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[SOLVED] Get rid of the active zone in transistor

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Carol Rein

Jan 8, 2022
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Let`s say that variable resistor is a photoresistor.

Transistor as switch.png

I need the transistor to get rid of the linear zone (work as a switch) and make the gate to ignore the voltages under the saturation threshold.
What do I have to add to this circuit?


Look for:
* schmitt trigger circuit
* comparator with hysteresis


Firstly, understand that an emitter follower configuration has no voltage gain so if you want an abrupt on/off transition you would have to provide a large change in base current and you will not get that with a potentiometer.

Secondly, understand that there is a PN junction between the base and emitter of the transistor so if you set the potentiometer to the 12V end of its travel, much of the load current will flow directly in to the base and possibly destroy it.

You need a circuit that compares the potentiometer output voltage with some threshold and changes the load current depending on whether that threshold is exceeded or not. You can do that using a comparator circuit and you can use the potentiometer, connected with its track between 12V and 0V to set the threshold voltage if you want with the photoresistor providing the other input to be compared.


Thanks, I got it.
Well, that circuit was for me to present the theoretical thing and understand the principle based on your answers.
Now this is my actual circuit, it`s a MOSFET, an ifrz44n
Mosfet switch.png

It's really working, the potentiometer is actually a protoresistor, but I got that linear behavior when the photoresistor is mid excited.
According to Everycircuit (the soft where I've captured the image) it is NOT working... but it actually works pretty well... may be the kp, to and lambda values are wrong set in EC, who knows.
Should I try adding something like this before the mosfet gate?
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please clarify:

What is a protoresistor?

Do you want
* ON/OFF operation, or
* speed control

In either case: a motor is an inductive device thus you need to add a freewheeling diode across the motor.
Do internet search.


In post #4, the right side of the potentiometer should be grounded to make it a voltage divider and limiting resistors added to its ends. Have you tried a zener diode in series with the wiper ?

Photoresistor (which I`m actually using)
Sorry, a typing mistake.
I need an ON/OFF operation, thats why I'm trying to get rid of the linear zone of the transistor.
Yes, the potentiometer could be used as a divider by grounding it, but not the photoresistor which has only two contacts such as a regular resistor. Photoresistor doesn`t show in Everycircuit, so I had to draw a potenriometer instead.
Yes, the diode is needed for inductive loads, you are right. Its just I wanted to simplify the diagram as much as possible, I am sorry for that... my bad.
Thank you guys for your answers.
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