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9v Protection Diodes?

Hi Everyone,

I have a friend's DMM that wouldn't power up. I opened the case and found that there was a power to ground short. The short is being caused by a SMT diode that is across the 9v battery wires on the PCB. I lifted one side of the diode and the diode is shorted and burned pretty badly. I powered up the DMM briefly and everything seems to be working ok. I want to know if anyone has any suggestions on what type of diode should I use to replace the shorted diode. It doesn't have to be a SMT diode. I am guessing that is a protection diode and not a zener since most of the ICs run off of 9v according to their datasheets. Any help would be greatly appreciated. Thanks in advance.

Jason

Reply to
me
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Hi Jason...

Can't think of any reason you'd need one at all, unless you plan on somehow installing the battery backwards :)

Take care.

Ken

Reply to
Ken Weitzel

Ken Weitzel wrote in news:DhRBg.332437$Mn5.252184@pd7tw3no:

Sounds like someone momentarily touched the battery terminals backward and the diode forward-conducted and blew out. Conclusion: a surface- mount diode doesn't work for backwards-connection protection, it just self-destructs. Unsolder the diode and leave it out of the circuit.

Reply to
Jim Land

Most reverse protection diodes do fail short-circuit. It's unusual to find one that survives when it does its basic job. The battery must have been connected backwards for the original to have failed, and as the rest of the meter seems to work ok, then IMHO, it has done the job it was designed-in for. For the minimal cost, and the protection afforded, I really can't see any reason NOT to replace it ... ?? Any old diode will do from a 1N4148 up.

Arfa

Reply to
Arfa Daily

I'd look for a convenient point on the PCB to solder in something more substantial like a 1N4001 or similar. Across the solder pads where the battery leads are soldered to the PCB would be fine, just make sure you get it round the right way, ie the white band to the positive terminal.

If your friend's a bit accident prone or absent minded I certainly wouldn't give him it back with no diode- the first time he touches the battery to the terminals the wrong way round he'll have an ex-DMM!

Dave

Reply to
Morse

Eh? The usual way is a series diode and that won't fail if the battery is reverse connected. Be a bit silly if it did...

--
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    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.
Reply to
Dave Plowman (News)

But not in this case as the OP said it is in parallel. I've seen loads of battery equipment with parallel reverse polarity protection diodes. I think the designers baulk at the losses involved in using a series diode, small though they may be in the case of a DVM.

Dave

Reply to
Morse

the

up.

I've asked this before here , but again. Does anyone know of series of diodes that are guaranteed to go open circuit on excess forward current or guaranteed go short circuit on reverse voltage overload.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Reply to
N Cook

Sorry to disagree with you on this one Dave, but that's not the case. All car radios have a diode connected in shunt with the supply. When you connect the radio backwards, the diode conducts heavily and blows the fuse. Usually, the diode itself fails also. The same applies to CB radios, ham radios and PMR radios. Likewise, a couple of pieces of portable audio equipment whose schematics I just pulled at random, also have a shunt diode. A series diode is seldom found because apart from anything else, it would have to be rated to carry the maximum continuous current of the equipment, and would also cause a voltage drop, which is wasteful of power, and not desirable for battery powered equipment. In any case, the principles are not in question here. This meter *did have* a shunt protection diode, which had failed as I would expect it to. The OP said so.

Arfa

Reply to
Arfa Daily

The forward drop may be enough to keep the meter from working.

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Michael A. Terrell
Central Florida
Reply to
Michael A. Terrell

What the OP did not mention was whether or not the DVM could also run off of a wall wart -- or, at least, make use of a wall wart-style charger. *That* could well be why the diode was toasted....

Jonesy

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Reply to
Allodoxaphobia

I'd agree with that if a high power consumer, but on something like a DVM?

Indeed. But only of use if also protected by a fuse?

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    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.
Reply to
Dave Plowman (News)

Thanks to everyone for their insights. My friend told me that he just turned on the DMM and it started smoking. The 1 amp fuse for current measurement was blown as well. Don't know if the blown fuse is related to the shorted diode. It is only powered by a 9v battery, no AC jack on the meter. It is a Mac Tools EM720 DMM. I will solder in a replacement diode. Thanks again, everyone.

Reply to
me

Hi Me...

Something a little wrong with that; can't imagine how you could get anything to smoke using a 9 volt battery (unless for some reason he was using an NiMh in it - even then not much)

Betcha a dollar to a doughnut he did the same thing that I did when I was a kid. Tried to measure the resistance of the AC line :)

Or, perhaps how much current was available from the AC line :)

Take care.

Ken

Reply to
Ken Weitzel

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