What does this diode do and what do I replace with?

[Monkeyman87]

I recently attempted to power a stereo head unit from a car with a 12v battery pack(lipoly). I accidently connected the power leads in reverse, and fried a diode on the pcb between the power leads.

I'm just wondering what the purpose of the diode could be, as this is not powered by ac and can't be for rectifying the input. I have no clue what to replace it with as it has no markings. Thanks for your help.

 

[ljcox]

My guess is that it is to protect against exactly what you did, ie. reverse the battery.

However, I would have thought that it would blow the fuse and thus protect itself. Did the fuse blow?

In order to decide on a replacement I suggest that you measure the current that the unit takes and check what fuse is in the circuit.

If the fuse is say 5 amp, then you would need a diode able to withstand at least a 10 Amp surge.

Alternatively, install your own fuse specially for the unit. So if the unit needs 100 mA, then install say a 200 mA fuse. Then a 1 Amp diode such as the 1N4004 would suffice.

 

[The Mad Professor]

Got a make an model number there ?,
with that info somebody might be able to dredge up a schematic

Would there be a resistor in series with the positive power line/ pcb track before the offending diode perchance ? Could be you've fried a Zener diode that was supposed to regulate the amplifiers supply volatage.

 

[audioguru]

Sorry if it offends, but the diode is called an idiot diode.
You also forgot to use a fuse, didn't you?
The diode acted like a fuse and now many things are fried.

 

[Monkeyman87]

I'm not offended audioguru!! I did bypass the fuse. Hopefully nothing else was damaged. I want to try replacing it anyway. I had a HD get fried by a bad psu and I replaced a diode similiarly and it worked perfectly. I'm just hoping I have the same luck this time.

The HU is an Alpine CDM-7478, and I did look for schematics, but had no luck finding anything. I don't believe there is any resistor in series with the V+, it's hard to tell by the way the board looks, but there is nothing but SMD resistors in that area of the board. I don't believe they would be able to handle the power if they were.

It has a 15 amp fuse so I could replace with say a 20 Amp diode? Another thing, should I use a schottky diode? Its my understanding that they allow higher currents at lower voltages.

Thanks

 

[dknguyen]

Why on earth would you bypass a fuse?

 

[Monkeyman87]

Because I didn't have a way to easily connect the lipoly connector to the wiring harness plug which the fuse is connected to. Careless mistake. To tell you the truth all I wanted to do was get the cd out of the player. It's not really a big deal as this HU is pretty cheap and really almost obsolete. If I had done something like this to say a CDA-9855 I would be pretty upset.

 

[ljcox]

It has a 15 amp fuse so I could replace with say a 20 Amp diode? Another thing, should I use a schottky diode? Its my understanding that they allow higher currents at lower voltages.

Thanks

I don't know how much current is required to blow a 15 A fuse or how long it would take. So the diode has to be able to cope with a current spike of 12/R where R is the total resistance in the circuit and withstand that current for the time it takes for the fuse to rupture (probably 20 or so milliseconds as a guess)

There is no point in using a Schotty diode in this case.

However, you could insert a Schotty diode in SERIES with the supply in order to block the reverse voltage.

 

[zevon8]

Approximately 3 times, or 45 Amps will blow a typical ATO style fuse in under 1/2 a second. A short circuit ( when connected via the reverse diode ) will produce even more, very quickly, and will blow the fuse even faster. The diode only needs to survive this short rush, so check the datasheet of some smaller diodes, they don't need to be 20 Amp jobs.

 

[Nigel Goodwin]

Approximately 3 times, or 45 Amps will blow a typical ATO style fuse in under 1/2 a second. A short circuit ( when connected via the reverse diode ) will produce even more, very quickly, and will blow the fuse even faster. The diode only needs to survive this short rush, so check the datasheet of some smaller diodes, they don't need to be 20 Amp jobs.

And to be honest, it's rare for the diode to survive anyway, it's mostly there as a 'crowbar' circuit - it's designed to go S/C and ensure the fuse is blown.

 

[zevon8]

And to be honest, it's rare for the diode to survive anyway, it's mostly there as a 'crowbar' circuit - it's designed to go S/C and ensure the fuse is blown.

Exactly, it's a "sacrificial" part. Alot of the time, in automotive gear, it's just a 1N4004.