# voltage regualtor

Discussion in 'General Electronics Chat' started by george medina, Oct 21, 2014.

1. ### george medina Thread Starter New Member

Oct 21, 2014
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a) What is the output of the voltage regulator if we set the power supply too low, i.e. 1 V? What about too high, i.e. 5 V? What about really high, i.e. 20 V?
b) Will this also work for negative voltage? If not is there a way we can modify the connection of the voltage regulator to the circuit so that it will work for -VCC?
c) How does this protect the circuit against reversed polarity?

2. ### MrChips Moderator

Oct 2, 2009
14,282
4,195
If the supply voltage falls below the rated regulated output voltage PLUS the drop-out voltage, the regulator no longer regulates. A linear regulator can only regulate downwards, not upwards.

Any excess voltage means that the regulator has to dissipate power as heat. For example, suppose the load takes 3.3V@1A.
If the input voltage is 20V, then 16.7V@1A = 16.7W must be dissipated as heat. If the regulator is not properly cooled with a very large heat sink it will overheat, burn out, or go into thermal shut-down.

(b) It will not work for a negative voltage. For a negative output you need a negative supply and and negative voltage regulator.

(c) It doesn't. You can end up damaging both the regulator and the circuit connected to the regulator.

3. ### george medina Thread Starter New Member

Oct 21, 2014
10
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then I should use possibly diodes, instead to protect the circuit from reverse polarity. if so, which didoes? WE want to limit input voltage to the circuit to 3.3volts

4. ### crutschow Expert

Mar 14, 2008
16,212
4,333
Here's a reverse voltage limiter using a MOSFET that has a very low forward voltage drop, much less than a forward biased diode for a properly selected MOSFET. For voltages less then 10V you need a logic-level type MOSFET that fully turns on (look for the ON resistance measurement value, not the threshold voltage, in the data sheet) at the minimum voltage it will see.

Oct 21, 2014
10
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k. thanks