pulse motor help

dorydan

Joined Jul 1, 2020
11
hello, i am trying to learn about transistors with this pulse motor project. can someone please explain if there is a way to estimate values of R1 and R2 below if V=1.2 and the coil=800 ohms? thanks, Dan

wayneh

Joined Sep 9, 2010
17,498
hello, i am trying to learn about transistors with this pulse motor project. can someone please explain if there is a way to estimate values of R1 and R2 below if V=1.2 and the coil=800 ohms? thanks, Dan
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A convenient rule of thumb is that a BJT needs a base current of about 1/10 the load current in order to function as a reliable switch - to saturate and conduct fully. In practice you can often get 1/20 or 1/50th, but you start with 1/10 as absolutely certain to work.

So at first pass, R2 might be 8k Ω to witch an 800Ω load. R1 might be 80k for switching the current in R2.

These would be starting values only and you might be able to increase both. The DC resistance of the motor is probably much lower than the running impedance, for instance.

I'd simulate your circuit in LTspice but the motor is a mystery. I don't have a simple DC motor model. Every time I look for one I get a headache.

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dorydan

Joined Jul 1, 2020
11
Thanks Wayneh, that is super helpful! Using those values, I was able to get it running and now I am tweaking it with potentiometers. It consistently stops running after a few hours. It is a super low speed, low torque motor (~30 RPM) which is what I want since it is just a conversation piece. I am not sure if it stops due to an electrical issue or a mechanical issue since it has very little "reserve" power, but now that it runs I will keep tweaking the resistance to see if I can find the ultimate sweet spot.

wayneh

Joined Sep 9, 2010
17,498
Thanks Wayneh, that is super helpful! Using those values, I was able to get it running and now I am tweaking it with potentiometers. It consistently stops running after a few hours. It is a super low speed, low torque motor (~30 RPM) which is what I want since it is just a conversation piece. I am not sure if it stops due to an electrical issue or a mechanical issue since it has very little "reserve" power, but now that it runs I will keep tweaking the resistance to see if I can find the ultimate sweet spot.
So is it stopping because the battery runs down? There's not a lot that can be done about that except to use a higher capacity cell. Starting at a higher voltage (alkaline cell, for example) might buy you a longer run time.

Increasing R2 until the current in the motor is throttled will minimize the base current in Q2, which is 'wasted'. But you don't want to have too much resistance in Q2 as that will cause power dissipation in Q2 and possibly heat. Maybe not so much at that low voltage.