Stepper motor controller current limiting question

[snippety snip]

Take a look at the discussion over at

for some recommendations.

--
Rich Webb     Norfolk, VA

a.pdf

for some recommendations.

Rich, thank you for your time and response. I am looking over the information at that site. It will take some time as there appears to be a lot there. I will read through all that and see where I am, but it appears, after only a cursory look, to address my main concern about current limiting PWM controllers.

Thank you again.

--HC

I'm no stepper motor guru, but I think the above is suspect. If you double the voltage (on a purely resistive load, at least) the power dissipation is *squared*. The equivalent duty cycle to maintain the same dissipation would thus be only 25%, not 50%.

But that's what you'd want the long-term average to be. Presumably, you would get much higher instantaneous power when starting up, which would then taper off to something like 25%.

Or am I missing something here?

Best regards,

Bob Masta DAQARTA v5.00 Data AcQuisition And Real-Time Analysis

Scope, Spectrum, Spectrogram, Sound Level Meter Frequency Counter, FREE Signal Generator Pitch Track, Pitch-to-MIDI DaqMusic - FREE MUSIC, Forever! (Some assembly required) Science (and fun!) with your sound card!

--
A slip of the tongue, I'm sure, but since:
                
P2 = P1 (E2/E1)²,
         
doubling the voltage across the load will cause the dissipation to just
quadruple, not square.

JF

the power dissipation goes up by *two squared*.

The inductance of the motor, if the PWM frequency is fast enough.

--
www.wescottdesign.com

.pdf

Since this is your first stepper motor project I would start by staying within the Voltage and current ratings of the motor. Once that is all working you can try pushing the boundaries for enhanced performance. But at least you won't 'let the smoke out' of the motor your first time.

George H.

a.pdf

Hello, Bob, thank you for your reply. I'm sure that's not right on my part; it was just a loose (and inaccurate) recollection I had at the periphery of my thoughts at the time. :(

--HC

d 7a.pdf t s

Hello, George, thank you for your reply. That makes the most sense to me. I will do that. In the meantime I am still trying to learn as much as I can and am still reading, as time permits, the site suggested to me by the first responder (Jones on stepper motors).

--HC

.pdf

at is purposefully driven at a voltage that is higher than its

Hello, all. Where I am now is I have learned a fair bit about programming Micrchip PICs and think I've got that well enough to finally implement a test design. I'm taking the suggestion that I start with rated voltages initially, so no need for current sensing yet.

I started looking for an H Bridge circuit for my test motor because running it bipolar yields higher holding torque than unipolar operation. I have a number of BUZ11 N-Channel MOSFETs that I wanted to use and, I've read, they offer less resistance to the load so less heat is generated when they are operating, particularly switching. Plus, I don't think I have a P-Channel MOSFET in captivity and I live in the sticks so I can't just run to the store and grab one.

So, after reading tons of stuff on H Bridges I kind of cobbled together a piece that works and I wanted to fly it out there and see if I've done any good or if I'm screwing up. I have the schematic done up and saved on Photo Bucket:

nunyabusiness11). I hope that link works. If not you can hit the site and search for my username listed at the end of that link.

I've wired it up and it works. It is simply 1/4 of an H Bridge, one of the upper legs supplying positive voltage to the motor. Currently I did not put in any clamping diodes around the motor because the MOSFET has one and it'll work for just seeing if the circuit works.

Basically, I'm using 5Vdc from the PIC (simulating it now by just touching the input to +5Vdc or Gnd) to activate a NPN phototransistor. That switches +12Vdc to ground via a 3k3 pull-up resistor (it is located before the phototransistor). That line (before the phototransistor) is connected via a 10k resistor to the base of a PNP 2N3906. The collector is tied to +12Vdc via a 1k resistor. The emitter is connected to ground via a 2k2 pull-down resistor. The N Channel MOSFET gate is tied to the line after the

2N3906 but before the 2k2 pull-down resistor. The N Channel MOSFET is tied to +12Vdc on the drain and the source ties to the motor which itself is tied to ground.

I used the phototransistor (SFH615A) to isolate the 5Vdc circuit from the 12Vdc circuit. When the input to it gets +5Vdc it switches on the phototransistor which allows the base of the 2N3906 to go low switching it on. That switches voltage to the leg with the 2k2 pull- down resistor and to the gate on the MOSFET.

I've tested it a few times, just powering the system up and touching the input directly to +5Vdc or Gnd. The motor holds securely when the input is energized and seems to freewheel correctly when the input is grounded (after I changed the pulldown resistor on the MOSFET gate from a 3k3 to a 2k2).

I'd appreciate input as to whether or not this is a good idea or if I simply got lucky that nothing smoked. I'm not an EE by any means and this circuit could be the silicon equivalent to a crack-baby.

Thank you.

--HC