Can anyone identify this resistor?

elcom ST6500) which is intermittently faulty. I thought the fault might be with the lead-acid backup battery as the old one was almost defunct, but w ith a new battery the fault is still there.

oks like a resistor has badly overheated. On removing it the legend on the PCB underneath looks like "l4.7R" but the dot and the R are indistinct. I s it a 14.7 ohm resistor? It might well be in the battery-charging circuit . At present its measured resistance is around 10k but it may well have bu rned out too much for this to be reliable.

A lot of old gizmos with small lead acid batteries just used a resistor to control charging. Basic battery data would also help.

NT

The battery is 12v 2.1 Ah but I don't have the circuit diagram (how many alarms come with them?). It's the spiral pattern on the resistor body which I didn't recognise, and the lack of colour coding doesn't help at all. I have a resistor in my junk collection of about 18 ohms, I might try that.

I suspect the battery had some cells fail and if this resistor was in series with the charging circuit it got hot and failed, so the battery then went totally dud. But that's only a guess.

Thanks for your help.

There doesn't seem to be anything visible on the device to indicate its value. The spiral is to increase the effective length of the resistive material (carbon film perhaps).

I think you will have to analyse the circuit to decide the correct value. Could the legend on the PCB be 14.7K? That would be closer to the measured value. It's possible that it wasn't the resistor's "fault" that it overheated, so there could be something else wrong.

Yes you do have the circuit diagram, it's just not on a bit of paper and takes a little more effort to interpet.

Spiral pattern is typical of high wattage resistors that have burn off their paint and colour bands.

Let's say it's a 1W resistor (though it's probably a 3W or a 5W): If the value was 10K, to develop 1W in the resistor we'd need: P=V^2/R sqrt(PR)=V sqrt(1*10000) = V = 100V across it

So the question is: where does this sit in the circuit?

If there's a transformer and it's on the secondary side, it would seem very unlikely it's ballpark 10K, because they could have used a much lower wattage resistor for the voltage that might be across it. If you put 12V across a 10K resistor it would only take 14mW.

If it's on the primary side, it's possibly an inefficient circuit that might have roughly mains voltage across it for some reason, and 10-20K wouldn't be impossible.

If it's a series resistor then it's much more likely to be a few ohms, with a small voltage drop across it (either primary or secondary side).

Theo

My guess is that may be a 4.7ohm resistor. Google image search for 4.7 ohm wire wound resistor shows at least one item that looks similar - without the burned-off coating of course.

Nick

all resistor values look similar! The spiral track is cut in production to raise the resistance value.

These resistors should be replaced with something of higher power rating, t hey can be a bit of a fire risk otherwise.

NT

It's a metal oxide power resistor. Metal oxide is deposited on the surface of a ceramic cylinder (or tube), and a spiral is cut in it to set the resistance to the required value.

Have you measured the resistance of it? There might be nothing wrong with it, even though the covering has fallen off. These are quite robust.

My guess would be it's a crude current limiter for charging the battery.

(Melcom ST6500) which is intermittently faulty. I thought the fault might be with the lead-acid backup battery as the old one was almost defunct, but with a new battery the fault is still there.

looks like a resistor has badly overheated. On removing it the legend on t he PCB underneath looks like "l4.7R" but the dot and the R are indistinct. Is it a 14.7 ohm resistor? It might well be in the battery-charging circu it. At present its measured resistance is around 10k but it may well have burned out too much for this to be reliable.

carbon film's cheaper

NT

They have a pretty colour strips on them though to indicate their resistanc e, which isn't much use of course when the colours have been burnt off. :-)

Might be the ealier verion of one of these.

I think it's likely to be 4.7 ohms if it were a larger battery it could be

From the satate of it it must have been overrarted for quite some time perh aps the battery shorted or soemthing but even then 12V across 4.7 is 2.5 a mps.

giving 2.5x12 = 30 watts that would be enough to a 3W resistor to make it look like your picture

.7 ohms. However I'd look at the circuit to see if that makes sense, or whe ther it could be 47R. If the PCB marking is definitely 4.7R rather than 47R , then 4.7 ohm it is.

they can be a bit of a fire risk otherwise.

If there's space for it, you could have it raised above the board as the le nght of the one show is only 11mm the one I've shown is 16mm so it wouldn't sit well on the board.

That is not a 1/4W resistor

Looks like a thin film resistor that has been trimmed to value. If it is then it may be a high tolerance resistor.

Does it actually read anything across a multimeter?

Is it in fact an inductor? Brian

Not intentionally.

That?s not a 1/4W resistor.

Looks bloody like one to me. Mind you the photo has no scale.

Mind you the photo has no scale.

The wire is good enough for that.

That's what a thin film resistor looks like under the paint. It's like a wire-wound resistor but it uses a ribbon of conductor instead of a round wire.