Pulse Width Modulation in context with controlling motors

hello everybody,

Monday an idea popped up from, my brain regarding controlling a dc motor(i m here talking about the small motors once used in video casette recorders or tape recorder which used magnetic tapes to record audio and video films .)

i thought that i could somehow controll the motor usin one relay and a comparator circuit which will compare the voltage with a reference voltage when the voltage will be less that the reference voltage then a negative voltage will be given to motor ,it will run in reverse direction else if the voltage is higher than the reference voltage than the motor will run in forward direction when no signal appears from the comparator thaen motor wont get any power.


i have planned to use a pc parallel port wire to give controlling signals to the relay .


my question :
1)how should i compare the reference signalwith that of pc?
2) can the motor withstand the fluctuation in the duty cycle?
3)is it possible to control the motor by varying its duty cycle alone?
with regards
shankbond

Hi there,

I don't think with one relay that is possible, a relay have two connections, one remains connected when the coil is not charged and then it is switched to the other when it is charged so the motor my switch between the direction of rotation but it certainly can not stop when you do not apply any signal.

so, how do u think i should go further?

not possible. the relay is too slow (5-10 milliseconds) and the motor inductance will arc the contacts.

Ubergeek63 said:

not possible. the relay is too slow (5-10 milliseconds) and the motor inductance will arc the contacts.

Click to expand...

please explain!

you can not use a relay as a PWM switch. you need a power FET

Ubergeek63 said:

you can not use a relay as a PWM switch. you need a power FET

Click to expand...

tell me more about it, i m only using a small motor,dont know about induction arc