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Over voltage protection circuit Answered

Hi everyone!

So i was making one of those water level indicator circuit:

I found the construction to be fairly easy for a newbie for me. I modified the circuit according to my needs, and have successfully constructed it. There is a small issue though. The place i live is susceptible to over voltages often and our fuse system is presently out, so my family has discouraged me from implementing the circuit at home in my main water tank. They want me to add a over voltage protection circuit so that they can feel safe. So anyone has any idea on how to construct an overload protection circuit(The circuit should be simple and understandable for a newbie like me)? My supply mains are 230v AC. It can either be before or after the transformer stage.

3 Replies

Orngrimm (author)2012-12-17

I can second the analysis of steveastrouk:
The given schematic is good and save.
why? The LM78095 is a linear-regulator and is placed correctly just after the transformer.
A linear regaulator may not be the most efficient thing (It is one of the more wasteful regulators but very simple and save) but it is really a workhorse and quite save.
It normally (and according to the datasheets) can withstand voltages up to 35V which would translate to a mains-Voltage of over 1600 Volts! And it is almost impossible to fool a LM7805 into a state where it lets more than those imprinted 5V by. The choice of a FIXED voltage regulatro like the LM7805 instead of an adjustable one like the LM317 eliminates almost all dangers of "wrong setup". Also the LM7805 is shortcircuit-save.
If you want an added savety, you can add a fuse to the system. this is done best as close to the mains as possible. So add the fuse in between the mains and the transformator. The value of the fuse should be something like 10mA if i take a close shave. But going to 50mA fuse will be more than stable enough to never have to replace the fuse only because one of your LEDs use a bit more power, but leaves you with a save 12W max-Power. And a big ammount of that will be heat in the transformator.

The biggest danger here is propably water-spillage to/on/in the circuitry and especially to the transformator.
So i suggest the following to eliminate even this almost 100%:
Buy a "wallwart"-type adapter to give you the needed 5V. Then go with the Lowvoltage near the tank. This way, even if you drop your entire circuit into the water, it is only 5V maximum.
If your socket (where you plugged in the wallwart) is fused, then you are preso clear and ready to roll. :)

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charmquark (author)Orngrimm2012-12-19

Thanks for the reply!
I have eliminated the water problem as my circuit is pretty far off the tank, almost 5 meters, so no issues. And even i was thinking of adding a fuse, so since you seconded my thoughts, i might as well add it.
And third, well, adapter is a far off thought since its not available in my place.

Anyways i guess these will suffice. Thanks guys!

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steveastrouk (author)2012-12-15

The circuit shown is well designed. The regulator protects your circuit from overvoltages, if you build it as shown. If we use a conservative design for the transformer, and say it has a 9V output, the regulator would be safe up to a maximum mains input of something like 1000V, sustained. You will not see even transient levels like that.

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