Combined digital panel Volmeter/Ammeter

[Yorkie]

I spotted some neat little panel meters that simultaneously display AC Volts from 75V up to 300V and current from 100mA to 50A. Ideal for building into say a variable AC power unit or to monitor the current and voltage of a variac.

A lot of these types of digital panel meters need a separate supply to power the meter, which must not share a common ground. These don't - they're powered directly from the AC supply which is being monitored. They only need a two wire connection to the supply being meaured, and the current is monitored rather like a clamp-on Ammeter, by passing the mains wire through the centre of a small core.

Neat eh, and cheap enough. More robust than analoge meters too in a workshop environment I dare say.

http://www.ebay.co.uk/itm/Digital-AC-300...1017630138

(I haven't poked around for an even cheaper supplier).

Hope that's of use and interest.

[Radio Fixer]

The Sutton ? valve tester (BVWS Bulletin) and the other Forum used such meters, don't know what they were though,

[Mark Hennessy]

It's amazing what you can get for such low prices! I've been looking to buy some panel meters for work, and of course we can't buy from eBay, so have to use Farnell, RS, etc, and you're looking at many 10s of pounds for just simple meters. For home use, the Chinese copies are perfectly OK, and considerably cheaper.

For example, I've just ordered one of these:

http://www.ebay.co.uk/itm/321104772389?s...K:MEWNX:IT&_trksid=p3984.m1439.l2649

This is actually a pair of them - for £10.70 including delivery! The cheapest LED panel meter that RS sell is £18+VAT, and that's a module that has to be mounted on a PCB behind a window or bezel that you provide yourself.

Of course, it remains to be seen how good they are - the 7107 is a standard chip, but the implementation matters. You have to use a good quality integration capacitor, for example. And if you use the internal voltage reference, the readings can drift slightly (because the LED drivers warm the IC). But, at that price, I don't mind doing a bit of engineering, and the picture implies standard through-hole components and a socketed DIL 7107 IC, so it should be nice and easy to work on. There's even a multi-turn preset! How do they do it?

Back to David's mains monitor, here's the same meter but with a BIN (£11.45 plus £1.30 postage!) - a little more expensive, but saves faffing around with an auction:

http://www.ebay.co.uk/itm/Digital-AC-300...7675.m1850&_trkparms=aid%3D222002%26algo%3DSIC.FIT%26ao%3D1%26asc%3D11%26meid%3D6929381957542187014%26pid%3D100011%26prg%3D1005%26rk%3D3%26sd%3D171017630138%26

Very tempting

[Refugee]

These are seriously cheap. I have bookmarked the link for next time i need one.

http://www.ebay.co.uk/itm/170846281102?s...K:MEWAX:IT&_trksid=p3984.m1423.l2649

[Mark Hennessy]

OK, I couldn't resist ordering a couple of these:

http://www.ebay.co.uk/itm/New-5-Digit-0-...uipment_ET&hash=item27cff14841

I've no idea how good these will be, but for less than a fiver each, it's got to be worth a go. I have a decent 5-digit DVM here that I can compare them to when they arrive...

These are interesting because 5-digit ADC/driver ICs are rare and expensive. I'm not sure which chip these use - perhaps it's a custom microcontroller instead. Just about every digit panel meter is 3.5 digits, based on the 7106 or 7107 for LCD or LED respectively. These chips are great, and have been second-sourced by manufacturers like Maxim over the years, but there are times when readings that only go to 1.999 are a bit restricting.

For example, the Thulby-Thandar PL series bench power supplies had meters that could go to 3.999. The metering is why these supplies were so popular. The IC they used went obsolete in the late 1980s, so they redesigned it using another ADC and a PIC microcontroller (I have examples of both here). Then a newer version came out with standard 1.999 displays (LCD as well), but this meant the voltage resolution was now 100mV instead of 10 (because the supplies went to 30V). I remember how people reacted to these at the place I worked at the time. No idea what newer models use internally, but the resolution is back to 10mV now.

Anyway, I'm waffling. If these 5 digit modules are any good, I'll let you all know (once I've snapped up a few more )

Cheers,

Mark

[Refugee]

I have bookmarked that one too as it is 5 digit.

[Neil F]

I see that the meters that Yorkie originally started this thread about have got even cheaper. Currently £7 buy-it-now, including postage, or on auction at a 1p start!

See e.g. eBay item number 330789414798

Neil

[Mark Hennessy]

Thanks for that Neil

I've just ordered a couple for £7 each. That auction wasn't as clear as it could be; many of the photos aren't working, for example. It does look a bit different to the one that David linked two initially, but it's worth a go at that price. It occurred to me that they might be easily modified. It seems that there are two PCBs, and I can't help wondering if one of them is the input conditioning, and the other is the meter proper. I would love to use these in DC bench power supplies if possible.

David, this thread has got expensive

Cheers,

Mark