Trying to use a brushed DC motor (same as pictured, but a larger model) as a variable load for a rotating drive mechanism by inducing Eddy currents that oppose the direction of the rotating magnetic field.
I'd like to PWM a MOSFET to connect a fixed resistor to the motor, thus applying a braking effect that can be controlled by the duty cycle of the PWM.
Are there disadvantages to this approach? And since I can't use a flyback diode, can I use a zener or diac instead?
Any thoughts are more than welcome!
You'd probably have better luck over at /diy/ for stuff like this, they have a whole general for amateur electronics
>disadvantages to this approach?
Don't think so, but eddy currents are merely a side effect, what you use here is the motor as generator and the resistor dissipates the energy. Zener diode or VDR as overvoltage protection parallel to the MOSFET.
>>7993963
So when the MOSFET goes to OFF state, won't the voltage over the motor spike because of the self inductance of the windings and the sudden change in current?
>>7994085
Of course it will, that's when the VDR or the Z-diode steps in to limit and absorb it. The value should be higher than the max generated voltage and lower than the max drain voltage of the MOSFET.
>>7994312
Typically you would choose a Schottky diode for this purpose, so would a VDR or Zener be fast enough for this purpose?
>>7994341
VDRs are quite fast and have a certain capacitance that slows things down a bit. Maybe you can use two, one directly at the motor/generator terminals and another one after the load resistor parallel to the transistor (CRC filter). If you have a scope you can test it with the motor in standstill.
>>7994497
Will do that, thanks a lot for the constructive feedback Anon!
>>7996535
It's about using a standard DC motor as a controllable torque/load. A case of creative problem solving if you will, but well within known principles and practices. I wouldn't be surprised if no text book covers this explicitely (like most things you do irl).
>>7996663
ok, but but ive never even heard of VDR/Z-diode/Zener
>>7996791
I have 2 subjects thats about how to type up agendas and minutes for meetings, and where we roleplay that we are in some big organization and need to do things like interviews and presentations.
>>7996780
Never seen such a thing? This one is for mains voltage surge protection, has a response time of less than 25ns and can absorb 8kA for 20µs. VDRs (voltage dependent resistors) are also called Varistors.
