Last check and question...

[WilliamD]

I rewired a microwave transformer, and got about 24vac. It was not fun rewinding the transformer! I hooked it up to the bridge rectifier, and only got about 21v DC. So then I hooked up my 40000 micro farad 40v capacitor, and it went up to 36 volts DC. So I was pretty happy. But, I'm going to want to make one for 60 volts DC for my gecko's, the 36 volts will just get me by for now. I know I'll need to add more windings to the transformer, and actually I might buy an actual 60v transforer, but will I need to get a higher voltage capacitor? Does it just store 40v at 40F, or is that the max you can run through it? If so, I can only find higher voltage capacitors, with low (10000) micro farad ratings. I'm only running 260 oz/in motors right now, but will be upgrading to some that pull 7 amps. So I know I'll pretty much have to get a different transformer. Also, the steppers are the ones from automation direct, and pull 2.8 or 3 amps, will they just draw what amps they need from the supply? I used 14 gage wire for everything, so I think the wire will be able to handle the heat. Sorry for the long post, but I'm getting so excited I'm near completion! Once I have this figured out, I can hook it up to the machine and start cutting stuff!!!

[ViperTX]

If you are aiming at 60 VDC as the output of the capacitor, then you should use a capacitor rated at 75 VDC.

I would look at getting a 48 VAC transformer or wiring two 24 VAC transformers in series this will give you about (48 VAC X 1.414) about 68 VDC. You can use the 24 VAC CT transformers at mpja.com click on the transformers link. Do you know what the ratings are for the steppers (current)? Use the 10A transformers.

[NC Cams]

Perhaps over simplified response to a semi comlex question involving transformer wiring and DC ciruits.

Most caps have what is called 'wvdc' (working volts dc or ac) ratings which pretty means just what it says. It is ok to run a cap with a higher voltage rating than what you plan to use. Running one that is too low could result in catatrophic failure.

You are actually taking a rippled voltage out of the transformer and smoothing it and storing the whole bunch in the cap (look at current with an oscilloscope).

This is why the voltage rises when a cap is added. Given enough time, the voltage at the cap will try to climb to the high point of the peak ripple voltage coming out of the transformer but can't due to internal losses - it won't limit itself to the wvdc or wvac rating, but will absorbe whats coming out of the transformer.

Since the instantaneous voltage is higher than the average voltage, you really need to run a higher wvdc rating than your volt meter says is coming out of the transformer. If you're working at 60vdc, I"d look for a 100wvdc rating - someone more learned in the science than I will correct this I"m sure.

Simply stated, current draw is established by E=IR where E is the voltage across the circuit from the power supply and R is the total series resistance in ohms.

Unless the amps have a lot of internal resistance (usually not) or built in current limit capabilites (quite often so), the amp will pretty much let the current flow that is established by E of power supply and impedance (simply expressed as R) of the motor/wiring down stream.

[pastera]

Just connect multiple caps in parallel.

When using a PWM driver current draw will depend on what the motor is doing. At low speeds the motors will draw very little current from the supply. At higher speeds, more current will be drawn as the back EMF of the motor becomes harder to overcome.

Aaron

[WilliamD]

Wow, some good information to soak in! I like the idea of wiring some caps in parallel, I didn't know you could do that. I also have a couple of microwave transformers, so setting them each up for 24vac and wiring in series doesn't sound too bad either. I'll probably just buy a cap I found for 85 volts and see how things work out. If I notice that there's not enough power, I'll add more caps. I read about matching the current to the motors, as not to overload them, I assume this means matching the amps. But I thought the motor just pulled whatever amps or current it needed, and the power supply just had to be capable of dishing it out. Is this thinking correct?

[NC Cams]

If you wire the caps in parallel, you ADD the capacitance - the WVDC rating doesn't change

If you wire them in SERIES, I"m pretty sure you ADD the WVDC rating sbut don't recall how the capacitance adds up.

It is analagous to connecting batteries in series or parallel.

Motor pulls current as a function of its internal impedance (resistance so to speak), or simply the R in E=I*R equation.

Effectively, you can't "shove" current thru the motor, it draws current pursuant to E=I*R relationship (over simplified yes, but adequate for illustrative purposes)..