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Protecting ICs against regulator failure

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ljcox

Well-Known Member
Does anyone have a circuit to protect CMOS ICs against the possibility of a short circuit occuring in the regulator? (I once had to replace an expensive mico controller in my nephew's audio system because the 7805 had gone S/C and destroyed it)

I don't want to have to re-invent the wheel. I did a brief search of these forums, but found nothing.

I know about crowbar circuits and once designed one for an 8V supply, but I'm looking for other options. For example, it could be done with a series transistor on the unregulated side. There would be a voltage sensor on the regulated side that senses when the 5 Volt rail is rising and thus turns off the transistor.

There would have to be a memory of some sort, eg. an SCR to keep the transistor off.
 
So you just want the power to shut off if the voltage after the regulator goes above ~5V, I would do just what you suggested, a voltage sensor tied to a transistor, or you could also use a 5v schmitt triggered inverter tied to a transistor. Unless you want to protect against current, then just use a fuse!
 
Yes I want to protect against over voltage. Yes I kow it is rare, but some ICs may not be available in the future and so I may not be able to buy spares.
 
Crowbar with a SCR and sensing zener which turns the SCR on at say 5.6 Volts and will take out the supply fuse.
Those 5 Volts regulator IC's rarely fail. they require good heatsinking over 300 mA even they are protected against high temp.
I built several TTL clocks in the 80's in which a 7805 runs at 800 mA and NEVER had a faillure of them in over 20 years.
 
Thanks for the responses. Yes my 7805 may never fail, but if it does it will blow some expensive ICs which may not be replacable in the future.
 
If your 7805 blows then the most likely result is its output will fall to 0V rendering your precious ICs unharmed.

If you don't want you 7805 to fail then don't reverse the supply polarity (or install a rectifier in series to prevent this) and add a protection diode from Vin to Vout (cathode connected to Vin) and lastly don't put it in the oven, microwave, toaster or expose it to neuclear radiation.
 
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