multimeter suggestions

[The Other Mike]

Or just get a clamp ammeter, although they are silly prices unless you have an AC battery :)

[Ron Lowe]

My old Fluke 87 chirps loudly and frantically if you select a voltage range whist the probe is connected to the current jack.

[Dave Plowman (News)]

Clamp meters are useless for small DC currents.

[dennis]

Not if they have a hall effect sensor in them, and you calibrate it properly.

[Dave Plowman (News)]

So tell us the price of such a unit and the method for accurate calibration?

[dennis]

Well I can't be bothered to do an extensive search so how about

we first measured DC currents on the SystemX PSUs we used a hall effect probe and an oscilloscope, calibration was by checking against known currents.

[dennis]

[Andy Dingley]

Same as any other bit of test kit. They're not excessive.

My Hall probe (addon for any voltmeter) cost about the same as my tacho probe did (=A350? Those were real old Imperial quids though), 20 years ago. You can also make your own, out of recycled lumps of ferrite (a scrap ignition timing light is good), a Hall sensor (couple of quid) and a few opamps. It's standard tech in the DIY electric car world.

[Dave Plowman (News)]

Amp. Which DVM is this supposed to replace?

By small current I meant the sort of thing you'd likely want to check for on a car - the quiescent current when checking why a battery goes flat. So it needs to be reasonably accurate at around 50 mA.

[Dave Plowman (News)]

What would the accuracy be when measuring say around 100 mA?

I don't know of any reasonably priced commercial unit that can do this with decent accuracy. Like, say, a 5 quid DVM could manage.

[dennis]

Which £5 DVM doesn't affect the current when you insert it into the circuit at those sort of currents?

Maybe the one connected to this?

[geoff]

In message , "dennis@home" writes

[Dave Plowman (News)]

Do you know how ammeters work?

[Dave Plowman (News)]

Have you read the spec? It doesn't give the accuracy for DC. Not surprisingly...

[Frank Erskine]

>40-400 Amp. Which DVM is this supposed to replace?

That reminds me of a Morris Marina (old M reg) 1·8TC Coupé which I had many fortnights ago. The battery started to go flat on a fairly regular basis when left for a few days. I connected a meter in the battery lead and found a discharge of almost exactly ½ an amp, which I thought to be rather suspiciously like a specific item rather than something random. It turned out to be a 6W festoon bulb in the boot - the pin switch to operate it had popped out of its bracket, leaving the boot permanently illuminated.

[Terry Casey]

I had a similar problem with a Pergeot Expert van, but in my case it was intermittent.

I only found the cause when a friend who was with me asked why the light in the cargo area came on when I went round a corner! This van was fitted out with racking so there was a sheet of metal behind the front seats. The only gap through which the chink of light could be seen was invisible from the driving position ...

Turned out to be an adjustment to the runner mechanism at the rear of the sliding door. It had come loose, allowing the rear of the door to move outwards due to centrifugal force on bends and corners. As the rear moved out, the front moved back slightly, allowing the pin switch to close.

If I opened the door after I got home, everything was ok, as the closing inertia operated the switch. If I didn't, there were two greedy lights left permanently on, yet so little light escaped, they were invisible under the street lighting ...

[Andy Dingley]

+/- 1.5% AFAICS

That's the sort of thing I've got. It has been a very useful bit of kit on occasion.

I note that the new one claims a frequency response of 20kHz. My old one was damped, but I did modify this to make it output the unfiltered signal as well. Quite useful sometimes into a 'scope. Mostly I was looking at drive currents into motors and a peaking current indicated a mechanical problem that shouldn't have been there.

[dennis]

Yes, is this some sort of quizz for the obvious? You break the circuit, insert a small value resistor and measure the voltage. How small depending on the actual meter and the connecting cables. Unless its a hall effect one for DC.

[The Other Mike]

Depends what you mean by small but I can resolve down to 1mA with mine and that's good enough in my view for checking stray drains on car battery circuits.

It probably cost the best part of 200 quid about 10 years ago though :)

RS 837- 155 the website says the last price was "£200" before it was discontinued

I just checked the leaflet in the box and accuracy is quoted as +/-

1% of reading +/- 2mA. It was regularly check calibrated with a NAMAS accredited facility until about 5 years ago. I've no cal certs but as the case seal is still intact I know it wasn't ever opened it so met the specs for all that time.

I've checked battery accessory drain with it a number of times and it's not significantly different to series testing with my usual (in cal) DMM, but a damn sight easier to use. I checked it on the bench today with a current source and my long out of cal DMM. It's within about 0.5% of an in cal Fluke bench DMM costing the best part of a grand, that's good enough for me.

[Dave Plowman (News)]

So no better than a 5 quid DVM. Most of the affordable clamp types are only suitable for large current DC measurement.