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blown diode?

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stevel

New Member
Hi all,

I have a piece of networking equipment that is designed to take 12vdc but we accidentally connected 24vdc. It looks like a diode on the input (located right next to the power connector) has blown and I was wondering if this is something that I could replace. Not sure if anything else has blown but this is the only component that shows visible signs of damage.

The diode has "RM" and "R807" on the case if this means anything to anybody.

Thanks in advance for any suggestions.

Steve.
 

Sceadwian

Banned
Could be a zener diode, can you take a good well focused pic of the board?
 

tunedwolf

Well-Known Member
More likely that it's a fast avalanche diode with a breakover voltage just higher than the normal expected supply, designed as a crowbar of sorts.

Sceadwian is right, take a close up photo of the board and post it.

rgds
 

unclejed613

Well-Known Member
Most Helpful Member
that sounds like an FR807. it's a 35ns 3 amp rectifier IIRC (i know it's a 3 amp device, but i forget the actual recovery time). the 7 at the end means it's reverse voltage is 1kV. it's common practice to put a high current diode across the power connector to prevent reverse polarity wall warts from damaging the unit. replace it with a similar diode (it can be an FR801,802,803, etc...), and double check your wall wart polarity, and use the proper voltage too.
the use of the diode protects the rest of the circuits by beginning to conduct (and then going dead short) in less time than it takes for the filter caps at the input of the regulators to begin charging (after they get a few volts reverse biased, the caps would begin conducting too, since they are polarized as well, but the caps would begin spitting smoke if that happened), limiting the damage to the diode only.
your device probably has internal regulators which will quickly overheat if run on too high a voltage. there may also be other devices that run directly off of the 12V input which will die instantly if you apply 24V. 99% of all data equipment run from a wall wart are center positive polarity. the ploarity of the plug is usually (but not always) printed next to the connector you will see what looks like a C with a dot or small circle in the middle. the C and the dot are connected by lines to small circles, one with a minus and one with a plus. the C is the outer shell of the connector, and the dot is the center pin. a center positive connector would be shown with the minus sign connected to the C, and the plus symbol connected to the dot. the same symbol is usually shown on the nameplate of the wall wart, or it will say "center positive" or "center negative". wall warts older than about 15 years may or may not have some indication of polarity, making identification of polarity a job requiring a meter or a device which had a diode in series with a lamp (or an LED and dropping resistor).
 
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