DC Motor Speed Controller





Introduction: DC Motor Speed Controller


today I’m gonna show you how to make a really simple DC motor speed controller.

Step 1: Watch the Video

the video is in my Youtube channel Chris' Project

Step 2: The Schematic

Step 3: Buy All of the Components

The components for this project are:

1pc IRF3205 mosfet

1pc 100k ohm potentiometer

1pc potentiometer knob

some wires

1pc old credit card

1pc DC motor

1pc heatsink

1pc skrew

1pc 9V battery clip and a 9V battery.

Step 4: Put the Heatsink on the Mosfet

Put the heat sink on the mosfet with the skrew

Step 5: Glue the Components to the Credit Card

Glue the mosfet and the potentiometer to the credit card

Step 6: Connect the Battery Clip

Connect the positive wire of the 9V battery clip to the drain pin of the mosfet and the negative wire to the dc motor.

Step 7: Connect the Potentiometer

Connect the left pin of the potentiometer to the drain pin of the mosfet, the middle pin to the gate pin of the mosfet, and the right pin to the negative wire of the 9V battery clip.

Step 8: The Last Connection

Finally connect the source pin of the mosfet to the other pin of the dc motor. Now just plug in the 9V battery and we are done.

Step 9: Testing

If you connect everything correctly than if you turn the potentiometer anticlockwise the motor will spin faster and if I turn the potentiometer anti clockwise the motor will slow down.

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Hi, I like simplicity of the speed controller. I m trying to do a speed controller for PC fans, so 12V.
can I use any power transistor or mofset ? any suggestion ? and don't you think that 100k potentio is to much??

would this work with 12 volts and higher?

can i use 12v DC power supply and a dc 12v motor



I have a motor that runs 9-12v (from a hobby shop-it's actually a motor to turn a propeller for a model airplane); tried to hook up a potentiometer to it (twice!) and both times the pot got really hot, then started smoking. Hoping this instructable will help solve my problem.

I like your enthusiasm but I think the reason why the potentiometer got hot and eventualy smoke because you connect the potentiometer in series with the motort and in that case it acts as a voltage divider. The pot can't handle the current load that the battery or your power supply delivers. Based on the above circuit, the pot controls the frequency or the width not the voltage. I hope I answers your doubt.

This circuit is a far from an optimal solution. This circuit is more suitable as mosfet test fixture (with modifications like adding a snubber) than as a real world motor controller. May I ask what your intended application is, what your motor's specs are, and those sorts of nitty-gritty details?

Not sure. The data sheet of IR is missleading. Important is how much current your motor can draw. According to current and voltage your are driving the MOSFET in the linear Mode with this circuit and MOSFET then often die because of a thermal runaway in the so called Spirito region. Some manufacturers show that region on their datasheets in the Save Operation Area, but IR does not and they don't even show the DC rating.

So if you want to figure out if it blows up, look for a data sheet where the linear mode performance for DC currents is shown in the datasheet together with the spirito region. I found this document explaining linear mode performance in more detail:


If you read the question carefully, it asks if this setup will work with a 12 v motor. Yes it will work with a twelve volt motor. It's not complicated. Its a 55v, 110a mosfet controlling a nine volt battery. Typically nine volt batteries can't supply 110a for very long, so barring the improbably ridiculous, and assuming it is properly snubbed for higher currents, it will work albeit inefficiently. Also I'm not the op.

When he asks for 12V I assume he is no longer using a battery as power supply. Anyway 110A is what you can get through the MOSFET if you turn it on completely (VGS=10V says data sheet), but if the MOSFET has handle a voltage drop in linear mode, that is a completely different matter. Then you reduce the VGS with your potentiometer and the RDSon rises and you cannot send 110A through it anymore... I guess you are right that the 9V block will probably not blow up the device, but you cannot be certain, just from the few information given by the datasheet. When force your device in linear mode and the MOSFET has to handle a VDS of just 9V, it will likely blow up when you drive more than 10A through it. I did not try it for this device, but I have bad experience driving MOSFETs in linear mode.

Actually the datasheet is quite adequate for its intended usage as a design tool. It is not a substitute for competent design, SPICE-type simulation, and nuts and bolts physical prototyping.

I would respectfully suggest to you that the reason you may have had trouble in the past with your mosfets is not the mosfet's fault. It might be, because mosfets are not generally intended to be operated in linear mode. Mosfets and igbt's are primarily intended to be operated in switch mode.

Basically there is no reason, imhto, to operate a power switching device in its linear region any more except technical naivete or laziness. Pulse width modulation in switch mode is the accepted alternative. But if you don't agree, I would be happy to discuss design specifics outside of this comment section. We are getting a bit off topic.

I totally agree that MOSFET are typically not intended to be driven in linear mode, but this circuit does drive the MOSFET in linear mode. So when then the question arises how much you can scale this up to bigger motors, the data sheet does not give the answer, because the data sheet does not anticipate such a usage of the device. The SPICE model also does not give the answer to the linear operation mode. There are however a few applications where MOSFETs do need some linear mode capability, so some competitors quote a more accurate capability on their data sheets, including a DC rating and the Spirito region. Look if the lines in the save operation area figure have a kink in line determined by the power dissipation (lines which go from top left to bottom right). If the lines shows such a kink then the Spirito region was determined and you can tell if the you can use that MOSFET in that region. Still this is typically a single shot rating, so you still need to leave some margin to that line. A factor 2 is probably enough. The data sheet of IR is therefore not adequate if you want to know if you can scale up this circuit. In general I would advice against upscaling this circuit and rather use a more complex circuit, as you propose.

Oh for goodness sake! This thread is done. I stand by my original answer even more than I initially did.

The way the transistor is set in the sink is not adequate, the right is that the body of the whole transistor is leaning against the sink.

This was already a thing, too much prior art and existing products. Perhaps you should credit your sources.

I use a LM 555 to drive the powerful N channel MOSFET IRF 540 N. (12-14 VDC supply, Motor 12 V 5.5 Amps- 60 WATTS). The circuit above is the simplest one for small DC motors- but with big 5 Amp Motors some more circuitry is necessary. Also, a fast diode over the Motor should protect the Mosfet from inductive back-strokes. If in the 555 driver circuit (Internet) the poti works in reverse- switch the end terminals where the diodes are located. Also good to experiment with LIN or LOG potis. In the other circuits, the Source of the N channel Mosfet goes to negative (Ground), the Drain goes to Motor Negative and the Positive Motor terminal goes to the + rail. Bridge in this case the Motor with a (fast) diode from Drain to the + rail. If you make a PCB, take care if your motor draws 50- 60 Watts that the copper lines are broad enough to handle the currents.

Or use one of these, if you have time to plan.


Out of curiosity why is the MOSFET needed?

why not simply connect power to the wiper of the variable resistor and power out to the motor?

I'm not understanding

1 reply

You'll burn out the potentiometer that way. They have *tiny* power ratings at the wiper contact, unless you use one that is designed for higher power.