PMW down. alternative?

[StarryNight]

hi, wondering if anyone has advice regarding my PWM for my homebrew DC winder (12-24v). It seems to have died. If I bypass the PWM and test the motor with a DC current it moves. I was wondering if anyone has a short term solution for creating a slow moving wind just to get me through a couple of jobs until I can order another PWM unit? If I hook up 12v directly she spins pretty quick making it hard to wind. thanks!

[Jim Darr]

StarryNight,

Am I correct in assuming that you use Pulse Width Modulation and vary the duty cycle to control speed?

Some of my winders use a Variac for speed control. Others use a Variable DC Driver Device to control speed. And on one, I use a A/C Dimmer which works well. It all depends on the type of drive motor you have on the winder and it’s ratings. All my winders are heavy-duty commercial models.

Your motor type and it's ratings will dictate the type of controller you need. A temporary fix might be as simple as using a lower voltage DC power source, say 9 volts, if the motor is a true DC motor (and if this voltage is within the motor's operating voltage parameter) and see if that slows your winding speed. You also need to consider the amperage rating of your motor and the amperage of your DC power source. Be careful and be safe in whatever you decide to try as a temporary fix (make sure you know what you are doing electrically and mechanically).

[StarryNight]

Hi Jim,
yes, that is correct. Not knowing much about DC motors and PWMs I managed to get this winder working but after a year the speed started fluctuating and has recently died. I probably was using the wrong power supply for the PWM or perhaps the wrong PWM module for the motor. I wonder what would be the ideal PS for a PWM to drive a 12v dc motor? I was using an old hard drive wall wart adapter rated at 12vDC and 1.7A.

[David King]

If you use a transformerless power supply to supply a pwm you may end up with very erratic behavior. I'd start with a regulated and filtered supply that gives you a smooth DC output. If your power supply is a cheap wallwart that chops up the Dc and you further chop up that supply in a PWM, you will end up with all kinds of hash and bizarre frequency interactions not to mention the noise you are broadcasting all over the neighborhood and back into your transmission lines.

[StarryNight]

Noise all over the neighbourhood? I knew winding pickups was a dark art but I had no idea I was casting spells! So a regulated PS is the way to go. Am I looking at something like this? http://http://www.ebay.ca/itm/380937855913

Or is there something more appropriate i.e. Cheaper? One thing, in my ignorance, that has always confused me is the amperage rating on different devices. A motor will "draw" so many amps while a PS will supply the required amps? Do they have to match or is there an ideal ratio between supply and demand? is the voltage affected through changes in amperage? I'm still on my first cup of coffee this morning so I might not be making any sense at all.

[rdrr]

You may have run into problems with amperage and burned out the pwm as well if your dc motor peaks at more amps then the PWM was rated for. Just because a wall wart states that it is 1.7A does not mean that it cannot spike at higher amps than that, unless it has built in current limiting which most do not. 1.7A would be the safe operating range of that particular wall wart.

[StarryNight]

You may have run into problems with amperage and burned out the pwm as well if your dc motor peaks at more amps then the PWM was rated for. Just because a wall wart states that it is 1.7A does not mean that it cannot spike at higher amps than that, unless it has built in current limiting which most do not. 1.7A would be the safe operating range of that particular wall wart.

My wall wart is rated at 1.7 amps but I didn't know they could be prone to spikes, interesting. Also, I just discovered browsing on ebay that some PWMs don't like reverse polarity. Perhaps that was part of my problem as well.

[salvarsan]

Plan C: cobble up a 555 timer in astable PWM mode to drive a power transistor.

Plenty of info on this google search URL.

Most of the circuits look like this:


Note that the output transistor can be any NPN Darlington with enough power handling. I suggest the TIP140 for its 10 amp continuous rating. It's cheap, too. All the parts are available at Radio Shack ... for now.

[salvarsan]

Ebay has 555 pwm driver kits.
They are inexpensive.

[J M Fahey]

If you have a wall wart and dare to build a simple 2 transistor regulator to sit between it and the motor, I can help you kludge something until parts (or a new regulator) arrive.
You'll need at least a medium/large power transistor capable of passing at least 3 A (if it handles 5 or 10A better) and dissipate >20W, plus some heatsink for it.
Post what you have in that area.

[Sock Puppet]

As an aside, Ebay has ready built PWM controllers at a fine price.

10A 6V 12 24V Pulse Width Modulator PWM DC Motor Speed Control Switch Controller | eBay

[StarryNight]

Great suggestions, thanks guys. I've got one on the way from ebay (where I got my first one). I have two winders so I'm ok in the interm. I'm more concerned about getting a proper power supply. I posted a link to one but if anyone know of something more appropriate I'm all ears. I'm also in the process of learning how to use my new syscomp oscilloscope so perhaps a PS that would serve double duty?

[David King]

I'd pick up a suitably sized transformer off ebay, get a bridge rectifier, regulators, some power conditioning caps and assemble it in a nice case or just buy a kit. Used regulated power supplies abound. Every recycled computer had one with outputs at 12V, and 5V and sometimes 3.3V as well.
Check out Power Supplies | MPJA.COM