Reverse Polarity Protection for an electronic DC load? - Page 1
Forum section: electronics / beginners

author: Bootalito
author_rank: Unknown
timestamp: February 12, 2018, 01:12:26 am
content: How do commercially design electronic DC loads (ones with 150V DC input and 60Amps+) implement reverse polarity protection?The easiest method I've come across would be to simply put a beefy diode in series with the load and strap it to a heat sink. A RURG5060 (500V 50A diode) would work. But then your DC load has a minimum voltage of at least 1V, and this would preclude testing any single cell batteries.Any mosfet design would draw current from the DUT such as this design I've used sucessfully for low voltage projects(5V or 12V, etc):But any mosfet based solution (even if its a high side pchannel with a charge pump) is drawing current from the DUT that the DC load's shunt isn't seeing, as the shunt would be to the right of the mosfet. Depending on how its implemented (like above) this would also limit the DC input to whatever Vgs is.Maybe something like this??:I'm not even sure this would work...The current from Vgs would flow....backwards to ground? I dunno.Maybe a relay would work? If you are measuring voltage directly on the terminals (which I am in my DC load project), I can safetly measure negative voltage no problem, then only turn on the relay when correct polarity is measured. But now you are dealing with a mechanical part that can wear, and only has a switching speed on the order of tens of ms., which....may be enough for protection, but doesn't really guarantee it.Anyway I'm wondering how this is done properly without

author: woodchips
author_rank: Frequent Contributor
timestamp: February 13, 2018, 10:10:04 am
content: All the low voltage loads I have seen put another supply in series with the load for voltages less than 2-3V. This bucks the cable, protection etc losses and is then usable down to zero input volts. Does of course need a 3-4V 150A power supply!Because you are loading current, the additional series voltage is irrelevant.

author: Kevin.D
author_rank: Frequent Contributor
timestamp: February 15, 2018, 02:43:41 am
content: Try this thread :-https://www.eevblog.com/forum/projects/input-protection-of-dc-electronic-load/msg1397141/#msg1397141Regards

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 15, 2018, 10:36:18 am
content: Quote from: Kevin.D on February 15, 2018, 02:43:41 amnew reply #24use relay with sense circuit upstream, no unnecessary voltage drop and unnecessary power elements (except the relays, 0V drop), fwiw...

author: grifftech
author_rank: Frequent Contributor
timestamp: February 16, 2018, 03:37:32 pm
content: Try the schematic below, if you connect with reverse polarity the fuse will blow, no diode voltage drop during normal use.

author: tszaboo
author_rank: Super Contributor
timestamp: February 16, 2018, 04:11:31 pm
content: Reverse voltage is tricky, if your load is a bunch of FET, because it could open the FETs and all kinds of unwanted current will flow.You want back to back FETs if you want protection, and isolated gate drive.Relays and fuses for 50A+ are also a problem, as they become very very expensive. Semiconductor fuses is basically the only ones in this range. Who likes to pay 50 dollar for a 50A 150VDC fuse?And then you think about overvoltage, that is another can of worms. I was working on a single cell 50A battery tester. Our solution was for the overvoltage problem: Well, then it blows up, and they have to pay for repairs. There was just no way to fit every single kind of protection into the price of the board.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 16, 2018, 05:40:27 pm
content: Quote from: NANDBlog on February 16, 2018, 04:11:31 pm$5 is good to go?https://www.ebay.com.my/itm/1pcs-50A-Car-Audio-AFS-Mini-ANL-Fuse-Auto-Stud-Fuses-32V-50A-50-AMP-Gold-Plated/172397094656?hash=item2823aae700:g:hnwAAOxycD9TRnjfhttps://www.ebay.com.my/itm/10-pcs-Relay-5-Pin-SPDT-SRA-12VDC-CL-DC-12V-Coil-20A-Universal-PCB-New-Black-T1/252603781034?hash=item3ad05bcfaa:g:rZ4AAOSw44BYDzXlsemiconductors (diodes, transistors) are harder to come by in my place (more expensive) ymmv...

author: tszaboo
author_rank: Super Contributor
timestamp: February 17, 2018, 01:08:05 am
content: Quote from: Mechatrommer on February 16, 2018, 05:40:27 pmFuses are only rated for 32V. And the relays only for 20A.I mean it will only be a problem, that the relay welds shut it's contact, and the fuse will simply refuse to fuse.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 17, 2018, 04:43:46 am
content: Quote from: NANDBlog on February 17, 2018, 01:08:05 amyeah? that is what written on it...Quote from: NANDBlog on February 17, 2018, 01:08:05 amwe need automation to not flow the current (offed mosfet) while relays are making contact, relays are not meant to activate constant load, but for reverse protection only. and use 10 of them in parallel you genius to avoid burnt marks. they are only $4 and good to 200A, well... thats what written on it. with hard brainstorming going on in this place, i can see people came up with ideas that turned more expensive than a simple thing yet less effective and less reliable, fwiw, ymmv.

author: tszaboo
author_rank: Super Contributor
timestamp: February 17, 2018, 10:39:57 am
content: Quote from: Mechatrommer on February 17, 2018, 04:43:46 amOP wants a 150VDC system. You cannot fuse that with a 32V fuse, because it is just not going to break the current.And you cannot just wire relays in parallel, even if they dont switch currents. It doesnt work that way, even if you insult me in every single thread on this forum. I suggest to get a beer or something and cool down

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 17, 2018, 03:13:32 pm
content: Quote from: NANDBlog on February 17, 2018, 10:39:57 amif differing to your opinion is an insult, then i'll leave your opinion to you and anybody behind you, but i have the right telling my opinion to the OP, and i surely will be tempted to reply to any uninformed or random speculations, no offense and nothing personal. i'm sorry and but i dont drink anything that will get me out of my mind. i prefer something that will make me stay alert such as caffeine

author: tszaboo
author_rank: Super Contributor
timestamp: February 17, 2018, 04:52:44 pm
content: Quote from: Mechatrommer on February 17, 2018, 03:13:32 pmNo, it is the condescending writing, and calling someone a "you genius" is what is insulting.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 17, 2018, 06:30:14 pm
content: Quote from: NANDBlog on February 17, 2018, 04:52:44 pmparalleling 10 relays will make it 200A, 100A to be safe. appeal otherwise.and remember, dont nitpick, i specifically said current is (should) not flowing during relays making contacts. and what is your reason a 35V 50A i linked cant be used in 150V circuit? other than the writing on it?

author: tszaboo
author_rank: Super Contributor
timestamp: February 17, 2018, 07:22:10 pm
content: Quote from: Mechatrommer on February 17, 2018, 06:30:14 pmBecause they dont break the current. An under rated DC fuse just arcs over after breaking, until it heats up and catches fire.If you connect a HV battery to this DC crowbar, it will simply short circuit it, delivering thousands of amps until something blows up more violently.And, once again, you cannot connect relays in parallel, even if you de-rate it 50%. There will be different contact resistances across them, making the current un-even. The ones which carry less current, will be even more contaminated, and their resistance will further increase, causing even more unbalance, and so on, until you overload one relay, which will either melt shut or not close anymore. And this might happen at 1000h normal operation.

author: Damianos
author_rank: Frequent Contributor
timestamp: February 17, 2018, 09:51:28 pm
content: In my opinion this is a good approach:Quote from: Kevin.D on February 15, 2018, 02:43:41 amIt is important that this protection must be independent of any control circuit (that may fail or "forget" to protect).Note what Kevin says about fuses ... This is also relative to the silly trick with the diode and the fuse that does not prevent the smoke.

author: chris_leyson
author_rank: Super Contributor
timestamp: February 17, 2018, 10:42:40 pm
content: Kikusui PLZ-72W 110V 10A 72W load has series diode for reverse polarity protection and a relay for over volts, over current, over power and over temp protection. Manual with schematicshttp://www.lost-manuals.com/equipment.php?id=12881

author: max_torque
author_rank: Super Contributor
timestamp: February 18, 2018, 05:12:39 pm
content: I think i'd put a suitable low volts drop diode in series, but put a beefy relay in parallel. You can have a software selection that shorts out the diode, removing the RPP, but allowing testing down to a lower input voltage. Suggest suitable caveat displaid on screen or on the front panel decal etc!You could combine this with the logic checking the polarity of the input before it shorts out the diode to bring an extra level of safety. (in most cases, the RPP will be required due to operator error during connection of the unit)

author: Eka
author_rank: Regular Contributor
timestamp: March 28, 2018, 05:47:14 pm
content: Google search for "high current DC contactor". Make the sense circuit redundant and fail safe.