Zener Diode Dilemma

[jjk]

Thanks for your comments Andrew.

I'm wondering why the zener would conduct at all at voltages as low as 1.4v?

Zener voltage regulation happens in reverse breakdown mode.

All zeners leak reverse current at voltages below the breakdown voltage.

With low voltage zeners, this leakage current is very high, up to 76mA

for this part number. This makes low voltage zeners unsuitable for

many applications, where you might expect a theoretically perfect

zener to work.

My goal was not to use the zener as a regulator, but to protect 3.3v devices from potentially external over voltage events (microcontroller driving 5v circuits).

If the 50mA or so leakage at 3V is an unacceptable issue for you,

you might instead consider a potential divider across the 3V supply,

with the centerpoint connected to an SCR gate to crowbar the supply.

Adjust the potential divider ratio to give the right tripping voltage.

A red led (forward biased) added at the top of the potential divider

might make it more accurate/sensitive.



I haven't actually tried this at such a low voltage, but I suspect it

may work better than a zener.



--

Andrew Gabriel

[email address is not usable -- followup in the newsgroup]

Thanks again Andrew. this is making more sense. I never used 3.3v zeners before.
The 3.3v protection I was talking about is to protect output pins of 3.3v devices. The gpio pins on a Broadcom processor on the Raspberry Pi is one example. If an external source that is attached to the pin fails and applies a voltage greater than 3.3v, the Broadcom device can be damaged. There are articles on the net describing the use of a zener for protection and that'swhat I attempted to do. I guess the safest thing to do is add buffers in between the processor and external devices.

 

[Andrew Gabriel]

Thanks again Andrew. this is making more sense. I never used 3.3v zeners before.
The 3.3v protection I was talking about is to protect output pins of 3.3v devices. The gpio pins on a Broadcom processor on the Raspberry Pi is one example. If an external source that is attached to the pin fails and applies a voltage greater than 3.3v, the Broadcom device can be damaged. There are articles on the net describing the use of a zener for protection and that's what I attempted to do. I guess the safest thing to do is add buffers in between the processor and external devices.

You can do that. When I've driven pi inputs from 5V outputs, I've used
a potential divider (1k8/3k3) to drop the input voltage.
If there's a significant risk of the pi's 3.3V supply failing whilst
the 5V output is still active, you could add a regular diode from the
potential divider's mid-point to the 3.3V supply, to limit the extent
the input can exceed the Vcc pin of the pi during such a failure, but
the pi isn't particularly expensive, and you could spend more trying
to protect one than it costs to replace it if you do damage it.