Replacing blown diode 12v FM radio

Thread Starter

billys1337

Joined Jul 15, 2021
2
Hey all,
Trying to repair a 12v FM radio that has been wired in reverse polarity and caused the reverse polarity diode to go short circuit.
I removed the diode from the PCB and the radio works fine without the diode, but I wish to replace it anyway.
The problem is I am unsure of what type of diode or what rating etc I should purchase (cannot see any ratings on diode or PCB), other than it needs to be a PCB surface mount type.
I know what a diode does but not a whole lot else, do Ineed a Schottky, a Zener, something else...????
rs-components has about 8000 different types of diodes.

I have attached some photos.
The radio is a (GME gr300bt) .... according to the spec sheet its "10.8 – 15.6 V DC negative earth", and has a "6 amp 3 AG fuse".

Any assistance would be much appreciated!

Cheers
Bill
 

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Irving

Joined Jan 30, 2016
2,311
Hmmm... normally the diode is meant to blow the fuse, not itself... assuming it didn't blow the fuse as well... errr... was there a fuse fitted as described in the manual (and ringed in red below)?

1626355498274.png

Its impossible to say what diode that was. You could try asking the manufacturer.

Failing that, measure the dimensions of the dead one to determine case size. Its almost certainly a TVS diode (transient voltage suppressor) probably rated at 18 or 20v breakdown. RS have potentially suitable 20v parts Hitachi DAM3MA27 or Wurth Elektronik 824550121 subject to case dimensions.

You might verify that the bar end of the diode is connected to the +ve battery lead and the other end to the -ve lead.
 
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Fuses can be bypassed. A unidirectional TVS makes a lot of sense. The automotive electrical system has -200V and +50V transients and you sometimes want to protect against 24V and 12 V systems in parallel.
 

Thread Starter

billys1337

Joined Jul 15, 2021
2
Thank you all for your replies, all very helpful.

Hmmm... normally the diode is meant to blow the fuse, not itself... assuming it didn't blow the fuse as well... errr... was there a fuse fitted as described in the manual (and ringed in red below)?
I forgot to mention that the fuse did blow and the original owner replaced it with a fuse that was twice the current rating as the original, tried again with the same result, this is obviously when the diode was fried.
 
Always remember that the transistor blows first to protect the fuse. If your APC, the thermal fuse blows to protect the suge suppression part of the surge protector and lets the surge go to the equipment. Well, it does turn on or off a LED when the protection fails, but you can't see the LED under a desk.
 

Irving

Joined Jan 30, 2016
2,311
Always remember that the transistor blows first to protect the fuse. If your APC, the thermal fuse blows to protect the suge suppression part of the surge protector and lets the surge go to the equipment. Well, it does turn on or off a LED when the protection fails, but you can't see the LED under a desk.
Had to read that twice.... lol :)
 
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