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# I have 12V control input source, and I want to switch 5V…

I have 12V control input...
I have 12V control input source, and I want to switch 5V circuit by the input. What is the best option for it? Is it safe to use MOSFET (gate voltage will be 7V higher than drain)? If no, than use optocoupler? I could use resistors-based divider to take 5V part from 12V input, but I don’t know existing resistance of the input circuit.
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3/28/2018
Russell H., Service Tech
Category: Consumer Electronics
Satisfied Customers: 12,259
Experience: 8+ years of professional experience
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Hi, thank you for contacting JustAnswer.com. My name is***** will do my best to provide the right answer to your circuitry design question.

You could use a transistor to switch that 5 V line, even with a 12 V input voltage as the control trigger... you would not use a MOSFET, they are rather sensitive, a common PNP or NPN transistor woul be OK.

To keep the 12 V from burning out the transistor used to switch it, you would need a fairly large current-limiting resistor on the Base of the NPN (or PNP) transistor.

Given that it was hooked up as a Base-to-Emitter gating, you would (if transistor were silicon) need a current-limiting resistor on the base, large enough to not exceed the maximum base-emitter current, at

E = I*R determining

(12 - 3) = I (maximum base-emitter current) * R (the resistor value needed.)

E.g. if the I were 20 mA (0.020 Amperes) then that would work out to

9 = 0.02 times R... or 450 Ohms for the current-limiting resistor.

(The '3' is owing to a silicon junction having a 2 Volt voltage-drop across it, i.e. across the base-emitter junction. The 5 V would be switched through the Collector-Emitter circuit of this transistor. I picture it with Emitter grounded, and an NPN type transistor, with the Collector-Emitter having a sufficient current capability to provide whatever current the 5 V circuit needs, that is being so switched.)

C.f. this page, which shows something like what I had in mind, though with very different value for the resistor: https://teachmetomake.wordpress.com/how-to-use-a-transistor-as-a-switch/

Thanks.

Customer reply replied 29 days ago
Thank you for detailed answer! Could you plz describe what’s the problem with MOSFETs? I have couple of BS170 right here. So if they are OK, I wouldn’t go to marketplace to buy transistors.

Actually, it was late last night when I gave my opinion. On further consideration, apparently a MOSFET might do fine. Just protect the gate from excessive voltage/current, and it should work just fine. I assume you know the ratings of the MOSFETs you have, as to gate voltage or current limits.

Thanks.

Russell H., Service Tech
Category: Consumer Electronics
Satisfied Customers: 12,259
Experience: 8+ years of professional experience
Verified
Customer reply replied 28 days ago
Got it. Thanks again!
Russell H., Service Tech
Category: Consumer Electronics
Satisfied Customers: 12,259
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