Will this reverse polarity protection circuit cause a problem? - Page 1
Forum section: electronics / general

author: onemilimeter
author_rank: Frequent Contributor
timestamp: June 26, 2011, 11:53:11 am
content: Figure below shows a reverse polarity protection circuit (a series fuse and a parallel diode).Since the dc power supply may be located far away from the electronic device, I wish to place an electrolytic capacitor just after the protection circuit. When the input supply is reversed, will it damage the electrolytic capacitor?Cheers

author: PeterG
author_rank: Frequent Contributor
timestamp: June 26, 2011, 12:38:18 pm
content: The way i see it, the diode will take the brunt of the power, the cap should see the voltage drop across the diode. Mose caps will tolerate this without issue.Regards

author: Simon
author_rank: Unknown
timestamp: June 26, 2011, 12:46:21 pm
content: yes the fuse will blow well before the cap goes, be sure to use a decent capacitor though

author: onemilimeter
author_rank: Frequent Contributor
timestamp: June 26, 2011, 12:48:01 pm
content: Quote[Source:http://en.wikipedia.org/wiki/Electrolytic_capacitor]This makes me worried...

author: Simon
author_rank: Unknown
timestamp: June 26, 2011, 12:50:40 pm
content: why not just put the diode in series ? what in parallel ?

author: PeterG
author_rank: Frequent Contributor
timestamp: June 26, 2011, 12:55:38 pm
content: The diode is there to short the power and blow the fuse. Depending on the diode, the cap may only see 0.6-0.8v, well within the limits of he cap.Regards

author: Unknown
author_rank: Unknown
timestamp: June 26, 2011, 12:56:21 pm
content: Disadvantage of the series diode is that you waste one diode drop of voltage, which may be an issue with low supply voltages (eg. 2 AA batteries).

author: Zero999
author_rank: Super Contributor
timestamp: June 26, 2011, 01:29:28 pm
content: It's a common way of doing it, just make sure the diode can handle the brief short circuit current.

author: Simon
author_rank: Unknown
timestamp: June 26, 2011, 01:53:47 pm
content: yes I've seen similar setups before. If a scotky diode is used it should stay well under a volt. But i sense that this circuit has been picked up off the net somewhere in a search for reverse polarity protection. If possible I'd use a beefy diode in series but we don't know the application

author: onemilimeter
author_rank: Frequent Contributor
timestamp: June 26, 2011, 02:01:10 pm
content: Quote from: Simon on June 26, 2011, 01:53:47 pmThe output of the dc power supply is +15V (fixed) in my application. The electronic device requires +15V too. Therefore, a diode in series may cause the effective dc power supply to be 0.5V~1.0V lower than +15V.I'm thinking of replacing the capacitor with a non-polarized electrolytic aluminium capacitor (or using two polarized electrolytic aluminium capacitor connected back-to-back). What do you reckon? Any disadvantage of non-polarized electrolytic aluminium capacitor compared to the its polarized counterpart?Cheers.

author: tecman
author_rank: Frequent Contributor
timestamp: June 26, 2011, 02:10:27 pm
content: Polarized cap is fine. The 1 volt or so of reverse polarity will have no adverse effect.BTW this is a very common way of protection. A series diode works, but you lose the diode drop, and on lower voltage circuits this can be a factor.paul

author: ejeffrey
author_rank: Super Contributor
timestamp: June 26, 2011, 05:26:54 pm
content: Quote from: onemilimeter on June 26, 2011, 02:01:10 pmIt is not needed, and the cost and size are larger than an equivalent polarized capacitor.The only thing you really need to worry about here is to make sure that the supply can provide enough current to blow the fuse. The short-circuit current should be around 2-3 times the fuse rating.+15 volt is usually an analog/op-amp supply voltage. Most op-amp circuits don't care much about their supply voltage within reason, so a series shottky diode probably wouldn't screw anything up either.

author: Simon
author_rank: Unknown
timestamp: June 26, 2011, 07:15:11 pm
content: how much power does all this draw ? if we are talking a few mA that's a supply that may not put out enough to blow the fuse. i think the idea proposed is designed for higher power stuff where a series diode could be impracticable and/or expensive and it could be quite large. this sort of circuit lends itself to using a lower speced diode as it only has to conduct for a fraction of a second

author: Zero999
author_rank: Super Contributor
timestamp: June 26, 2011, 07:42:39 pm
content: You could use a Schottky diode which will drop half the voltage of a silicon diode.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 27, 2011, 08:12:37 am
content: An electrolytic capacitor will have no problem with the 0.7 volt or so across a conducting diode.If the diode goes open before the fuse blows,placing full reverse volts on the supply line,what is the worst case scenario?(1)You cook an electrolytic worth $1?(2)You cook some of your active devices ?My feeling is,if the protection circuit fails,you have more problems than worrying about an electrolytic.Occasionally,manufacturers get it wrong & connect electrolytics in reverse,& this is only found out after many years of service.In these cases they are probably subject to considerably more than 0.7volts reverse.On the other hand,electrolyticswillblow up if reversed across power rails of similar voltage to their normal ratings.VK6ZGO

author: scrat
author_rank: Frequent Contributor
timestamp: June 27, 2011, 11:10:22 am
content: What about a series MOSFET and a pair of resistors/diodes for polarization?

author: 74HC04
author_rank: Contributor
timestamp: June 27, 2011, 12:44:29 pm
content: QuoteI like this idea. Wouldn't the body diode present in most (all?) MOSFETs cause an issue though?

author: Zero999
author_rank: Super Contributor
timestamp: June 27, 2011, 04:37:10 pm
content: Quote from: 74HC04 on June 27, 2011, 12:44:29 pmThe idea is to connect the MOSFET backwards so the body diode is forward biased and short circuited by the MOSFET.Be careful though, the maximum gate voltage is normally something like 10V to 20V so if the power supply exceeds this, you'll need a resistor and zener diode or a potential divider to limit the gate voltage and make sure the power supply voltage is high enough to fully turn on the MOSFET.

author: Simon
author_rank: Unknown
timestamp: June 27, 2011, 09:23:05 pm
content: Quote from: vk6zgo on June 27, 2011, 08:12:37 amyes I did this, nearly got acid in my eye

author: Jimmy
author_rank: Regular Contributor
timestamp: June 28, 2011, 02:04:44 am
content: opWhy not use a H bridge and automatically reverse the polarity instead of blowing a fuse

author: vk6zgo
author_rank: Super Contributor
timestamp: June 28, 2011, 04:28:05 am
content: You guys reallyareengineers,aren't you?Everybody & his cat have used the series fuse,shunt diode circuit for years,without any major dramas,but youhaveto design something complex to replace it, "just in case".VK6ZGO

author: scrat
author_rank: Frequent Contributor
timestamp: June 28, 2011, 12:09:43 pm
content: Quote from: vk6zgo on June 28, 2011, 04:28:05 amIn fact, after posting about the MOSFET, I asked myself if it was really appropriate. This isn't a new solution, just solves a slightly different problem.The MOSFET can completely save your circuit, while the "diode" relies on the fuse blowing.The fuse records the fact you reversed the polarity, the MOSFET doesn't.

author: Zero999
author_rank: Super Contributor
timestamp: June 28, 2011, 05:21:12 pm
content: The MOSFET circuit is more convenient because there are no fuses to replace when an idiot connects the supply voltage backwards.

author: Neilm
author_rank: Super Contributor
timestamp: June 28, 2011, 06:42:11 pm
content: Quote from: Hero999 on June 28, 2011, 05:21:12 pmyes, but if he is stupid enough to connect the supply backwards he should have to pay the stupid tax - in this case changing the fuse. With luck they won't do it again.Neil

author: vk6zgo
author_rank: Super Contributor
timestamp: June 29, 2011, 02:00:28 am
content: If you can stand the voltage drop, the series diode does the trick,or even better,a bridge rectifier.Of course,if the device is used in a car(CB,Ham radio,etc),& has one side of the chassis connected to (supposedly) earth/negative,& you wire it backwards,you can still short out the car battery via the mounting screws.VK6ZG0

author: ejeffrey
author_rank: Super Contributor
timestamp: June 29, 2011, 09:27:59 pm
content: Quote from: Neilm on June 28, 2011, 06:42:11 pmMeh. The idiot is probably me at a later date. And the real idiot was me not when I plugged it in wrong, but me when I built the thing such that it was easy to plug in wrong. I don't mind replacing the fuse when I screw up, but if there is a cheap and effective solution that prevents damage -- or better yet actually work either way -- I am all for it.

author: Simon
author_rank: Unknown
timestamp: June 30, 2011, 05:30:25 am
content: why not change the plug ?

author: Zero999
author_rank: Super Contributor
timestamp: June 30, 2011, 05:06:24 pm
content: Quote from: vk6zgo on June 29, 2011, 02:00:28 amGood point, you'd need to put the diode on both the negative and the positive to stop that.Quote from: Neilm on June 28, 2011, 06:42:11 pmYes, but if this is for a customer you'll have to put up with more crap from them if you tell them to replace the fuse every time they mess up.

author: onemilimeter
author_rank: Frequent Contributor
timestamp: June 30, 2011, 10:09:42 pm
content: If the electronic device requires +/-15V, can the circuit shown below be used for reverse polarity protection? I'm not sure about the negative supply part...Cheers

author: Time
author_rank: Frequent Contributor
timestamp: June 30, 2011, 10:47:44 pm
content: Why not just use a full diode bridge and it doesnt matter which way you hook up the polarity?

author: Zero999
author_rank: Super Contributor
timestamp: July 01, 2011, 04:00:32 pm
content: Quote from: Time on June 30, 2011, 10:47:44 pmYou might as well do that if you've got two voltage drops anyway.