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Is there such thing as an electronic overload circuit?

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fabbie

New Member
Is there such a thing as a electronic overload?
Since im using PIC to drive some motors and i dont want the current to exceed a certain value. Is there any kind circuit i can built to act as an current overload ?
Any recommendations? thx
 

spirosd

New Member
You might want to google for 'chopper circuits'. They acts as a switch, opening and closing the circuit, ensuring that the current does exceed a predetermined level.
 

fabbie

New Member
I dont really understand how does this sense resistor works. Do u mind explaining? It just seem like an ordinary resistor
 

k7elp60

Active Member
In the circuit posted by Russlk the current drawn by the motor flows thru the resister. The resistor is in parallel with the base emitter junction of the sense transistor. When the voltage drop across this resistor reaches the turn on voltage for the transistor(about 0.7 volts) the transistor turns on and and the collector emitter junction turns off the control transistor.

I you were to put a fuse in place of the sense resistor and eliminate the current sense transistor you would have a shorted control transistor. A fuse IS NOT FAST ENOUGH TO PROTECT A TRANSISTOR.
 

Oznog

Active Member
Self-resetting circuit breakers work well for motors.

Many PICs have an ADC input that you can use to read motor current. If it's too high, the program can shut the motor off or, if it's driving the motor with PWM, reduce the duty cycle.

A prob with the transistor shutoff schematic shown is also that this will probably just drop voltage across the transistor rather than turning off. This can generate a lot of heat in the transistor.
 

fabbie

New Member
hey i dont get 1 thing though about this sense resistor. If my control transistor is shut off, there wouldnt be any voltage drop across the sense resistor therefore this would cause the sense transistor to turn off. With the sense transistor turn off, the control transistor would turn on again. this will repeat over and over again in a loop. I dont see how this helps, unless i got the whole concept wrong :?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
fabbie said:
hey i dont get 1 thing though about this sense resistor. If my control transistor is shut off, there wouldnt be any voltage drop across the sense resistor therefore this would cause the sense transistor to turn off. With the sense transistor turn off, the control transistor would turn on again. this will repeat over and over again in a loop. I dont see how this helps, unless i got the whole concept wrong :?

It limits the current, it doesn't turn it off. As Oznog says, this will result in the transistor getting hot!.
 
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