Re: Voltage regulators fail high

On Thu, 12 Feb 2009 11:48:35 -0500, Hammy <spamme@xxxxxxxxxxx> wrote:



You can use a SCR crowbar which blows the fuse,or something like this.

http://i44.tinypic.com/103iypt.png

This will a least save you on fuses if its just due to wiring errors.

The comparator senses the output voltage and compares it against the
reference if it's greater the comparator pulses the base of Q1 which
with Q2 makes a latch this switches off the P-FET at the 7805
regulator input shutting it down.

I didnt include any caps other then C1 for some noise imunity, I also
didnt calculate resistor values but you should get the idea.

Theres allso dedicated monitoring IC'S.

You may instead want to figure out why they keep failing do you have
them properly heatsinked?

Two things I forgot; put a small signal diode at the comparators
output cathode towards the transistors just 1n4148 will do . The other
thingis place a small capacitor across R6.If you don't have that cap
there the transistors will latch as soon as you apply power to the
circuit.Start out with 10pf increase as needed.

Usually most devices operating off 5V can take an absolute maximum of
7V so set your protection to activate at around 5.8 or 6V. But check
the datasheets on what the devices are that the 7805 is supplying and
Check the absolute max voltage they can withstand. Then set your
Protection accordingly.

You could also place an led in the transistor circuit so you know that
it has been tripped.
.

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