Value of a large SMD resistor marked R050 ?

[Jim Hayes]

Apologies for the stupid question, but having had a good look around I can't satisfy myself that I know the value in ohms of this resistor.

The background : I stupidly connected a new 12volt water pump with reversed polarity, which resulted in a nasty popping and frying noise :(

Damage seems limited to a can capacitor, a small smd capacitor and a large smd resistor which broke in half. It's about 6mmx3mm in size and marked R050 as far as I can tell from the 2 scorched bits of it I've got!

The caps I can replace (I'm unsure of the value of the 805 one, but if I replace with one the same size it will probably work I hope, unless there is a way of measuring it's brothers on the board?).

The resistor I can't decide if it's 0.05 ohm or 5 megaOhm, which are both values I've seen for similar sized resistors for sale with that marking.

It's near the blown can capacitor, fairly close to the 12v power in leads, I'm guessing it might be a power sensing part bit I can't be at all sure.

The pump cost me fifty quid and I can't really return it, so I'm willing to try replacing those parts and seeing if it will still work (I can't see any other signs of damage, but that's not to say that there are not other problems with it now).

5 mOhm I probably have, although in through hole (but I can probably attach that to the smd pads easily enough).

Any thoughts? Many thanks in advance.

Note to self - make sure the leads on your multimeter are plugged in the correct way when checking polarity :(

[Mike Harrison]

On Sun, 25 Aug 2013 12:30:27 +0100, you wrote:

>Apologies for the stupid question, but having had a good look around I
>can't satisfy myself that I know the value in ohms of this resistor.
>
>The background : I stupidly connected a new 12volt water pump with reversed
>polarity, which resulted in a nasty popping and frying noise :(
>
>Damage seems limited to a can capacitor, a small smd capacitor and a large
>smd resistor which broke in half. It's about 6mmx3mm in size and marked
>R050 as far as I can tell from the 2 scorched bits of it I've got!

0.05 ohms

[Jim Hayes]

Thanks Mike! Much appreciated.

I'm embarrassed on 2 counts : Having to ask and having got myself in this position in the first place :(

off to ebay it is then... 

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[Nick Johnson]

Definitely 0.05 ohms.

--

[Nigel Worsley]

That 0.05 ohm resistor will connect to a chip that is highly likely to
now be deceased. Just because it didn't physically burn doesn't mean
that it will work, although power supply chips usually do fail rather
visibly. Curious as to why a pump has an internal switching power
supply!

[Nick Johnson]

What makes you think it has a switching supply? The 0.05 ohm shunt just implies it has current monitoring/limiting.

[Nigel Worsley]

> What makes you think it has a switching supply? The 0.05 ohm shunt just
> implies it has current monitoring/limiting.

True, but I can't think of why a pump would need to do this except as
part of a switching supply. Maybe I am over-extrapolating :)

Nigle

[Nick Johnson]

Possibly to detect stalls? At any rate, current monitoring isn't mandatory for switching supplies either. :)

[Jim Hayes]

I'm not confident, but it's going to cost me about 3 quid and an hours work to replace the obviously burned components, so I'll probably give it a go.

I don't really expect it to work to be honest.

It's a magnetic drive kind of pump, by the way. It has static coils and a switching pcb mounted on the outside of the impeller, a bit like an RC brushless outrunner(?) motor.

Anyway, I messed it up by being an idiot, thanks for the help with the resistor value and the advice that it's probably burned out elsewhere!

[Jim Hayes]

To my surprise (and pleasure) having replaced those components it now works!

Many thanks for the help,

Jim.