Need Help!- choosing the right power supply

[cyclotron]

Hi all,

I was wondering what other people might have done when
choosing a power supply for their geckos and their steppers.
I am running 3 pacsci powermax II's
( ~190 oz-in, wired series 3.3 Amps, 2.8 V, .85 Ohms)
The literature on the gecko site says that each drive will draw
1/3 of the motor current (so a total of 3.3 Amps for all 3 drives plus about 10
Amps for all the motors) Is it wise to get a power supply that gives only the current that you need (in this case about 14 Amps) or, should one pick a power supply with a lot of extra current? ( in the event that a 4th axis is added for example, or for extra control boards that turn on coolant and things like that) Insight into this area would be greatly appreciated, my knowledge of electrical issues like this is very limited. Thanks!

[olf20]

When building my setup / power supply, I used the information at Gecko, and Bob Cambel's web site. Both of them together gave me more than enough knowledge. After I decided on what I was going to build I discussed this with the supplier of my motors just to make sure I was in the ball park. Built the supply and never looked back.
If you have any specific questions, I would be happy to help out where I can.
olf20
Atlas 9 x 42
Mach3
Dell Optiplex

[LeeWay]

You might want to consider how often that all three motors will be pulling full power at the same time. I think this is seldom. I am running 3- 3 amp motors on a 12 amp supply with no trouble at all. I think your motors could easily get by using 12 amps as well.

I haven't yet added any fourth axis, but you could always use a smaller cheaper supply for it. The trouble you may see when buying high amp power supplies is cost. If you plan on having a fourth axis, get a bigger supply initially. That would be the cheapest way around it I think.

[tauntdesigns]

I've read 1/3rd and I've read 2/3rd of the current. The one I have has about twice amps that I need. You set the amps on the Geckos with the current set resistor. ((edit)If your planning adding a forth axis.... Like was said above 12a or so)

I'd use the 4 20 rule by getting a power supply that is between 4 times and 20 times the voltage of your stepper motor.
You say your motors are 2.8v, so
4 times 2.8v equals 11.2v
and
20 times 2.8v equals 56v
so I'd get a power supply between 12v and 56v ((edit) the bigger the better)

Later,
Jack

[ger21]

I think you misread the Gecko literature. When it said the drive will draw 1/3 the rated current, that's all you need for each drive and motor. 1.1 amp for each motor, 3.3 amps for all three. A 5 amp supply would be fine for those motors running bipolar series.

If you want to plan for a 4th axis, then sure, it's wise to do it now. But you're stepper motors will be the only devices using that power supply. Additional current capacity doesn't hurt anything, so if you want to build a bigger supply planning for the future, it won't hurt anything (except your wallet).

Bottom line is that 5 amps should be sufficient for even 4 of those motors. I personally would add a few amps for a safety margin, and look for something around 8-10 amps.

[LeeWay]

Ah yes.
I forgot that I was initially running some 269 oz motors with only a 6 amp 62 VDC lab supply and 201's. That worked pretty well too. This is actually going to be running my lathe pretty soon.

[cyclotron]

I am confused then, originally, when I read the gecko material, i thought exactly as Ger21 suggests: I only need to add up the draw of the drives, and not the current rating of the motors themselves. Later I started second guessing that. From the perspective of my limited electrical education, it seems strange to accept that a motor rated for 3.3 amps is going to get all the power it needs from a drive only getting 1.1 Amps. Can anyone clarify this for me?

I think since tomorrow is a business day, I will call Gecko and ask
them for advice.

[Torchhead]

A Gecko with no drives attached just draws milliamps of current. The power draw occurs from running the motors so you don't count them twice. Since the current is limited by the drive and depends on loads and speeds, the average current (the number you use for building power supplies with transformers) is a number lower than the fully loaded sum of all the motors....in most cases a lot lower. What most people do is just build the supply with overkill and use a finger-in-the-wind design set of rules. If you drag out the current probes and scope to really look at the real amount of current it's using under normal operation you could better profile what you need.....but then who has the time for that?

In our new "Smart" supply we measure the actual current being drawing from the PS (it's used to detect a short or overload and shut the supply down in milliseconds rather than relying on fuses. The system load is also displayed on a realtime bargraph and in amps on the optional Operator Control Pod, and we can profile any set of motors quickly.

Tom Caudle
www.CandCNC.com

[cyclotron]

I finally got a power supply and all of the input has been really helpful.
I ended up getting a supply with an overkill number of amps as I anticipate
running more than 3 motors eventually. as for voltage, I picked a supply that was about 13 times the rated voltage of the motors. hopefully this will give a little versatility if I want to connect less powerful motors to this supply.

ALSO gecko is very helpful and I recommend calling them for anyone who is viewing this thread looking for answers of their own. big ups to gecko.