Another Reverse Polarity Protection Question - Page 1
Forum section: electronics / beginners

author: trilerian
author_rank: Regular Contributor
timestamp: December 21, 2023, 04:03:40 am
content: I have a circuit and I have reverse polarity protection on the main power source. But this project is a battery discharger, so not only can you hook up the power source incorrectly, but you could also hook up the battery to be discharged incorrectly.What do I need to do to add RPP for the battery to be discharged?

author: SuzyC
author_rank: Frequent Contributor
timestamp: December 21, 2023, 04:52:49 am
content: You could use a full-wave bridge rectifier and connect the AC bridge rect pins to the battery and either way the battery is inserted, the polarity would be correct.

author: Teledog
author_rank: Regular Contributor
timestamp: December 21, 2023, 05:03:45 am
content: Selenium rectifier stack!Or, if high enough HV, ..the old mercury arc units!

author: EPAIII
author_rank: Super Contributor
timestamp: December 21, 2023, 07:24:32 am
content: Just use a polarized plug and socket.

author: trilerian
author_rank: Regular Contributor
timestamp: December 22, 2023, 05:25:06 pm
content: Quote from: EPAIII on December 21, 2023, 07:24:32 amI will be using XT90 connector for the main battery leads and JST XH for the sensor connection. But that doesn't stop the end user from connecting the main leads to their battery incorrectly.I'll have to look into the above posts to see what that entails.Thanks

author: perdrix
author_rank: Frequent Contributor
timestamp: December 24, 2023, 04:06:55 pm
content: This works pretty well...When the power is the right way round the Fet is turned on hard and is effectively not there.When the power is connected the wrong way round the gate is pulled -ve and the Fet is turned OFF hard.

author: trilerian
author_rank: Regular Contributor
timestamp: December 25, 2023, 07:32:14 am
content: Could anyone walk me through what would happen in my circuit if I plug the battery to discharge in backwards? So 8.4V would connect to ground, but the negative on the battery would the connect to the external load and through that to the drain on the mosfets. The gates on the mosfets have a 10k pulldown on them. Would the 8.4v going to ground change the ground and apply a voltage to the gate, essentially completing the circuit, which would be a short? I don’t really know what would happen here.

author: ArdWar
author_rank: Frequent Contributor
timestamp: December 25, 2023, 07:39:47 am
content: The (reversed) battery current will be conducted by MOSFET's body diode if their gate is in off state. Your gate drive appears to be completely isolated from the load so reversed load shouldn't affect the switching operation itself.On another note did you just tried to parallel four MOSFET directly without ballast resistor at all?

author: trilerian
author_rank: Regular Contributor
timestamp: December 25, 2023, 01:43:58 pm
content: Quote from: ArdWar on December 25, 2023, 07:39:47 amThis circuit is for switching and not operating in the linear region. From what I read I didn’t need the ballast resistors as they will for the most part regulate themselves.

author: coromonadalix
author_rank: Super Contributor
timestamp: December 25, 2023, 03:28:08 pm
content: all this to save a diode ? or even a rectifier bridge ?a simple 3-4 amp diode is way sufficient if you regulator as 1 -1.5 Amp max outputdont tell me you are afraid of a 0.6v dropand for the second parta reverse diode is okay on the regulator pins, once again the simple cheapest is another diode in serial with the output, nothing will come back in reversenormally put before the mosfets loads but you have may schematics on the web about variable loads, check them and adapt your power stage

author: trilerian
author_rank: Regular Contributor
timestamp: December 25, 2023, 08:18:55 pm
content: Quote from: coromonadalix on December 25, 2023, 03:28:08 pmPlease forgive my ignorance, but I'm not quite following. The VIN has a mosfet for rpp, but that was because I wanted to learn how to do that, and it was my first design with a mosfet that I chose. I just keep using the same supply for my projects. I think that is pretty common for hobbyists to stick with what they know. However, I do realize something like a Schottky diode would work in place of the mosfet. But the mosfets are $0.36 each, so I am not worried about the price. i suppose if I were making thousands of the units...For rpp of the battery that is discharged, anything that I use has to be able to withstand the current that would be carried. In this device, up to 45A. It also cannot cause a significant voltage drop. I do intend to be able to read IR of the battery that is being discharged, and will use the secondary leads from the battery that connect to the voltage sensor connector. This is essentially a balance connector for the battery. Ultimately, it probably will fall to the end user to not plug the battery to be discharged in backwards. But some people will. It will be good for me to know the likely parts that will get damaged so I can replace them if needed.