Have you ever accidentally inserted your batteries in reverse and destroyed your circuit with reverse polarity? Or perhaps applied too high a voltage to your circuit?
Over voltage or reverse polarity can destroy your hard work in seconds and it's one of the first things you'll want to add to any circuit design. In this article we'll go through how to use just a few passive components to protect your circuit from magic smoke.
The simplest over reverse polarity protection circuit is to use a diode. It's super easy and cheap but it does come with a cost of some power leakage resulting in some voltage drop. In most simple projects this is totally acceptable but you might find this doesn't work for low voltage apps.
The simplest over voltage protection circuit is to use a Zener Diode.
If you aren't familiar with zener diodes, they are a particular type of diode that have very useful characteristics that make it act like a voltage regulator.
With an ordinary diode, they will stop the flow of current until the voltage applied exceeds their reverse breakdown voltage. At that point, the diode usually gets destroyed.
A zener diode when used in revese biased conditions, won't get destroyed even if the voltage is exceeded. Above its zener voltage it allows the current to flow and most importantly, the zener diode keeps the voltage at it's zener breakdown voltage value.
That is a handlful, so let's put it a bit simpler.
Let's say you have a Zener diode of a breakdown voltage at 5.1V.
It is the same a normal diode but if you place it in reverse, it will act like a voltage regulator. It will stop the flow of current until the voltage reaches ~5.1V. Once above 5.1V, current flows through and further it regulates the voltage to 5.1V.
Well not so quick. One issue is that we aren't still protected reverse polarity. Of course, you could put a regular diode in series to do this but this would not be very efficient.
To get over voltage AND reverse polarity protection, you can use a low-side or high-side configuration for your protection circuit which you'll find below.
One uses an n-channel mosfet and the other uses a p-channel mosfet. The n-channel mosfet is probably less commonly used because it has a disadvantage of disconneting ground which may not be ideal for some circuits.
In both cases, the mosfet is positioned so that it is open only when polarity is correct. The zener diode acts as a voltage regulator so you would select a zener diode with an appropriate zener voltage breakdown based on your circuit requirements.
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