Will this reverse polarity protection circuit cause a problem?

[onemilimeter]

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

[PeterG]

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

[Simon]

yes the fuse will blow well before the cap goes, be sure to use a decent capacitor though

[onemilimeter]

Most electrolytic capacitors are polarized and require one of the electrodes to be positive relative to the other; they may catastrophically fail if voltage is reversed. This is because a reverse-bias voltage above 1 to 1.5 V[2][3][4] will destroy the center layer of dielectric material via electrochemical reduction.

[Source: http://en.wikipedia.org/wiki/Electrolytic_capacitor]

This makes me worried...

[Simon]

why not just put the diode in series ? what in parallel ?

[PeterG]

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

[alm]

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).

[Zero999]

It's a common way of doing it, just make sure the diode can handle the brief short circuit current.

[Simon]

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

[onemilimeter]

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

The 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.

[tecman]

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

[ejeffrey]

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?

It 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.

[Simon]

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

[Zero999]

You could use a Schottky diode which will drop half the voltage of a silicon diode.

[vk6zgo]

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,electrolytics will blow up if reversed across  power rails of similar voltage to their normal ratings.

VK6ZGO

[scrat]

What about a series MOSFET and a pair of resistors/diodes for polarization?

[74HC04]

What about a series MOSFET and a pair of resistors/diodes for polarization?

I like this idea. Wouldn't the body diode present in most (all?) MOSFETs cause an issue though?

[Zero999]

What about a series MOSFET and a pair of resistors/diodes for polarization?

I like this idea. Wouldn't the body diode present in most (all?) MOSFETs cause an issue though?

The 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.

[Simon]

On the other hand,electrolytics will blow up if reversed across  power rails of similar voltage to their normal ratings.

VK6ZGO

yes I did this, nearly got acid in my eye

[Jimmy]

op

Why not use a H bridge and automatically reverse the polarity instead of blowing a fuse

[vk6zgo]

You guys really are engineers,aren't you?
Everybody & his cat have used the series fuse,shunt diode circuit for years,without any major dramas,but you have to design something complex to replace it,  "just in case". 

VK6ZGO

[scrat]

You guys really are engineers,aren't you?
Everybody & his cat have used the series fuse,shunt diode circuit for years,without any major dramas,but you have to design something complex to replace it,  "just in case". 

VK6ZGO

In 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.

[Zero999]

The MOSFET circuit is more convenient because there are no fuses to replace when an idiot connects the supply voltage backwards.

[Neilm]

The MOSFET circuit is more convenient because there are no fuses to replace when an idiot connects the supply voltage backwards.

yes, 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

[vk6zgo]

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