Help with reverse polarity protection in simple circuit - Page 1
Forum section: electronics / beginners

author: trilerian
author_rank: Regular Contributor
timestamp: September 09, 2023, 05:08:12 am
content: Hello, I would like to add reverse polarity protection in my circuit. I know I need to use either a diode or mosfet, and I know the diode is the easier method. However I think I want to go the mosfet path so I learn how to do it. The circuit is powered by a 2s lipo normally, but can use a 12v dc power supply. Actually since I am using the 7805, it can be powered by more, but typically these are the voltages used. Also because of the 7805 and the drop out voltage on it, I don't think a schottky diode would work with a 2s lipo. I am using the 7805 because of the low drift since I am not using a voltage reference in my circuit. So if I replace that with something, the drift is important, or I need to add a reference. The design measures the magnetic strength of a bldc rotor. It works fine as is, I have assembled quite a few of them already. But for my v2 board I would like to make some changes. I am attaching a pic of the schematic to this post.Thanks

author: Zero999
author_rank: Super Contributor
timestamp: September 09, 2023, 09:29:46 am
content: The L7805 is marginal with two lithium cells.A diode in reverse parallel and a fuse can be used for reverse polarity protection, with minimal voltage loss.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: September 09, 2023, 11:12:50 am
content: The simplest solution would be to use a better LDO regulator with an in-built reverse polarity protection. It doesn't look like your circuit consumes a lot of current, so, for exampleLT1761-5should do just fine. Plenty of stock at Mouser (LT1761ES5-5#TRPBF).CheersAlex

author: langwadt
author_rank: Super Contributor
timestamp: September 09, 2023, 11:24:47 am
content: a pfet

author: ledtester
author_rank: Super Contributor
timestamp: September 09, 2023, 11:53:46 am
content: This should get you started:https://components101.com/articles/design-guide-pmos-mosfet-for-reverse-voltage-polarity-protectionhttps://www.onsemi.com/download/application-notes/pdf/and90146-d.pdfhttps://www.monolithicpower.com/designing-a-reverse-polarity-protection-circuit-part-i

author: BillyO
author_rank: Super Contributor
timestamp: September 09, 2023, 02:49:00 pm
content: Quote from: ledtester on September 09, 2023, 11:53:46 amNot sure about that first one. First they talk about the poor inefficiency of using a Schottky diode due it's forward voltage drop (usually less than 0.5V) then they show a far more complex and expensive MOSFET circuit that in the simulation drops over 1V! Plus allows more than 1.2 volts through in the reverse case. Hmmm... this article would have me running to the Schottky solution with glee.

author: Peabody
author_rank: Super Contributor
timestamp: September 09, 2023, 03:10:39 pm
content: I think that voltage drop must be an artifact of the simulation. It looks like what would happen when the mosfet is off, and current is flowing through the body diode. Anyway, below is my favorite explanation of the mosfet method. If you pick a mosfet with a 20V gate voltage spec, you shouldn't need the zener diode or the resistor - just connect the gate directly to ground.

author: BillyO
author_rank: Super Contributor
timestamp: September 09, 2023, 03:16:20 pm
content: Quote from: Peabody on September 09, 2023, 03:10:39 pmAgreed. Just a dodgy article.One thing you must be aware of with a MOSFET solution is the reverse current flow with a supply voltage drop if there is a lot of capacitance on the load side. For general, simplistic, fairly low current use the Schottky diode is the best way to go. For anything less than 2 Amps I would not bother with the MOSFET.

author: Peabody
author_rank: Super Contributor
timestamp: September 09, 2023, 03:30:54 pm
content: Well, the whole point of using the mosfet is to avoid the voltage drop. If that's not important, then the diode would work fine. But if the diode is a schottky which has warmed up because of the voltage drop, you could get some reverse current there too.

author: langwadt
author_rank: Super Contributor
timestamp: September 09, 2023, 03:48:18 pm
content: Quote from: BillyO on September 09, 2023, 03:16:20 pmyes, the concept works, but not when pick a fet with 1/5 the needed current rating and a 5R on resistance ..

author: mon2
author_rank: Frequent Contributor
timestamp: September 09, 2023, 07:13:53 pm
content: Apply a full wave bridge rectifier. It will self correct the polarity of the power source to the target circuit.

author: BillyO
author_rank: Super Contributor
timestamp: September 09, 2023, 07:24:34 pm
content: Quote from: mon2 on September 09, 2023, 07:13:53 pmI've certainly done that in the past. As long as you are okay with a voltage drop of between 1.5V to 2V it's fine. Also keep in mind that in the case you are using some current, there will be power dissipated across the bridge. For instance, 6A will burn up to 12W.

author: langwadt
author_rank: Super Contributor
timestamp: September 09, 2023, 07:49:21 pm
content: Quote from: BillyO on September 09, 2023, 07:24:34 pmand there will be diode voltage difference between the supply ground and the circuit ground

author: trilerian
author_rank: Regular Contributor
timestamp: September 10, 2023, 11:31:58 pm
content: Quote from: Peabody on September 09, 2023, 03:10:39 pmI have watched that video a half of dozen times now, lolI think the following part will work for me.SI2393DS-T1-GE3VDS 30vCurrent 7.5aRDS on 22.7mohmsVGS -20v +16vgate source threshold 2.2vMy circuti probably draws slightly less than 100ma, so 7.5 is more than enough. At most I will have a 13v power supply, and with VGS above 13v I shouldn't need a diode and resistor. The gate source threshold is the voltage needed to turn the mosfet on?Am I correct in my assumption that this part would work?Thanks

author: Peabody
author_rank: Super Contributor
timestamp: September 11, 2023, 01:01:47 am
content: The threshold voltage is the voltage at which the mosfet just begins to turn on. So you would want the G/S voltage to be several volts higher than that to turn it on fully. But with a minimum power supply voltage of two lipo cells, and only needing 200mA or so, that mosfet should do fine. The zener and resistor are not needed. Just make sure you orient the mosfet correctly.

author: trilerian
author_rank: Regular Contributor
timestamp: September 11, 2023, 01:29:02 am
content: Quote from: Peabody on September 11, 2023, 01:01:47 amThank you very much. And thanks to everyone else who posted in the thread. I appreciate how helpful people are on this site!

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: September 11, 2023, 08:05:58 am
content: The elegant solution is the LTC-4368 and a pair of MOSFETs, these ICs are widely used and available from Mouser. I've used them many times and they "just work". Be aware they are tiny!https://www.analog.com/en/products/ltc4368.htmlSJ

author: alligatorblues
author_rank: Regular Contributor
timestamp: September 14, 2023, 04:34:19 pm
content: Quote from: Peabody on September 09, 2023, 03:30:54 pmI always put the diode on the input ground.

author: Zero999
author_rank: Super Contributor
timestamp: September 14, 2023, 07:00:00 pm
content: Quote from: alligatorblues on September 14, 2023, 04:34:19 pmWhy?Note that doing that will cause problems, if the circuit interfaces with anything on the other side of the diode, otherwise it doesn't make any difference, other than it's unconventional.There are two reasons why I would put the diode on the negative:1) If all of the voltages are referenced to positive i.e. it has a positive earth.2) It makes wiring easier, but the circuit mustn't interface with anything on the other side of the diode.

author: trilerian
author_rank: Regular Contributor
timestamp: October 09, 2023, 01:18:27 am
content: Just wanted to reply to this thread and let everyone know that the mosfet I chose works for reverse polarity. Thanks a lot!But I do have another question. Can I leave it plugged in backwards indefinitely, or is their a time limit I am missing in the data sheet?

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: October 09, 2023, 07:55:13 am
content: Quote from: trilerian on October 09, 2023, 01:18:27 amThe Analog Devices data sheet shows two mosfets "back to back" in series with the supply, so I guess using one backwards is not a problem. Beware that some power mosfets include a series diode from source to drain.I use DMT6002LPS-13 mosfets with an LTC4368CMS-1#PBF, if you use only one of these (as opposed to two back to back), it will only give over Voltage/current, not reverse polarity protection as the internal diode will conduct.Both the LTC4368CMS-1#PBF and DMT6002LPS-13 mosfets are currently stocked by Mouser.SJ