Zener protection

You've got to know (or guess) how much over voltage you are protecting against. Is it 10 Volts? or just 1 volt? With this maximum over voltage you can then pick an R value such that the maximum current (and power) rating of the zener is not exceeded. (Get a big beefy Zener.)

A poly fuse may work for your over current. (as long as you are not in a hurry... poly fuses take some time before they trip.)

George H.

On a sunny day (Fri, 29 Oct 2010 06:55:35 -0700 (PDT)) it happened Mahen K wrote in :

No

Forget that diagram.

You will need a crowbar, say a thyristor with a voltage sensing circuit, and a fuse in series. Maybe like this:

• - fuse -------------------------- + | | | --- | \ | / \ Zener --- --- power \ / /---- R1 --| board --- | | Thyristor R 2 | |

- --------------------------------- -

The zener voltage should be the trigger voltage of the circuit (where it starts shorting the supply) - the voltage needed to turn the thyristor on. R1 should be very low, theoretically zero, but better a few Ohms, current limit while the thyristor has not killed the fuse yet, but is triggered. R2 can be about 1 k, it is just to keep the thyristor gate low in case of any leakage. You could also use an integrated circuit in place of the zener to get a more precise cut of voltage.

A capacitor of about 0.01uF across R2 will help cut down spurious triggering due to noise.

--
"For a successful technology, reality must take precedence 
over public relations, for nature cannot be fooled."
                                       (Richard Feynman)

That won't give you the protection you want. One simple circuit you could use is a "crowbar", like this:

+5 ---[Fuse]----+-------+------+ | \__|__ | ---- / \ \ | SCR \ / --- Zd | -- | | | \ | [Load] | +----+ | | | | | [4.7K] | | | | Gnd ------------+-------+------+

Use a 1.25 or 1.5 amp fuse. The SCR should be able to handle 2 amps or more - a C106DG or equivalent would be fine. Zd is a

1N4734A 5.6 volt zener diode. The 4.7K ohm resistor finishes the circuit.

When the voltage rises above ~ 5.6 volts, the SCR turns on and shorts the 5V line to ground, which blows the fuse and removes the voltage from the load circuit.

More precise control of the trip voltage - that is, the voltage at which the circuit will operate to blow the fuse - can be achieved if you want by using a TL431 and 2 additional resistors in place of the zener.

Ed

Thanks all for explaining. I will go with SCR/Zener solution as you all suggested.

m.jnk