Thanks to all for the info and advice!

Here is what I have decided to do:

For now I am going to buy a 36v NEMA 23 servo motor, matching 300 line encoder and a Gecko 320

I'll use the power supply that I have which one of you graciously calculated for me that I still have some 'room' on.
 
Hopefully I will be able to get everthing proved out and working good and then I can think about getting a bigger motor and probably a separate power supply for it.

Here is an update on this project:

I got my servo motor, encoder and Gecko 320 drive. Got everything running and adapted the servo motor onto the 4th axis.

Everything works great!. I can turn (within reason) and threading is perfect. Holding power is not as good as it was with the big stepper, but that's a compromise with the smaller servo motor and only a 300 line encoder. Both can be remedied if the need arises.

Here's the problem. I fried the power supply. This was totally my fault and I knew full well I was overloading it badly with the servo added in there. The power supply is advertised as having overload and short circuit protection (theoretically Simpson proof), and the fuse did blow, but not untill after the power transistors were toast. At least that what I think is wrong with it.

So, I ordered new transistors for it and that may or may not fix it. I was planning anyway to get a larger power supply, and the one that I cooked is not expensive, so no biggie.

Eventually I will be replacing the little 36v NEMA23 servo with a larger 90v NEMA34, but in the mean time, I'll need to power the 36v servo, hopefully with the larger 72v power supply I am planning to get.

Here is the question: can I power the 36V 4A servo with the 72V power supply if I put a 9ohm 140 watt resistor ahead of it? That's what I come up with, but I don't have confidence that these are the correct numbers or if I'm heading for frying something else.

I have my limit switch issue resolved, so the power supply is the last upgrade and I'll be done with the CNC mods for a while.

That would be a really bad idea. The voltage drop across a resistor is directly proportional to the current through it. So when your servo driver is drawing less current it will have more voltage across it. Not a good thing.

If looking to get a larger supply, I would seriously  consider getting or putting together a linear supply with a Toroidal transformer, If fused right they are pretty much bullet proof, switching supplies for steppers or servo's is not really necessary.
There is a Guy on ebay that makes them to order, if necessary,  AnTek.
And if you need a low current auxiliary supply, you put an over-wind on to suit.

Hi,

Another option to get higher DC voltage is to use a "voltage multiplier circuit". It's a very simple, commonly known, circuit, utilizing  two diodes and two capacitors. I used it on my lathe since I didn't have a suitable outlet from the transformer to get 80VDC. So I got them from 28VAC using this circuit.

Daniel

Hi,

Another option to get higher DC voltage is to use a "voltage multiplier circuit". It's a very simple, commonly known, circuit, utilizing  two diodes and two capacitors. I used it on my lathe since I didn't have a suitable outlet from the transformer to get 80VDC. So I got them from 28VAC using this circuit.

Daniel

Yes, but compare to a proper full-wave rectified supply it will have only half the current capacity, and will require double the filter capacitance to maintain the same ripple voltage.  Overall, not a good solution for a high-current device like a motor.

Regards,
Ray L.

http://www.kelinginc.net/SwitchingPowerSupply.html   Model KL-7220

This is what I bought.

I ride the short bus when it comes to electronics, I'm afraid . . and while I can sort of reason out what an 'over wind' is (a tap basically?), the term  'toroidal transformer' is alien speak. 

Yes, I know now that switching power suppies are not only unnessesary, but not recommended by Gecko. I suspect they may also cause a lot of noise, but that's just speculation.

If the new transistors fix the 36v supply that I have now, I could use that to power the little servo for the time being.

Eventually, I will probably replace the little servo with a larger NEMA34 90v ort 72v servo, but meanwhile, there must ne some way to get 36V from a 72V transformer easily. I have ordered a pair of 9.1ohm 100watt resistors. I read that a 'voltage reduction' circuit uses a resistor on both the positive and negative wires and I have also read that the resistor goes only on the + wire, I have no idea which is better or why. Seems to me a DC motor  swithces polarity depending on the direction, so there is no '+' wire in reality. Is that correct?

I would like some more explaination as to why  the voltage drop across a resistor based on current draw is going to pose a problem. I have a little better  understanding of the difference between 'switching' and 'unregulated' now, and my understanding is that 'unregulated' supplies do not compensate and the voltage drops when the draw goes up. Gecko recommends 'unregulated' supplies, so I presume they must be designed to handle the voltage drop.

So I don't understand why a votage drop on a  . .  36v  'tap' would be a problem for the servo motor if I put the resistor upstream of the Gecko. What am I missing?

http://www.kelinginc.net/K23-120-36.pdf

This is the little servo motor. Quite the little brute actally. If a resistor was sized to give 36V @ 4A and the voltage was geater at less draw and fell off over 4A, what would the result be in performance and would it damage the motor? I'm fairly certain it would not bother the Gecko drive. The drive is rated for 80V and and it has an adjustent to limit output current anywhere between 0 and 20A.

Ray,

What determines the current capacity is the capacitors. And as you know, a capacitor can deliver enormous currents. As to the capacitance, you're right - you need the same capacitance twice - one for each half of the rectified signal.

Daniel

Simpson,

You got me confused... are you seeking to get 72VDC from 36VAC or vice versa?

In any case, voltage reduction by using resistors is not good for the purpose of powering motor drives, since the current the drives draw is not constant and there is no way to calculate the resistors required.

Daniel

Ray,

What determines the current capacity is the capacitors. And as you know, a capacitor can deliver enormous currents. As to the capacitance, you're right - you need the same capacitance twice - one for each half of the rectified signal.

Daniel

Daniel,

Sorry, but that's not correct.  Take a 24V/5A, or 170W, transformer, connect  it to a full wave rectifier and filter, and you'll have a 34V/5A, or 170W,  power supply.  Connect that same transformer to a voltage doubler and filter, and the power the transformer can deliver remains 170W, but you've douibled the voltage, so the current must be halved.  There's no free lunch here.

Regards,
Ray L.