning out.

A very important point though is that a linear power supply (tranformer/rectifier/capacitor) copes very well with short duration overloads and maintains a good voltage while pumping out a few extra amps. A switching supply on the other hand would cut its output voltage drastically to save itself from pumping out even a milliamp extra. Thus, for driving steppers in a typical router, you could use a smaller VA rated linear supply than a switching supply, and this ratio is probably near the 1.414 factor, but that is coincidence.
  #4  
Old Thu 22 June 2006, 17:38
Mike Richards
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Part of the confusion in electronics is caused by the way things are measured. For instance, the 120VAC assigned to the normal line voltage in North America is really the root-mean-square reading (or average reading). The peak-to-peak reading is 1.41 times higher, or about 170VAC. What that means is that the power company is actually supplying 340VAC because the sine wave is going both positive and negative relative to neutral. With that bit of trivia out of the way, it's easier to understand what the various components in the power supply actually do. The bridge rectifier converts the positive/negative 60 cycle per second sine wave (in North America) to a positive 120 cycle per second sine wave (assuming that we want positive voltage instead of negative voltage). Since the output is a sine wave, the capacitor acts as a battery (of sorts) to store energy between sine waves. If we use a big enough capacitor, we effectively change the 120 cycle AC sine wave into Direct Current (DC). That's why we have to use capacitors with lots of capacitance in power supplies. When the load on the power supply is greater than the capacitor can handle, we get some ripple, which is an AC component to the DC voltage. Depending on the amount of ripple, the power supply is said to be efficient or non-efficient. The important thing to know is that nothing in the circuit magnifies the voltage by a factor of 1.41. Most volt meters read the RMS value of the voltage, but an oscilloscope would show the peak to peak value of the voltage.

From a practical standpoint, a 'square' stepper motor uses more current than a 'round' stepper motor. It's always better to assume that you'll need more current than specified by stepper manufacturer so that performance doesn't suffer by draining the capacitor(s) too much between AC cycles. My test is to feel how hot the power supply is running. If it is at room temperature or slightly warmer, I don't worry. If it is too hot to touch, then it's time to add more capacitors.
  #5  
Old Sat 24 February 2007, 11:58
reza forushani
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I just ot a MONSTER power supply. I ordered a 500 VA at 70V and this thing is big. It will fill the whole top part of the board. I'll have to move everything else down. Or move the PMDX card somewhere elese? Maybe I should have gone a little smaller.
  #6  
Old Sat 24 February 2007, 13:13
Gerald_D
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Reza, can you re-package (re-arrange) that supply to make it taller instead of wider? Take photos of what goes where, strip it down and re-assemble it more efficiently for space? If you go this route, please post a photo for comment before switching it on.
  #7  
Old Sat 24 February 2007, 16:52
reza forushani
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  #8  
Old Sat 24 February 2007, 16:53
reza forushani
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  #9  
Old Sat 24 February 2007, 21:29
Gerald_D
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Can you tell us the supplier and part number of this beast? What voltages does it put out?
  #10  
Old Sat 24 February 2007, 21:38
reza forushani
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Hi Gerald

The Beast or MONSTER is 500 VA with three outputs
70 VDC, 24 VDC, 9VDC
Custom made for me by a guy in NJ. He wanted $150.00 but I talked him into $100.00 including postage, promissing him more busines.
His name is John and his email is john@ango.com
Check out his websites. He has many transformers, power supplies, etc
http://www.toroid-transformer.com/

remind you this thing is probably a ton?! Realy many many kilograms or pounds, very heavy.
  #11  
Old Sat 24 February 2007, 21:42
reza forushani
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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
  #12  
Old Sun 25 February 2007, 09:58
Gerald_D
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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?
  #13  
Old Sun 25 February 2007, 14:14
reza forushani
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9V for the PMDX board and Relays, etc
24V in case I want to do some other lights, etc
Just to CMA
  #14  
Old Sat 03 March 2007, 10:47
Jay Waters
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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?
  #15  
Old Sat 03 March 2007, 11:23
Jay Waters
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Found what I was looking for here
  #16  
Old Sat 03 March 2007, 11:33
Mike Richards
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Jay,
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.
  #17  
Old Sat 03 March 2007, 11:36
Gerald_D
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Alternatively, avoid buying stuff that needs "odd" voltages. eg. If you have 115V in the system, use a 115V light.
  #18  
Old Sat 03 March 2007, 15:02
Jay Waters
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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.
  #19  
Old Thu 15 March 2007, 16:18
bugmenot dillbert
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Just out of curiosity, if the motor only ever draws 2/3 of the rating, is there any penalty for using a 2/3 of the amp rating for the motor for the driver, ie. a 2A driver running a 3A rated motor?
  #20  
Old Thu 15 March 2007, 22:25
Gerald_D
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Bugmenot, my first post in this thread was misleading - I have edited it now. The driver must match the motor current. But all 4 motors never work at 100% together, continuously.

However, I don't know what happens when the driver's "holding current reduction" is turned off - theoretically the motors would all draw max current when the machine is standing still. Let's say the conventional wisdom (and the first post) is only true if the drivers have a current reduction (min 33%) when the motors are idle.
  #21  
Old Tue 20 March 2007, 13:33
Loren Gameros
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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:

Hi,

"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.
Thanks.

John

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.
  #22  
Old Tue 20 March 2007, 13:50
Gerald_D
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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.
  #23  
Old Tue 20 March 2007, 14:38
Loren Gameros
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Ok, Thank You.
  #24  
Old Wed 21 March 2007, 13:45
Loren Gameros
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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.

Loren
  #25  
Old Wed 21 March 2007, 23:45
Gerald_D
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Relax, you will be okay
  #26  
Old Thu 22 March 2007, 07:58
Bob Cole
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Gerald, Anyone:

I have (4) Applied Motion Stepper motors. They are rated @ 2.47Volts, 5.9 Amps. Could someone give me an idea based on the math that you use to compute the power requirements, what size power supply should I be looking for to give me plenty of power with a "comfortable" reserve. Meaning I only want to do this once, Correctly.

thank you in advance.

Bob C.
  #27  
Old Thu 22 March 2007, 09:27
Gerald_D
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Bob, here is my stab at it:

- Volts: 2.47 X 20 = 49.4. Therefore a standard nominal 50V DC supply is just right. (The transformer will be 35V AC)

- VA: 35V X 5.9 Amps X 4 X 0.67 = 553VA if using using drivers with current reduction (Gecko's). Otherwise, the .67 factor cannot apply and you need 826VA

A nominal 50VDC 1000VA supply should laugh at anything you throw at it....hope you can fit it in the case and keep it cool.
  #28  
Old Sun 08 April 2007, 22:34
Michael Cunningham
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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 or should the supply only
supply 20X max.. ie 50v? Why?
  #29  
Old Mon 09 April 2007, 00:25
Gerald_D
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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).

I personally don't understand where the voltage limit of 20x comes from, but I have heard often enough that too high a voltage causes the motors to overheat.
  #30  
Old Wed 10 October 2007, 16:19
driller
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If you want more information on the GECKO drives, check out the white paper on www.geckodrives.com Mariss explains how the motor works and how to size the power supply to the motor.

But as Gerald said on this thread....

Add all the motor amps together, then multiply by 0.66 and that is the AMPS you need.

Look at the motor voltage, it should be between 1 volt and 12 volts, multiply by 20 to find the maximum voltage for that motor. Do not use more voltage and don't use over 80 volts with a Gecko.

The closer your 20x and power supply are to each other the better the performance.

If you have 4 motors at 2 amps each, then 4x2=8 further, 8 x 0.66= 5.28 amps

If you want a 70 volt power supply and to deliver 5.28 amps, then multiply 70 times 5.28 to get 370 VA or 370 watts.

A 400VA power supply is all you need. If you want to over-do it, then multiply the original 8 amps times 70 volts to get 560VA. You cannot use anymore than that no matter what you do with the drivers and motors, so anything more is not possible to use. Actually, you cannot use more than the 370VA, but no matter how many times you tell some people, they will not believe you, or just want to buy something bigger for some other reason.

Why convert to watts ? To find out what power you need from the 115volt wall outlet.
400 watts divided by 115 volts = 3.5 amps. So you need 3.5 amps from your wall outlet.

- - - -
Another question was why the 80 volt limit ?
When you decelerate a motor, it acts like a generator. The motor you just powered up to move the gantry will act like a generator when it is slowing that gantry down. That voltage can easily spike up over 20% of the voltage you can get from the power supply. So, Gerald is correct in his observation that a Gecko has components rated for 100 volts, but under operation, the voltage can get close to that !

Hope this is not confusing.

Dave
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