Diode question - Page 1
Forum section: electronics / projects

author: Wilksey
author_rank: Super Contributor
timestamp: January 25, 2022, 03:22:52 pm
content: 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!

author: TimFox
author_rank: Super Contributor
timestamp: January 25, 2022, 04:02:54 pm
content: 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.

author: Wilksey
author_rank: Super Contributor
timestamp: January 25, 2022, 04:12:17 pm
content: Thanks Tim,There is a 200mA resettable fuse in series with the VCC rail.

author: TimFox
author_rank: Super Contributor
timestamp: January 25, 2022, 04:49:57 pm
content: 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.

author: Wilksey
author_rank: Super Contributor
timestamp: January 25, 2022, 05:00:13 pm
content: 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

author: TimFox
author_rank: Super Contributor
timestamp: January 25, 2022, 05:11:26 pm
content: 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.

author: Wilksey
author_rank: Super Contributor
timestamp: January 25, 2022, 05:14:57 pm
content: Thanks Tim, very useful information.

author: ajb
author_rank: Super Contributor
timestamp: January 25, 2022, 07:38:41 pm
content: 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.

author: Wilksey
author_rank: Super Contributor
timestamp: January 25, 2022, 10:03:37 pm
content: Hi ajb, I don't have the schematic, it is a 3W 5.6V Zener from the SMD code.

author: Zero999
author_rank: Super Contributor
timestamp: January 28, 2022, 08:32:22 am
content: Quote from: Wilksey on January 25, 2022, 04:12:17 pmPresumably you're talking about a PTC resistor? If so, they're very slow. It might be not quick enough to reliably protect the diode.