Re: Using zener diode to protect test circuit?

Andrew wrote...
>
> I am designing a test instrument that will be powered from the circuit
> under test. Consequently there is a high probability of the incorrect
> voltage or polarity being supplied to my circuit. I cannot protect
> against reverse polarity by using a series diode as this will drop the
> supply voltage too much.

You didn't say how high the voltage is... If it's above 4V, you
can use a bridge rectifier to present the correct polarity to your
circuit. Not content to see a 1.2V drop in the bridge? Use four
MOSFETs instead, two each p- and n-channel. I think this works.
Tie the drains of both types to both inputs, with each FET's gate
to the opposite input. Tie the p-channel sources together, this is
the + output. The n-channel sources are the - output. Like this:
..
.. IN --+------- D S ----+---- + out
.. | p G |
.. | _______| |
.. | | |
.. | +---- D S ----' low-voltage active bridge
.. | | p G Vin >> Vgs(th), and < 20V
.. |__| ______|
.. | |
.. +--| --- D S ----, uses two each p-channel and
.. | | n G | n-channel MOSFETs, as shown
.. | |_______| |
.. | | |
.. IN --| -+---- D S ----+---- - out
.. | n G
.. |__________|

When power is first applied the FET's substrate diodes conduct
to present the proper voltage polarity to the output. Two of
the FET's also immediately turn on, shorting out their diodes
for a low voltage drop. The opposite-polarity FETs remain off.


--
Thanks,
- Win
.

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