Faulty multimeter + can you identify this component inside?

[Agent24]

I have recently discovered that the mA range for my multimeter doesn't work, and before you ask: Yes I have checked the fuse and the fuse is fine

It has two settings, one for 200uA to 200mA and one for 10A (both DC)

Connecting up a circuit which should be drawing 100mA I get a 0.10 reading on the 10A DC range which looks all OK.

However on the 200mA range where I should get 100 I get nothing.


It's not just the meter isn't reading the current, the current doesn't appear to be flowing at all. A simple circuit with an LED lights up on the working 10A range but on the other ranges the LED doesn't light and the display shows nothing.


When I opened it up I noticed a strange component I don't recognise. It's soldered where the silkscreen shows a resistor symbol but I don't think it's a resistor.

I have attached photos... Its the thing marked "75S" which looks like a cross between a Tantalum and a Disc Ceramic

[stj]

Re: Faulty multimeter + can you identify this component inside?

that looks like a self-resetting polyfuse.

[Agent24]

Re: Faulty multimeter + can you identify this component inside?

Interesting, I had not thought of a polyfuse. But I thought polyfuses looked like Disc Ceramics rather than this one? Besides, the meter is quite cheap and uses a standard fuse already.

In any case I think I have found out why the meter isn't working. When I say I 'checked the fuse' I didn't actually test it, just looked at it.

Turns out that while the fuse wire appears intact, the fuse is actually open circuit. Maybe it broke when the meter was dropped? (happened a few times)

[smason]

Re: Faulty multimeter + can you identify this component inside?

I learned a long time ago not to trust a visual check of a fuse.

I won a $40.00 bet with a friend who insisted a standard glass fuse was fine. I tested with a meter, and it was open. If you really looked hard, you could see under the metal end cap ot was blown.

[Agent24]

Re: Faulty multimeter + can you identify this component inside?

I learned a long time ago not to trust a visual check of a fuse.

I guess I learned that just now But I'm kicking myself because I know better and I know that I should have checked it properly anyway

Open circuit rectifiers and switching transistors look fine from the outside too...

[Krankshaft]

Re: Faulty multimeter + can you identify this component inside?

The filament probably broke where it connects to one of the end tubes.

I once had a fuse that still passed current but was blown the filament vaporized but the vaporized metal still made contact across the end caps.

It was a slow blow so I guess it could have also vaporized along the ceramic center insulator slow blow filaments are wound around.

The fuse looked black and silver inside.

I caused the blown fuse by accidentally shorting VCC and ground. It was a cheap Kinyo 5.1 sound system they actually used a bunch of diodes in forward configuration to step down the voltage to the amp ICs. I jumped one of these diodes during testing.

The result was low supply voltage from the transformer since it couldn't pass full current the voltage dropped.

It caused hours of headbanging for me since the fuse was soldered inline with a transformer secondary lead and completely heat shrunk over. I thought there was no fuse the amp was so cheap. Luckily eventually I found it.

It's funny how one of the simplest of electronic components can cause so much trouble.

[stj]

Re: Faulty multimeter + can you identify this component inside?

it should have been a sand-filled ceramic fuse in a meter btw.
cheap bastards!

[Agent24]

Re: Faulty multimeter + can you identify this component inside?

it should have been a sand-filled ceramic fuse in a meter btw.
cheap bastards!

It's a cheap meter, only $20 from DSE...

What is the advantage of a ceramic type? Should I get a ceramic one for the replacement?

[stj]

Re: Faulty multimeter + can you identify this component inside?

ceramic fuse wont explode into a mess of glass bits if something goes very bad!