# TV electrics - blown Diode?



## PSC (May 7, 2009)

I (stupidly) have tried to reset the sky box in my Ducato by unplugging/replugging it and in doing so touched the power lead to the body of the box creating an arc/short circuit. Both sky boxes (UK and Europe) and the Sat Dish controller box all promtly died. 8O 8O I've pulled panels out of the backs of cupboards looking for in-line fuses and can't find any, I've checked all of the fuses on the dash fuse panel and separately the MH fuse panel - all appear to be intact and the only thing I can find on the power cable are a switch (not much to blow in that) and a diode. I've looked it up and it's a Ripca/Ripaults 2842D diode. Now this somewhat taxes my (30 years ago) physics O level knowledge so a quick read of wikki indicates that the diode ensures no reverse polarity or something and that they can fail if there is a massive reverse voltage jolt - all of this is double dutch to me...

Do diodes blow?

If so, where can I get another one? A quick look at the manufactures web site and the minumum number they sell is 10! 

Do any electric whizz's out there know if (a) you can actually blow a diode by arcing the power cable on a 12v supply? (b) whether this would be the only thing protecting the circuit to the sky boxes? (c) where can I get a new part...?

Cheers all............


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## teemyob (Nov 22, 2005)

I have Diodes, if you want some, send me a PM with your address and I will pop you some in the post.

TM


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## philoaks (Sep 2, 2008)

PSC said:


> I (stupidly) have tried to reset the sky box in my Ducato by unplugging/replugging it and in doing so touched the power lead to the body of the box creating an arc/short circuit. Both sky boxes (UK and Europe) and the Sat Dish controller box all promtly died. 8O 8O I've pulled panels out of the backs of cupboards looking for in-line fuses and can't find any, I've checked all of the fuses on the dash fuse panel and separately the MH fuse panel - all appear to be intact and the only thing I can find on the power cable are a switch (not much to blow in that) and a diode. I've looked it up and it's a Ripca/Ripaults 2842D diode. Now this somewhat taxes my (30 years ago) physics O level knowledge so a quick read of wikki indicates that the diode ensures no reverse polarity or something and that they can fail if there is a massive reverse voltage jolt - all of this is double dutch to me...
> 
> Do diodes blow?
> 
> ...


If more than one appliance went off when it happened then I would suspect a supply fuse has gone rather than a diode to protect against reverse polarity.

What you really need is a meter (or a mate with a meter) to work your way through the circuit and identify the fault.

Edit: just re-read your post. Does the "power cable" with the switch feed all three items that you mention?

Meant to add that the purpose of a reverse polarity diode is to provide a virtual short circuit should the wiring be accidentally reversed. Normally that short circuit would cause a fuse to blow rather than the diode itself.


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## Codfinger (Jan 11, 2008)

*Blown fuse*

I did exactly the same thing when I did mine nearly tore my hair out but eventually found a blown fuse in the skybox power lead which whoever had fitted the box had hidden behind the false rear of the cupboard where the Skybox is located, have a good look at all your wiring I bet some genius has hidden a fuse somewhere, are you sure you have checked all your fuses in your fuse panel? if in doubt put a meter across them to check continuity, its easy on our Rapido as a blown fuse lights up in the fuse panel ..........bloody good idea :idea: 
Chris


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## philoaks (Sep 2, 2008)

Looks like the component you've found is a relay with diode protection as opposed to just a diode http://www.ripca.com/catalogus.php?toon=product&welke=755&template=-1


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## PSC (May 7, 2009)

we've got blown fuse lights too, but non appear to be lit. Good idea to check the fuses with a meter though. I've got a meter but hadn't thought of digging it out of the shed to check the circuits properly - was doing it all by sight and logic!  I will have better hunt back along the cables to see if there is an inline fuse, but suspect that using a meter might be the best way of fault finding! I had electricity - gas and water are fine as you can smell/see them! 

Thanks for the all the replies so far...... TM, I might take you up on the offer, but let me get more techical first! Sadly, this might be next weekend.....


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## PSC (May 7, 2009)

Philoaks - yes that's exactly the unit I've found.... so what does it do? I'm a complete numpty when it comes to wiggly amps!


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## Mouse44 (Aug 23, 2010)

Hi

I believe diodes are like a non return valve,the current can only flow one way,so if you reverse the polarity it blows protecting the circuit beyond that point ,so replace it and it should be ok. 

PS always switch of sky boxes when removing cables or working on it.

Hope this helps


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## philoaks (Sep 2, 2008)

PSC said:


> Philoaks - yes that's exactly the unit I've found.... so what does it do? I'm a complete numpty when it comes to wiggly amps!


The relay will (I assume) just be an electrically operated switch. Often used for a high current load being controlled by a low current switching circuit. I think the diode would just be for reverse protection as mentioned earlier. If the diode blows and goes "open circuit" then it would have no affect on the setup other than the fact that you lose the reverse polarity protection.

If it blows and goes "closed circuit" then everytime you replaced the fuse it would keep blowing.


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## clodhopper2006 (Aug 13, 2006)

The diode does not protect against wrong polarity. It's all to do with a thing called back EMF. It's basically a big voltage that develops the instant the relay coil is de-energised. The diode allows this voltage to disipate safely and quickly.
The fault you have with both pieces of equipment has to be fuse somewhere and as has been said, you really need a meter.
Without seeing your circuit though I cant say what the purpose of the relay is.


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## Codfinger (Jan 11, 2008)

*Blown fuse*

Correct me if I'm wrong but I've cooked diode's on stuff before and the result as I recall was that as soon as I conected the power the fuse blew as when a diode is powered up the wrong way it blows and goes to earth?
PSC check you have power to that relay, it should have a fused supply from the m/h fusebox before you go any further and start ordering new relays, some leccy whizz on here should be able to tell you which poles to bridge to by-pass the relay to confirm relay pooped.
Chris


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## PSC (May 7, 2009)

thanks again folks.. I think the next step is defo to dig my meter out


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## bigfrank3 (Mar 26, 2007)

When a diode blows it can go 'open' or 'closed' circuit. If it is open then no current (or voltage ) will flow in the circuit. If it is closed then the circuit will pass current and voltage, but where as when the diode was good it would only pass electricity in one direction. 

I know we are talking about DC and electricity will only flow in one direction but as previously said it can prevent back emf and and also wrong polarity connection. 

To check the diode with your test meter connect it across the diode you should get a reading one direction and not get one if you connect it with the leads in the oposite wires.

Keep testing, bigfrank3


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## clodhopper2006 (Aug 13, 2006)

bigfrank3 said:


> To check the diode with your test meter connect it across the diode you should get a reading one direction and not get one if you connect it with the leads in the oposite wires.
> 
> Keep testing, bigfrank3


No, the diode is connected directly across the coil. Test one way and it will be a short, test the other way and you'll get the resistance of the coil.


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## PSC (May 7, 2009)

update.....

the unit previously described in the OP is more of a switch than a simple diode. It has a diode in it to control the operation of the switch. I've now tested the unit with a meter and it's not working.

In truth I am not 100% sure what the damn thing does, and when I take it out of the circuit all together my sat boxes all light up, alive and well! I think it is to allow the switching from one cupboard (where my boxes are currently located), to another location on the van where the TV/Sat boxes etc may have been originally installed. I can't trace the other part of the circuit as I CBA to pull every panel off..... so am going to rely on the well tried and tested principles of "ignorance is bliss" and "if it ain't broke, don't fix it"!!

Having proved that the switch has failed I am going to source a replacement/alternative and maintain the circuit as it was.

Lessons learnt - buy a meter! It's been great fun crawling all over the bus testing fuses and circuits with the one I borrowed and I've found a few other problems which I've managed to fix too! 

Thanks everyone for your help.

On on!


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## CliveMott (Mar 10, 2008)

There are several safety applications of diodes which are simple electrical one way valves.
.
In lots of equipment a diode is connected directly across the incomming supply after the in-line fuse. In normal use the diode does nothing, however if some plonker connects the DC supply backwards the "wally diode" will appear as a short circuit causing the fuse to blow and so protect the rest of the circuit from reverse DC polarity. If the diode fails short circuit then thats fine although the diode will require replacing. If however the diode blows open circuit because someone has fitted a bigger fuse then the reverse voltage will be applied to the electronics of the unit and its probably a bin job.
OR
The diode connected across the relay coil is provided as a suppression component to protect whatever is controlling the relay coil from a high voltage spike when the relay is de-energised. In this application its referred to as a "Free Wheel Diode"
OR 
In military equipment a diode is connected in series with a relay coil. When the DC supply is of the correct polarity the diode will conduct causing the relay to energise. The contacts of the relay will then connect the DC supply to the equipment. Get this back to front and no problems and no blown fuses. But it costs a hunky relay.

C.


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## PSC (May 7, 2009)

thanks - I think its the second of the 3 you describe..... that would make sense anyway.

Notwithstanding that I got my kit to fire up again by by-passing the unit, I wasn't planning on leaving it permanently connected. Clearly it was there doing something and will therefore be replaced. 

Great explanations though.... I love the military option. In otherwords, it makes sensitive electronics "soldier proof"!!!


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## PSC (May 7, 2009)

latest... as the unit being discussed is a relay of some form or other, and as I can't work out why it is there... I've simplified the circuit and am back up and running. Odd that the relay seemed to control the power into 1 of 2 cables. One powers up all my sat controllers, the other disappears behind the panelling and doesn't reappear anywhere so I've taken the relay out therefore rendering the second circuit redundant. I suspect it is a secondary feed to another part of the bus in the event that I want to use an alternative TV mounting point. But as I can't be bothered to pull every panel out I can't work out where the second feed point is. Anyway, it's academic for now as there is no power going to it.

The "new/modified" system is protected by the fuse box and I've left an inline power switch so I can issolate the sat control boxes in the future to prevent this mysterious problem .

TV/Sat system tested and working!  Another job done.


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