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Measuring DC Motor Load Resistance - Selecting the Right Switching Transistor Answered

I am an electronics noob, although I have done several Arduino based circuits, the stuff I have done is all a bit "painting by numbers" and I feel the need to understand more about what I am doing. I have searched the internet for an answer but can't quite get all the info I need. Time to ask for help!

I am building a circuit (without a microprocessor) where a PIR sensor will start a small 3v DC motor (from the junkbox) when the sensor is activated (PIR output pin goes high).  The most helpful page I stumbled upon so far (because I think I understand it) is here:


Under the heading: Choosing a Suitable NPN transistor, where it  gives a procedure for selecting a transistor.

The first step is to ensure that the transistor's maximum collector current is greater than the load current, where load current  is calculated by dividing the supply voltage by the load resistance.

The thing that is tripping me up is: how do I measure load resistance of the motor? My hunch is that I just measure  the resistance across the positive and negative terminals of the motor whilst it is disconnected.  Can it be that simple I ask myself.

I appreciate, I could just try any old NPN transistor and see if it works, but that will leave me no better off for the next time.

Thanks for any help you can give, be they  answers or other places to look!

Softy Squirrel



7 years ago

With you I agree MOSFETs are better and easier to use then bipolars.
( Strong are your feelings on this Padawan  :-)
But running and measuring does not give you stall current at all.
Stall, a current which is always greater and can be ten times the
no load current on larger machines.



7 years ago

"Just measure the resistance across the positive and negative terminals of the motor whilst it is disconnected." Like Steve said, It is that simple !

And do it while the motor is cold turning the shaft slow to see the low point..
This way you get the lowest motor resistance that \you are bound to see.

Unless your load becomes a drive source, but that is another story.

Size the current of your NPN to handle 2 times the current for safety sake.
I always look to pick more current then I need. Keep in mind bipolar NPN
transistors have more gain in the middle of their current ability then at the top.


....and don't forget the freewheel diodes !

I disagree with Andy, the supply volts of the motor divided by its DC resistance gives you the absolute maximum it can pull from the supply: yes, you measure it on a multimeter.

If you don't have any data on the motor by far the easiest thing to do is to connect the motor directly across the battery and measure the motor current with a multimeter set to amps. You will find the motor current varies considerably with the load it's driving. Just measuring the resistance won't tell you much.
If it's only a standard 3V motor driving a light load then a transistor with a max. 500mA collector current would do you.