Input protection for a high current power source - Page 1
Forum section: electronics / beginners

author: ziggyfish
author_rank: Regular Contributor
timestamp: May 04, 2017, 11:29:25 pm
content: Hi,I want to protect someone from connecting a 22V battery with the wrong polarity to a circuit that draws 100A in normal operation.What is the best way to do this?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 05, 2017, 12:36:03 am
content: Is the battery less than 100A S/C, or more?Tim

author: ziggyfish
author_rank: Regular Contributor
timestamp: May 05, 2017, 12:57:30 am
content: Quote from: T3sl4co1l on May 05, 2017, 12:36:03 amIt's s 16Ah (16000 mAh), 10C discharge, 20, 160A max.

author: john_c
author_rank: Contributor
timestamp: May 05, 2017, 02:45:58 am
content: How about using a MOSFET in full conduction as a diode? They can make FETs with less than 1 mOhm of on-state resistance, now. Put an n-channel one in on your ground return, and you only waste about 5 watts. Compared to a regular diode (60 watts) or even a Schottky diode, that's not bad.If it's a sensitive load, then this protection may need to be on the high side. Perhaps you could use a p-channel FET, but there aren't as many options. I Googled "reverse polarity protection FET," and it looks like what's sometimes done is to use a small charge pump in order to provide sufficient gate voltage to turn on an n-channel that's on the high side.

author: Zero999
author_rank: Super Contributor
timestamp: May 05, 2017, 01:05:58 pm
content: There's nothing you can do to the battery to guard against this.The circuit needs to be protected against reverse polarity. If a Schottky diode has too greater voltage drop, then a MOSFET can be used, as mentioned above but note the maximum rated gate voltage is normally <20V, often <15V for low voltage, low RONtypes.Another way to do this, is use a large diode, with a much higher current rating than the circuit, connected in reverse parallel with the circuit and a fuse before the diode.

author: ziggyfish
author_rank: Regular Contributor
timestamp: May 05, 2017, 10:40:46 pm
content: So its not possible with circuitry.Would it be better to use physical construction( i.e key the connector) to solve this problem?

author: Zero999
author_rank: Super Contributor
timestamp: May 05, 2017, 11:44:04 pm
content: Quote from: ziggyfish on May 05, 2017, 10:40:46 pmYes, brilliant idea. Use a keyed connector, wired up by someone who knows what they're doing.The battery just supplies current to what ever is connected to it. There's no way it can know whether it's frying the device or not. A fuse is a must but even then, the current required to fry something connected in reverse, is normally a tiny fraction of its normal operating current so that offers no protection whatsoever.Designing a system so a fault can't occur in the first place, is always much better than attempting to protect it against the fault.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 06, 2017, 12:02:44 am
content: I guess you could have a low and high power circuit. Low power would be fairly easy to protect and used to drive a large contactor if correct polarity. Fairly low loss but large, expensive, mechanical.... Key it.

author: tszaboo
author_rank: Super Contributor
timestamp: May 06, 2017, 12:37:35 am
content: Yes, this is much better. Use anderson power connectors, pretty standard for batteries.The other option would be to place a crowbar circuit on the input. That is a fuse, and a diode, which normally does not conduct. Bear in mind, I have seen improperly sized crowbars, where the diode goes bang first.

author: james_s
author_rank: Super Contributor
timestamp: May 06, 2017, 12:42:16 am
content: I sometimes use both a keyed connector and a crowbar diode and fuse. It depends on how much damage is likely to be done by reverse polarity and how likely some idiot is to connect it backward.