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PWM not causing motor to turn until 50%

I built a Pulse Width Modulator to control the speed of a 12V DC motor, but the motor doesnt even turn until the potentiometer is at 50%. How can i fix this?


Let me give more background. I took apart a Skil 12VDC 1 Amp Drill and removed the motor, gearbox, transmission, and chuck all in a single unit, and wanted to control that motor much like the trigger on the drill did...slow to fast, at full torque.

So I built a PWM to do it...  I got it from here....http://www.bakatronics.com/shop/item.aspx?itemid=383

The PWM turns the motor but not until the pot resistance has been reduced by 50% or so...if I leave it at 50% the motor turns slowly at first then gains speed...this was not the control level I was looking for...

can anyone help me figure out what is going wrong? 

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So it seems like I could measure the amperage at the load, while the motor is on, then remove the motor, and measure the voltage with a load under the 400mA fuse(( ( a 12v LED or some such) for measuring DC Voltage...So I'll have numbers in a few hours


If you modulate the frequency, you have frequency modulation (FM). If you modulate the width of the pulse, you have pulse width modulation (PWM).

Some PWM controllers also modulate the frequency as a cheap filter to irritating audiable frequencies.

You're right in that you get full voltage/current, but only for brief pulses so that the average is representative of the mark to space ratio (the highs vs. the lows).

Question: are there any remnants of the orignal drill controller electronics, or do you have your new controller wired directly to the motor?

Possibility: is the pot linear or logarithmic? Most (inexpensive) ones are logarithmic.


I did indeed hook up the original speed controller to the drill and to a 12V 1.5 Amp power supply. It didnt have enough juice to break the friction from the planetary gears in the transmission. 

Short Answer...not enough Amps...the battery supplies enough amperage. However the battery is only a 1AmpHr battery...and I need sustained Amperage for up to 30 minutes.

So the hunt is on for a better PWM and a 250W power supply 

The PWM drive mentioned doesn't appear to tell what PWM frequncy is being used, so only an oscilloscope (or a special multimeter) might be able to discern that.Might or might not be important.

The motors it typically drives appear to be model train motor, which are probably in the low amp range fit for this driver. The Skil drill on the other hand should draw substantially more current, as well as being a different inductance (hard to measure) than train motors. I would guess that there is a combination failure of the power supply cutting out, saving the PWM speed control from getting over-currented and smoking. The power supply cuts out as the system approachs or passes 1.5 A and the PWM driver is not even able to supply much voltage (since power is down) until the motor is turning (making the current draw less). As the inertia of the motor allows it to draw less, the supply can stay on longer, and the PWM starts working so the motor can turn at a spped appropriate to the duty cycle. 

I have some Harbor Freight 9.6 volt drill motors that have a rated stall current of 41 A, which is why I think the Skil drill motor draws a bit more than 1 A just starting out.

It seems that either the PWM is not functioning as predicted, or my multimeter averages values. I first hooked up the motor and decreased pot resistance until the motor started to turn. It read roughly around 1 amp, then raise the resistance and the amperage dropped, lowered resistance and the amperage increased enough to spark the leads of the meter.

Unhooked the motor and hooked up a 12v led, at maximum pot resistance. The volts were reading about .6 and it increases all the way up to around 13 or so at 1% resistance.

So as I said, either the PWM doesnt behave as intended, or my meter reads in averages.

I also tried to hook up the power source ( 1 amp 12VDC ) to the original motor controller from the drill and have the motor be the load. It barely, and I mean barely had enough power to break the planetary gear friction. Some how the battery was providing alot more power to the motor than my power source. How could it do that without burning out the motor.


Should I try a battery pack instead of a wall mounted transformer, will that burn out the PWM, its only rated at 12VDC and 2 Amps


so I might need to build the 12V >10 Amp PWM that I have seen online.

The behavior of your meter is to be expected. At minimum duty cycle (you describe that as max pot resistance), the pwm controller puts out very short bursts of 12V. Your meter is not an oscilloscope, so it can only present you one reading: an average voltage of 0.6V.

Both controller and meter are functioning as predicted.

It sure looks like your motor needs at least 1.0A to start turning. After that you can throttle down a bit for slower RPMs.

Well my friend,

To be honest, I can't answer this problem, but I can give you another. I have a couple 12v dewalt drill motor/ trans assemblies I have been playing with. One thing I have found is that these drills pull SERIOUS amps! You can probably get the motor to turn (unloaded on a workbench) but I can assure you as soon as you apply a load and start sucking amps that little PWM is going to be toast. --I have seen such smoke on more than a few prototypes I have made. I do however have (maybe) a solution... I have been looking at, but not tried, some R/C motor controllers. It seems that these controllers handle massive power and are recognized by the PIC as a standard servo. --I'll make you a deal: You go buy one, test it and if it works, tell me! Then I can buy one myself! :)

These Doo-Hickeys are designed to be plugged into the receiver of an RC car in place of a regular servo, so if you buy one that advertises itself as for this purpose, then you can send it a srandard servo pulse from your picaxe (or other servo controller).

I found a schematic for a 12VDC-24VDC >10 Amp PWM, do your 12V DeWalts pull more than 24 Amps?

Here is the link for the schematic...


perhaps this is the PWM that I should be using


Append: After more research some of the 12VDC drill motors I've seen have outputs of 250 Watts or higher, meaning almost 21 Amps at max load. It would seem that I would need to use a power supply for a computer to run this motor....surely that cant be correct...I would be better of with a battery pack that is rechargable. 

Many Battlebots guys use NiCad packs that can dump current at a higher rate many wall power supplies. Lead Acid does very well too. A controller that can handle that much current is really needed as well. Innovation First, Robot Power, Vantec, Robot Solutions all have various devices that can run pretty high current for a price. If speed control is all you need (not direction), paralleling a number of FETs could get you to the current handling needed, but a FET driver woulld be needed too.