Simple Preventive Measures For Accidental Reverse Polarity? - Page 1
Forum section: electronics / beginners

author: Trurl
author_rank: Regular Contributor
timestamp: September 22, 2024, 02:06:41 pm
content: When I commented in another thread that I intend to use a modified server PSU for automotive diagnostics & ECU Programming to keep the battery charged up, I was warned by a member(Gyro) as follows:"... Beware of using a server PSU in an automotive environment that it wasn't designed for, where it may be subject to accidental reverse polarity, load dump etc. At the very least you're going to need a [Edit: large parallel reverse protection diode, maybe a TVS, and a] big fuse."Currently, I have a 100A "kill switch" for the negative side, so I can have the PSU ON and the external cables connected to the car's +/- terminals, but the PSU NOT connected to the car when the "kill switch" is OFF, during which I can set the PSU's voltage to be higher than that measured on the car's +/- terminals, to prevent reverse current into the PSU.I'm wondering if there is a way to have such "large parallel reverse protection diode, maybe a TVS*" and "big fuse" inline with the cables?*What is a TVS?If someone would post photos/examples of such components with their typical installation methods, I'd be grateful.Thanks...

author: Xena E
author_rank: Frequent Contributor
timestamp: September 22, 2024, 04:07:17 pm
content: TVS = transient voltage suppressor.Can you link your original thread, (it's best to keep questions on a subject together).X

author: Trurl
author_rank: Regular Contributor
timestamp: September 22, 2024, 04:11:52 pm
content: Thanks for the clarification of TVS.https://www.eevblog.com/forum/beginners/best-way-to-solder-copper-terminals-onto-psu-output-tabs/msg5650807/#msg5650807Note: Reply #19 by GyroIts a long thread and the topics are unrelated, hence my starting this new thread.

author: Gyro
author_rank: Super Contributor
timestamp: September 22, 2024, 07:32:59 pm
content: Yes, Transient Voltage Suppressor. There will be one in the ECU, and probably smaller ones in other sensitive electronic bits, which clamp the supply voltage at some level above highest charging voltage but low enough to prevent damage, possibly around 24V but there is a slope from initial conduction point and the maximum current it can handle.As I mentioned in the other thread, a really hefty reverse polarity protection diode across the output of the PSU would hopefully blow a reasonable sized series fuse between it and the battery, worst case it would hopefully fail short and still blow the fuse. The PSU can probably handle itself safely if its output is shorted, the important thing is to prevent the, for all intents and purposes, infinite short circuit current discharging into it.Unidirectional TVSs (there are unidirectional and bidirectional) act like big Zeners with optimized characteristics for handling heavy discharges, so it can be used for both voltage limiting and reverse protection. For overvoltage (load dump) it should either limit the voltage to a safe level or, again, blow the fuse.The important thing is to find a TVS big enough.P.S. A high current series diode instead would solve a world of problems. The problem is that it will have a forward voltage drop. I don't know how much PSU output voltage adjustment range there is to offset this - say, 1.5V at 100A.

author: Xena E
author_rank: Frequent Contributor
timestamp: September 22, 2024, 09:13:34 pm
content: Da ja vu... there's been similar questions before if I understand the OP here. Anyway, I'll give a similar answer!For reverse protection a diode/contactor combination can be used on the vehicle side of the output. The polarity having to be correct before the contactor pulls in, therefore isolating the output of the PSU until it is correctly connected.Contactor may be a problem for the currents being suggested, possibly a vehicle starter solenoid? I don't know if they're able to have their coils powered for extended periods.