VHF reverse polarity diode

Waterman · 11 May 2011, 12:34

VHF keeps blowing fuses every time it's switched on. From research I suspect it may be the reverse polarity diode that has blown (which would mean the polarity was reversed at dome stage????). I've nothing to lose now so going to Open up the radio (cobra 55) and remove the diode.
Anyone ever try this???

ashbypower · 11 May 2011, 12:49

Quote:

Originally Posted by Waterman

VHF keeps blowing fuses every time it's switched on. From research I suspect it may be the reverse polarity diode that has blown (which would mean the polarity was reversed at dome stage????). I've nothing to lose now so going to Open up the radio (cobra 55) and remove the diode.
Anyone ever try this???

Doubt it very much, Normal failure mode on a Diode is to go open circuit!!
The system works by the Diode being normally reverse biased, when it sees reverse polarity the diode conducts thus blowing the fuse and not the radio. I assume polarity is correct.

Waterman · 11 May 2011, 14:51

Yes and by removing this now damaged diode I am hoping the radio will function again although I will now not have the benefit of this check valve fail safe and if polarity is reversed again the entire circuit will fry.....

I think its worth a shot, out of curiosity if nothing else, as otherwise it's heading for the bin.

ashbypower · 11 May 2011, 14:57

As I said the normal failure mode for FMECA purposes is that a diode fails OPEN !!! i.e blown !!!

jyasaki · 11 May 2011, 15:25

If you're going to open it up, do a bit more poking around than simply removing the diode that, as Ashby says, likely isn't the problem.

I'd suspect (though I've never investigated) that the radio has some voltage regulating circuitry just in from the power feed; that's a more likely culprit than the diode, which would simply block any voltage induced reverse current.

You may be able to find schematics on-line, if you're lucky.

jky

Waterman · 11 May 2011, 16:57

Found this
VHF blowing fuses... - The Hull Truth - Boating and Fishing Forum

bingomike · 11 May 2011, 21:40

Many years ago, I was involved in CB radios (when they were a bit illegal!!!)

Reverse polarity was a common problem..

Yes you are right, you should be able to remove the diode - hopefully it did its job and protected the radio - if correctly rated fuse all should be well!

Try and replace diode (easily got from Maplins or RS components) as without it there will be no protection..

Mike

jwalker · 11 May 2011, 21:58

I don't understand the hull truth thread... I suspect it's not the whole truth. Nor do I understand the comments in this thread re the diode. Normally a reverse polarity protection diode is simply in series with the power supply. Reversing the supply polarity should do no damage because the diode will not conduct in that direction. Diodes do not fail when reverse biased, that's their job, they behave as one way valves for electrical current.
I guess it's possible your radio is in some way unusual but I suspect a different fault if it keeps blowing fuses.

bingomike · 11 May 2011, 22:19

Quote:

Originally Posted by jwalker

Nor do I understand the comments in this thread re the diode. Normally a reverse polarity protection diode is simply in series with the power supply.

The diode is in parallel with the power supply.

When radio is connected properly current cannot flow through diode as it only allows power to flow one way.

If you connect the supply wrong way round, it allows the current to flow directly across the input causing the fuse and normally the diode to blow - normally quick enough to protect the equipment.

Although you may then connect power supply correctly, the fuse will continue to blow as the diode has "blown" and is allowing current to flow both ways.

Over 30 years or so ago, I used to make a fair bit of pocket money 'Fixing' broken CB radios that had been connected wrongly. (about 1 in 25 were damaged beyond repair, normally because the wrong in-line fuse had been used!)

Hope you can understand this explanation

Mike

Waterman · 11 May 2011, 22:42

Yes it took me a while to get my head around it too but bingomike has explained it very well. Cheers Mike.

Took the radio apart this evening and removed the diode this. I will try fitting it tomorrow and see how it goes.

As regards replacing it, I may look into it but reckon you would need to be fairly confident with your soldering to to it, it is in an awkward spot and tiny,

bingomike · 11 May 2011, 22:53

Make sure you have the correctly rated fuse, lower rather than higher! if you haven't the correct one!

Mike

PS let me know how you get on

jwalker · 12 May 2011, 11:06

I understood what you were saying but it makes no sense to do that unless that diode had another purpose or it's simply poor design. Also Diodes normally fail open circuit so it hints at some other device or purpose. The only reason I can think of for that system is if the charge voltage and the internal battery voltage can't tolerate the voltage drop across a series diode. Anyway good luck with it.

Just a thought, if your replacement diode has a higher current rating than the fuse you use then you'll have a better chance of not damaging the replacement.

bingomike · 12 May 2011, 11:22

Quote:

Originally Posted by jwalker

I understood what you were saying but it makes no sense to do that unless that diode had another purpose or it's simply poor design. Also Diodes normally fail open circuit so it hints at some other device or purpose. The only reason I can think of for that system is if the charge voltage and the internal battery voltage can't tolerate the voltage drop across a series diode. Anyway good luck with it.

Just a thought, if your replacement diode has a higher current rating than the fuse you use then you'll have a better chance of not damaging the replacement.

The only use the Diode has is to protect the radio from being connected the wrong way round.

The diode is wired so the flow of power goes from negative to positive, so under normal circumstances power can not flow as Diodes only allow power through one way.

When you connect the power the wrong way round, it allows the power to flow and will blow the fuse and normally the diode as it has quite a low current rating. (just like connecting a piece of wire straight across the terminals)

Replacing the diode with a higher rating will allow damage to your gear if wrongly connected in the future. You can in theory use the radio with no diode but risk blowing it up if you reverse the polarity sometime in the future!

Hope i have explained how it works in an easy to understand way.. Its probably 25 years since I have had to think about this subject!!

Mike

jyasaki · 12 May 2011, 16:18

Quote:

Originally Posted by bingomike

Replacing the diode with a higher rating will allow damage to your gear if wrongly connected in the future. You can in theory use the radio with no diode but risk blowing it up if you reverse the polarity sometime in the future!

No; a higher diode current rating will simply conduct more of the reverse current through the diode before it dies. Nothing should happen beyond the diode (no current drop to generate voltage.) In truth, you'll get the same amount of current as before, assuming you've got the proper fuse fitted.

jky

bingomike · 12 May 2011, 18:38

Quote:

Originally Posted by jyasaki

In truth, you'll get the same amount of current as before, assuming you've got the proper fuse fitted.

jky

Its the current you don't want going backwards through the equipment, so if you use a higher rated diode it will conduct more current before blowing the fuse therefore allowing more current to pass through the other components in the equipment.

Mike

Waterman · 12 May 2011, 19:25

Well removed it and it worked.

may look at replacing the diode now, any one want to point me to a particular one that I might use?? diode : Maplin Electronics

Managed to leave the two ends of the old diode on the board so hoping I can solder it direct to them.

bingomike · 12 May 2011, 21:15

Thats good.

Can you read the size of the original diode? If not I will check an old radio that i have got knocking around.

Mike

Waterman · 13 May 2011, 08:58

Cheers Mike,

Old diode broke up when I was removing it so cant use that as a ref.

JABS · 13 May 2011, 12:28

You could add a diode bridge. It wouldn't then matter if you did connect the radio the wrong way round. They are cheap!

jyasaki · 16 May 2011, 05:45

Quote:

Originally Posted by bingomike

Its the current you don't want going backwards through the equipment, so if you use a higher rated diode it will conduct more current before blowing the fuse therefore allowing more current to pass through the other components in the equipment.
Mike

Assuming a crowbar configuration (which I didn't early on in the thread), if the diode shunts enough current to blow the fuse on the input power, the diode's current rating doesn't matter, as long as it's enough to get the fuse to blow. If it's say, a one amp fuse, it doesn't matter if the diode is rated at 2 amps or a hundred: when it sucks more than an amp, the fuse blows.

jky