Alright, so I've been trying to figure this out for awhile, and I believe I've made headway. I have a small motor (6VDC, 3A) that I want to ramp from off to full power in roughly 30 seconds. I finally understand what a PWM does conceptually. Originally, I was using a circuit by AnalogKid from the following thread:
http://forum.allaboutcircuits.com/threads/simple-dc-motor-ramp-up-circuit.112540/ (thread)
http://forum.allaboutcircuits.com/attachments/rampdriver-gif.87260/ (actual circuit image)
With a few tweaks, I was able to build it in an online SPICE simulator:
http://goo.gl/LnyXGA (click reset to start the simulator)
However, when I built it, the circuit doesn't hold the output voltage - the motor ramps up, drops off (but doesn't shut off), ramps up, drops off, etc. Did I do something wrong? I understand that this is how a PWM behaves, but the simulation shows a linear ramp-up output. Why is it pulsing when I actually breadboard it?
After SIGNIFICANT searching and learning, I found an op amp integrator circuit that is supposed to do exactly what I want: ramp up and hold the voltage:
http://www.edn.com/design/analog/4314544/Integrator-ramps-up-down-holds-output-level (article)
http://m.eet.com/media/1126960/12071-figure.pdf (actual circuit image)
Once again, I built this in the SPICE simulator, and I added a PNP transistor that I would use to drive the motor:
http://goo.gl/fJd6yj
However, I noticed a few things (and please forgive my sincere ignorance of electronics). First, the PNP's output starts at 5V and then ramps up to 15V. Why doesn't it start at 0V? Second, where is the 15V coming from in the first place since I started with 5V? I mean, I get the fact that these are operational amplifiers, but I assumed that the output would go from 0-5V. What did I do wrong?
Ideally, I'd like to figure out how to make the first circuit work properly - it's simpler, smaller, and I'm not so grand at electronics. If that circuit won't do what I'd like, I'd like to understand where I'm going wrong with the second circuit before I sit down to build it.
Thanks for whatever help you can give!
http://forum.allaboutcircuits.com/threads/simple-dc-motor-ramp-up-circuit.112540/ (thread)
http://forum.allaboutcircuits.com/attachments/rampdriver-gif.87260/ (actual circuit image)
With a few tweaks, I was able to build it in an online SPICE simulator:
http://goo.gl/LnyXGA (click reset to start the simulator)
However, when I built it, the circuit doesn't hold the output voltage - the motor ramps up, drops off (but doesn't shut off), ramps up, drops off, etc. Did I do something wrong? I understand that this is how a PWM behaves, but the simulation shows a linear ramp-up output. Why is it pulsing when I actually breadboard it?
After SIGNIFICANT searching and learning, I found an op amp integrator circuit that is supposed to do exactly what I want: ramp up and hold the voltage:
http://www.edn.com/design/analog/4314544/Integrator-ramps-up-down-holds-output-level (article)
http://m.eet.com/media/1126960/12071-figure.pdf (actual circuit image)
Once again, I built this in the SPICE simulator, and I added a PNP transistor that I would use to drive the motor:
http://goo.gl/fJd6yj
However, I noticed a few things (and please forgive my sincere ignorance of electronics). First, the PNP's output starts at 5V and then ramps up to 15V. Why doesn't it start at 0V? Second, where is the 15V coming from in the first place since I started with 5V? I mean, I get the fact that these are operational amplifiers, but I assumed that the output would go from 0-5V. What did I do wrong?
Ideally, I'd like to figure out how to make the first circuit work properly - it's simpler, smaller, and I'm not so grand at electronics. If that circuit won't do what I'd like, I'd like to understand where I'm going wrong with the second circuit before I sit down to build it.
Thanks for whatever help you can give!