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Electronics n00b Needs Advice on Motor Control

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Gabe Billings

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Howdy. I've got an electronics project I need some assistance with. I think it's pretty basic stuff, but the extent of my electronics knowledge is what I learned in my 9th grade shop class 15 years ago. I've tried to track down answers through on-line electronics guides, but I'm still a little hazy on what I'm doing.

I've got a couple small electric motors; 3v / 290mA and 4.5v 920mA. They were scavenged out of battery powered toys. I got the current readings by putting my multimeter in the circuit while it was energized.

I'd like to make a little project box that I can use to control the speed of both motors as well as power them off of AC current instead of batteries. While doing my research online, it seemed that rather than using a potentiometer for this a better solution was some sort of PWM device. So I tracked down one of those that looked as though it would work. This is what I bought. I put one of the kits together last night and not only did I not burn the house down with my new soldering iron, I managed to get it to work.

My question is how to deal with the incoming voltages into either motor control. I wanted a wall transformer big enough to run both motors and ended up getting **broken link removed** from Digikey. It's rated at 6v 1250mA. Oddly enough, when I checked it with the meter it shows an output of 8.65v. I'm not quite sure what's going on with that.

So I need to know what to do to make sure I don't blow up the motors, especially since the PS I bought seems to be significantly higher voltage than I expected. If I hooked everything up as is it seems as though it would work, but I'd need to be careful not to crank the motor control's pot too far or else I'd overload the motor. Ideally I'd like the correct voltages going into the motor control so I can use the full range to adjust the motor speed.

So this is where I get hazy on what I'm doing. It seems as though I just need a resistor in each circuit to get the voltage where I need it. I don't know if that's the correct answer to this problem and if so if I'm doing my calculations correctly.

3v motor: 8.65v - 3v = 5.65v across the resistor 5.65/.29= 19.5:eek:hm: at 1.6W

4.5v motor 8.65v - 4.5v = 4.15v across the resistor 4.15/.920 = 4.5:eek:hm: at 3.8W

So that's what I've got. I don't know if I'm close or if I have no idea what I'm doing. At least I'm pretty sure I'm not going to electrocute myself. Sorry for the lengthy post. Any assistance would be greatly appreciated.
 
unregulated wall warts always read above their rated voltage when unloaded. In theory, they should deliver the rated voltage at the rated current.

So your 6V WW should read 6V when you draw 1250 mA. Put a 5 ohm (10W) resistor across the output of that WW and you should measure 6V.

For what it's worth, your motors may handle 8.65 V just fine. Especially if you are PWMing them. I'd try it and see if they heat up. If they don't you may be ok. Understand that when you turn both motors on, they will see a lower voltage than with just one on and thus turn at lower speeds. If you care about precision, I'd build a regulated supply (using LM317s) to supply exact voltage.

Also, are you using the same supply for the PWM circuits - that will be a problem as the motor noise will cause all sorts of problems.
 
philba said:
Also, are you using the same supply for the PWM circuits - that will be a problem as the motor noise will cause all sorts of problems.

Are you asking if the PWM is powered by the same supply as the motors? It is, and I'm not quite sure how to get around that. The circuit boards only have two pair of terminals for power in and out to the motor.

What kinds of problems is this going to result in, and how might I go about fixing it?
 
A resistor in series with a DC motor seriously reduces its torque. It might not even start and anything will stop it if it is already running.
Keep it from over-reving so it doesn't fly apart and keep its very high stalled current from frying it.
 
You could attach a thermistor to the motor housing and set it up to cut the motor power if it overheats. They're relativly expensive for decent ones but non contact Infrared thermometers are a great thing for a hobbyist to have as it's a point and shoot way to determine how hot something is without accidentally losing your finger prints.
Motors don't instantaniously explode if their voltage rateings are surpased, they just heat up signifcantly faster. As long as the insulation resistance isn't violated (very hard to do) you can send double tripple or more voltage to a motor for short periods of time for increased torque, but you have to keep in mind how much heat that's going to generate. If you test the motor leads as if it were a resistor you know it's basic DC resistance which you can calculate how much power the resistance will disipate at a given voltage, and hence how much heat it creates and figuring out the copper mass you can determine exactly how fast it will heat up (if you want to go overboard)
 
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Whoops

So last night I hooked up both motors to see if they would work. The first odd thing I noticed is that there doesn't seem to be an 'off' on the motor control. There's always a small amount of voltage getting through, even with the pot turned all the way down.

Other than that it seemed to work as promised. At one point I took the motor control out of the equation because I was curious to see how the motors would run hooked directly to the power supply. And apparently I fried the power supply. Everything stopped working, and I started checking things with the multimeter, and found that the power supply was now putting out about .01 volts. I don't know what cause that, but it seems to be toast.

So I'm thinking I should give up electronics fiddling and take up knitting.
 
DC electric motors have a very high stalled current. The motor is stalled when it starts or when something stops it. The very high current fried your power supply.
Use a power supply with a much higher current rating. Then your motors will fry.
 
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