Resistor power rating

[Toby]

I have just swapped out the electronic temperature controller in my mum's fridge, as the old one had failed, and was appearing to brown out when it tried to turn the compressor on, so it would reset its self, and then wait a few seconds, turn the compressor on in a continuous loop.

I now have the old module apart, and there is a 4K7 resistor that is measuring about 10 ohms (out of circuit). I expect this is part of the step-down voltage regulator arrangement, and is the cause of the problem.

It looks like a standard carbon film resistor, except it is much larger than a 1/4w one - probably about 8 x the size. Here is a link to some pictures of it, one comparing to to a standard

1/4w one

It measures 6.9mm wide by 13mm long

Any idea how I would find out the power rating of this so I can get a replacement?

Thanks!

[The Natural Philosopher]

that should be marked: Id guess at 5W or 10W from the size.

[meow2222]

Unusual for an R to lower in value. Just put something big in, if you cant calculate it from the circuit.

NT

[Theo Markettos]

You sure that isn't an inductor? It's a little unusual to have a light brown background, which is usually the reserve of resistors, but if it were an inductor that would be 4.7mH. Inductors are usually fatter than resistors and a 10 ohm series resistance wouldn't be that far off.

Is there any marking on the board that confirms it's a resistor (R-something not L-something)? Do you have any means of measuring inductance?

Theo

[Mike Tomlinson]

En el artículo , Andrew Gabriel escribió:

Looks like metal film to me; the squared-off ends and a slight spiral indentation along the body are the give-away.

Agreed that resistors will never reduce in value when they fail.

[Tim Watts]

4-5W I reckon. 10W are almost always white ceramic "boxes" or metal clad.

[Andrew Gabriel]

This is a possibility - there are inductors which look like 1/2W resistors, although I haven't seen one this size before - when they get that big, they're usually more obviously inductors.

[The Natural Philosopher]

4.7mh would be a thousand turns on a substantial ferrite pot core 4.7uH is more like that size.

[tony sayer]

In article , Andrew Gabriel scribeth thus

Just had a rummage thru the RS book of words .. online. Take Inductors leaded ones excluding surface mount, leaves 1200 odd, filter out any other than Radial 82 items, take a 8 to 12 ohm resistance range only brings up a few and nothing resembling that one in the pix. Not to say it doesn't exist of course but it does look like a ressy never seen inducts quite like that one.

Course if you were to take it to bits or just use a grinder to take some of the coating off you might be able to see copper wire or similar in there but I rather think that you won't somehow.

I have known resisters to go low in value but it's very, very, rare!...

Next time you present a pix like that put a ruler next to it so we can determine the size accurately.....

[Toby]

I have uploaded some more pictures now - showing it in the digital calliper ;)

I will take my meter over and measure the known good one this evening, but I cant take that out of circuit, as to remove the board from it's mount, the plastic weld mounts needs to be cut off :(

[Graham.]

If that is an inductor, the manufacturer gets full marks in the FUD department.

[Matthias Czech]

Am 25.06.2014 09:13, schrieb Andrew Gabriel: [..]

Don't know if its a resistor or inductor, but if its the latter, it might be one of these:

[Graham.]

That's pretty conclusive. 4,700 microhenrys, 10 Ohms DC resistance.

As there is bugger all else on the OPs board, it is almost certainly the triac that is faulty.

[cl]

Are you sure it's 4k7 and not something with (confusing) extra bands.

[tony sayer]

In article , Matthias Czech scribeth thus

Yes quite remarkable!, the last ones in that list do look identical and the resistance is right too!....

[whisky-dave]

They don;t normally mark colour coded resistors with wattage ratings. I'd say somewhere between 3-5 Watts. The DC caps are 25V & 16V. if you could trace where the resistor is in the circuit you'd get an idea of what current might flow through it. i,e 25/4700= 5ma so 25X.005= .13W could it be a crude dropper from mains 240V or 110V 240/4700 = .05 = 12W bit unlikely to have the whole 240V drop across it.

[The Natural Philosopher]

sheesh must be a lotta turns of very fine wire on that one!

[whisky-dave]

I'm now imagining loads of little vacumm cleaners with smiley faces......

[mick]

There's a lot of stuff on the back of the board, including a PIC16F76 microcontroller, crystal, loads of resistors & capacitors, several (probable) transistors, a hefty diode and a couple of pushbuttons. :)

[Theo Markettos]

Looking at the first pic of the board, I thought it was a dead cert to be a simple linear regulator: crappy single diode rectifier, smoothing cap, series resistor/inductor, 3 terminal regulator, relay coil.

But then turning it over it turns out it's covered in other things that change the game completely.

Moral of the story is, always post hi-res pics of both sides of the board...

Theo