ammeter and voltmeter needed

[williamwright]

I've been ages searching and I can't find what I want, though it sounds so simple. It's a digital ammeter/voltmeter combination. The max voltage to be read is 16VDC; the max amps to be read is 20A. The supply voltage is nominally 12VDC. It would be perfectly OK for the voltmeter and ammeter to be separate units.

I found a product priced at around £20, which seems a bit cheap to me, but delivery is 20th - 29th Oct, and it only reads up to 15A

Well, 15A would be OK at a pinch, but I can't wait until 20th Oct. I'm assuming the ammeter is 15-0-15 as well, although it doesn't say so. I do need centre zero. I do have a 30-0-30 analogue meter but 2A scarcely moves the needle, and I will need to see a confirmation of that sort of low current use/charge.

I don't want a hall effect device; I want something with a shunt.

I've looked all over. I must be looking in the wrong places. Can anyone who is conversant with such things and knows the suppliers help?

Bill

[Andy Burns]

they do cheaper ones for 20A rather than 30A, but they only have the fuse-blade hardwired, no croc-clips

[tabbypurr]

Oh, then you'll need 2 separate meters. Don't overlook the possibility of converting an ammeter into either type you want. Or add switching so you only need one.

NT

[Fredxx]

That looks very nice, having a remote shunt at the point of measurement is a nice feature.

[Brian Gaff (Sofa]

This where we mourn the loss of Tandy on the high street. They used to stock the sort of things that vehicle customisers wanted, though in them there days they were mostly analogue of course.

Brian

[Chris Green]

It's not something that you'll get 'off the shelf' I don't think. You

*might* find something in the [small] boat electronics field but it will probably cost an arm and a leg - boat stuff always does.

Take a look at:-

All my boat electronics are 'home made' though. I use a BeagleBone Black single board computer. The voltmeters are just simple voltage dividers feeding into ADC inputs. The ammeters are hall effect devices, they really are the best way to measure (fairly large) currents nowadays. You can get ready made transudcers, things like this:-

[Brian Gaff (Sofa]

I actually threw out an old ammeter when I cleared the shed. It was made by Westinghouse, and was a bit retro looking with a kind of curved bit of case for one connection that pushed against the metal car body as you did up a clamp and there was a screw for the other end. Now it was centre zero and the needle was very bouncy indeed.. I originally got it from a bloke who fitted it to his sports car, but found it totally unusable due to the bounce in the needle making measurements totally useless. Stupid design. Brian

[Dave Plowman (News]

Ones designed for car use are normally 20 amp - but being more specialist units, rather more pricey than a basic one. And usually do things like dwell and RPM etc. I keep one of those in the old car.

The snag with using a DVM to measure high current is they are normally fuse protected. And inrush current can cause that fuse to blow - and it may not be a standard cheap fuse.

For higher currents, a clip on type may be better - but more expensive and tricky to get an accurate one to measure DC current.

Another way is to get a suitable shunt with leads and crock clips etc (perhaps build it yourself?) and use an ordinary DVM across it. You'd obviously need to make a calibration table. Or trim the size of the shunt. A length of ally would what I'd try - and file the width down to calibrate.

[Dave Plowman (News]

Ah - old fashioned analogue?

[Andrew]

At this time of year there are (were) usually auto-jumbles all over the place where old vehicle ammeters can be found.

[alan_m]

Judging by the many on Ebay I assume it be 0 to 15 and not 15-0-15. Most(all) don't seem to have any display ability for a -ve indication.

Perhaps consider a cheap 0 to 20V panel meter for your Voltage display

and then find a -199.9mV 0 +199.9mV voltage meter for your Ammeter by adding your own external shunt of the appropriate value to make the

+/-199.9V = +/-19.99Amps

Looking on Ebay the only combined meters that meet your requirement for centre zero amps appear to use a hall effect sensor where you need to pass the wire through the sensing ring

example

or

[John]

I've been ages searching and I can't find what I want, though it sounds so simple. It's a digital ammeter/voltmeter combination. The max voltage to be read is 16VDC; the max amps to be read is 20A. The supply voltage is nominally 12VDC. It would be perfectly OK for the voltmeter and ammeter to be separate units.

I found a product priced at around £20, which seems a bit cheap to me, but delivery is 20th - 29th Oct, and it only reads up to 15A

Well, 15A would be OK at a pinch, but I can't wait until 20th Oct. I'm assuming the ammeter is 15-0-15 as well, although it doesn't say so. I do need centre zero. I do have a 30-0-30 analogue meter but 2A scarcely moves the needle, and I will need to see a confirmation of that sort of low current use/charge.

I don't want a hall effect device; I want something with a shunt.

I've looked all over. I must be looking in the wrong places. Can anyone who is conversant with such things and knows the suppliers help?

-----------------------------------------------------------------------------------------------------------------------------------------

Try CPC - their prices are competitive

Otherwise try RS Components or Farnell

[alan_m]

Further reading of some data sheets about using a shunt with a centre zero voltmeter suggest that the power supply for the panel meter may/will have to be isolated from that used for the current through the shunt. In a DMM this is easily achieved by using a PP3 battery as the power source.

In these modules this may be easily achievable by using a dc to dc converter that isolates output from input to supply the panel meter's power. This would allow the one source of power to be used such as using a car battery as the power source with the meter (with additional dc to dc converter) measuring both the charge and discharge current.

See pages 15 and 16

LCD

from around £15)

LED

When selecting individual modules in the above links there is a data sheet link on the right hand side of the page and towards the end of the sheet some application notes.

Possibly the reason for cheap centre zero digital ammeters having a hall effect sensor is the ease of isolation between the meters power supply and the current being measured.

[williamwright]

Thanks. I might end up using a CPC voltmeter. But they don't have a suitable centre zero ammeter.

Farnell is CPC but with higher prices isn't it?

Bill

[Tweed]

Farnell is dearer, but carries a *much* larger range. Both owned by Avnet.

[williamwright]

I think Farnell has big discounts for certain customers.

Bill

[Tweed]

They offer 15% discount to the higher education sector, and anything purchased in significant quantity is subject to negotiation.

[Harry Bloomfield, Esq.]

A bi-polar cap across the movement would smooth that out.

[Harry Bloomfield, Esq.]

Dave Plowman (News) used his keyboard to write :

On a car, you can put your meter end to end across the main battery thick cable, using that as the shunt. You can calibrate the meter using a resistance in series with the meter.

[The Natural Philosopher]

Not exactly corrected for temperature and not ideal unless you do it AFTER the starter motor takeoff