Diode question

[Wilksey]

Hi,
Can a Zener diode also act as a reverse polarity protection diode?
I am trying to see if a circuit meets spec (not my circuit/design, someone else's), the spec says it needs to provide over voltage protection and reverse polarity protection, there is a single device across VCC + GND in the form of a 5.1V 3W Zener and that is it, so will that cover reverse polarity protection?

TIA!

[TimFox]

The normal use of a Zener diode is in the "reverse-bias" direction, where the diode (almost) does not conduct until it reaches a relatively large voltage.
In the "forward-bias" direction, it acts as a normal diode and conducts with a current roughly exponential in the voltage.
The data sheets may not discuss this forward-bias operation in great detail, but the power rating should be roughly the same as in the normal reverse-bias direction.  Therefore, it should protect against reverse-polarity fault, to the extent of preventing the unwanted reverse voltage in the circuit to reach, say, 1 V.  This will be true unless the current through the diode in that mode overheats the diode due to excess power dissipation--you need to see what would limit the current in your circuit for both modes of the diode.

[Wilksey]

Thanks Tim,
There is a 200mA resettable fuse in series with the VCC rail.

[TimFox]

That would limit the power in the diode to < 200 mW, but the short-time power could be higher, depending on how long it takes the fuse to react.

[Wilksey]

Usually the device is powered from a 100mA LDO output so it would never see that kind of current.

I am looking at the device as it is reported to occasionally be "not responding", whatever that means in real terms, so I am looking at the spec and the circuit, I have already found a few anomalies such as no ESD protection on certain lines and resistor values wrong, and I was dubious of the Zener acting as a reverse polarity protection device, but it sounds like it might work even though not ideal is that a fair summary?

Thanks

[TimFox]

Yes.  Also note that Zener diodes usually have higher junction capacitance than signal diodes (e.g., 1N4148), but that can be a good thing for protection circuits.  Also, the leakage current is probably higher.

[Wilksey]

Thanks Tim, very useful information.

[ajb]

Is it truly a zener, or is it a TVS diode?  If you're looking at the schematic often the symbol is the same, and they're functionally the same at a basic level, but the TVS diode will have specified power/current ratings in both directions.

[Wilksey]

Hi ajb, I don't have the schematic, it is a 3W 5.6V Zener from the SMD code.

[Zero999]

Thanks Tim,
There is a 200mA resettable fuse in series with the VCC rail.

Presumably you're talking about a PTC resistor? If so, they're very slow. It might be not quick enough to reliably protect the diode.