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Old 11 May 2011, 18:53   #11
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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
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Old 12 May 2011, 07:06   #12
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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.
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Old 12 May 2011, 07:22   #13
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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
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Old 12 May 2011, 12:18   #14
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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
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Old 12 May 2011, 14:38   #15
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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
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Old 12 May 2011, 15:25   #16
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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.
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Old 12 May 2011, 17:15   #17
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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
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Old 13 May 2011, 04:58   #18
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Cheers Mike,

Old diode broke up when I was removing it so cant use that as a ref.
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Old 13 May 2011, 08:28   #19
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You could add a diode bridge. It wouldn't then matter if you did connect the radio the wrong way round. They are cheap!
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Old 16 May 2011, 01:45   #20
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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
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