Query on forwarding biasing of a clampingTVS/ Zener on reverse polarity - Page 1
Forum section: electronics / beginners

author: x4ce
author_rank: Contributor
timestamp: August 24, 2024, 07:57:11 am
content: Hi,Given the attached circuit what will be the behavior of the D8 diode when polarity is accidentally reversed? Should I need to include a current limiting resistor?Regards

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 24, 2024, 08:12:46 am
content: When voltage is reversed, the diode will conduct, and thus limit the voltage that the rest of the circuit sees. This conducting diode will short circuit the power supply, and it will take all the current that the power supply can deliver. And something will break. Either the diode will release it's smoke or the power supply. Such a diode normally works in combination with a fuse.The circuit around the FET is a separate reverse voltage protection circuit. It's more common to see these with a P-channel MOSfet in the other power conductor, but this works too.But the diode is not intended as to be used as a reverse polarity remedy, but as a TVS instead, so there are not two independent reverse voltage protection circuits. But the diode will still conduct, and thus overheat and likely to get destroyed. Instead you can use a bipolar TVS. TVS diodes are also not rated for (forward) DC current at all.

author: x4ce
author_rank: Contributor
timestamp: August 25, 2024, 06:47:44 am
content: Quote from: Doctorandus_P on August 24, 2024, 08:12:46 amThanks for your explanation and that's why curious in case reverse polarity this could be damaging.I will use a bipolar/ bi-directional TVS. Also, is it possible to use a limiting resisting in series with uni-polar TVS?Regards

author: PGPG
author_rank: Frequent Contributor
timestamp: August 25, 2024, 12:32:23 pm
content: Quote from: x4ce on August 25, 2024, 06:47:44 amEverything is possible. The question is how it will workI have tested that even small 0.5A PTC withstand 25A 50us pulse that I need them to withstand (1kV Surge pulse through 40Ω resistor).But PTC are slow.I have not tested what will happen if you connect DC strong source to PTC in serie with TVS. Do first TVS will be broken or do PTC will work first.If you will test it and then be sure that in such situation PTC will switch off supply enough fast to protect TVS then you can use them that way.If your circuit after TVS can withstand being negative polarized with may be 1.5V (you need to read from TVS characteristic a point for your supply source maximum delivered current) than you can skip the negative voltage protection after TVS.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 26, 2024, 05:04:09 am
content: Yes, a bidirectional TVS seems the more likely choice there, given the polarity circuit.Regarding fusing, a diode will generally handle enough, but to be sure, you must compare fusing ratings -- TVS don't give this parameter, so you can only guess; and beware that PTC fuses can take quite a lot more I^2t than conventional types, due to their large bulk.For an educated guess, compare the largest rectifier you can find in the same package (i.e., about 3A in SMA), and use its I^2t rating as estimate. TVS generally have somewhat lower Vf (due to the thinner junction, hence lower avalanche voltage), but only very moderately so (like, 10s of mV lower, maybe up to 100?), and "largest in package" should get similar on die area, making this a reasonable estimate.Series resistance should be added to the circuit to limit maximum current, both to protect the PTC fuse, and to honor the surge rating of the diode. Often, this can be in the PTC itself, but just check and make sure everything is happy.Tim

author: CaptainBucko
author_rank: Regular Contributor
timestamp: August 26, 2024, 05:19:38 am
content: Quote from: Doctorandus_P on August 24, 2024, 08:12:46 amIn certain circumstances, specifically current limited power supplies that are the same or lower than the fuse, the fuse does not blow. In this case, the TVS diode fails, and they normally fail short circuit, and they will then dissipate a lot of power. I have seen them de-solder themselves from a PCB and fall off. Just something to be aware of.