Tiny digital ac volt/amp meter

[camallison]

I'm perfectly capable of misleading myself David - on reading your post again, I can see I have!

Colin

[camallison]

I'm also thinking of using one as it comes in exactly the same way for my workbench. One thing that's nice about this particular variant is that it fits a 1U panel nicely...

I guess the black/red wires go to the LED backlight. Perhaps options for changing the colour, etc...

Thanks again,

Mark

The 1U height gave me a clue - I looked on some server rack photos I have and there they were! They seem to have been used on mains input panels for server racks - maybe the original intended purpose.

I disconnected one of the black/red wires, and sure enough the backlight went out. Options are more likely programmed in to the display driver "blobs" on the board - can't see any options jumpers.

Colin

[Nick]

Its worth noting that on many LCD displays, the backlight is not LED - rather, the trend is to use electroluminescent (EL) sheeting as emission is even and distributed, rather than point source.

You can buy EL sheeting and wire in different colours - its fun to play with. Downside is that unlike LEDs which have a simple DC supply, EL requires an AC voltage (typically at a few kHz) of between 60 and 600V at a very small current - such inverters are now commonly available very cheaply (few quid) and extremely compact - often built into the LCD display panel. NOte that with such inverters, you need good screening to minimise the risk of interference with radio and audio devices...

Cheaper LCD displays therefore tend to use LEDs, more whizzo ones use EL backlighting with a correspondingly (slightly) higher price.

Search eBay for "el sheet" and "el inverter" for loads of hits...

PS. General point about these cheap meters - they are incredibly easily confused by RFI - they completely lose the plot - I've had this when driving class-E plasma speakers etc. Any semblance of accuracy in the readings goes completely out the windows - both volts and amps can be 50 or 100% out...

[camallison]

The meter I have bought is purely for an indication of brownout and I am not relying on absolute accuracy, so not too bothered about RFI.

As an aside, I contacted a friend of mine who is involved in the electronics sector in Shenzhen (we were both working at opposite ends of a joint venture in the 90s) to ask him for a guesstimate of the ex-works price of my meter. His response shook me - USD 3.50 only.

Colin

[Mark Hennessy]

Interesting Nick - I thought that EL was pretty much a thing of the past. Certainly, I can only name one piece of kit with one (our Neutrik A2D audio analyser - and that is pathetically dim). Everything else is LED or CFL or OLED. With modern GaN LEDs, efficiency is very good, and designers have got better at making diffusers that evenly distributing the light over larger displays.

I'm spending this week in BH, mostly in workshops and apps rooms, so can report this with some confidence

Thanks Colin for the further info. I thought there might have been "blobs" to set options - the expensive Lascar modules have the same - useful for when you know the decimal point won't need to move, so saves wiring it up externally. Sometime you have to cut a tiny track between blobs. Some of them have pads for SM resistors, so you can scale them without external resistors. Very neat.

From what you say, it sounds like these could be surplus units that were designed for a reputable design. Or perhaps they are clones? Either way, they sound pretty promising. I'll do some testing and investigation when mine arrive. I might start by changing the blue LED to white - blue LEDs are so "last decade"

All the best,

Mark

[Nick]

No question that the future (at the moment) is OLED.

I've been playing with a small OLED display and its lovely - currently a bit pricy, but you can get ex-mobile-phone ones cheaply to have a play with....

OLED is a bit tricky for your average electronics punter though - all the drive electronics I am using are SMDs... SOT-23 and smaller...

[Rascal]

Check out:-

http://www.amazon.co.uk/99-9V-Digital-Vo..._1?ie=UTF8&qid=1370505453&sr=8-1&keywords=panel+meter

Order 2 x - arrived from HK 10 days later.

Read 00.0 to 99.9 volts - and run off a 4.5v to 25v supply. These are well made units

£1.62 each - how do they do it?



Paul

[Mark Hennessy]

I bought a load of those from probably the same seller via eBay for a very similar price. I agree; they are exceptional value for money. The only problem is the measurement is referenced to 0V, and there are occasions when you'd like it to float. But that's nit-picking, given the price.

Since this thread started, I've bought many different types of these, with an intention to review them all. I've got 3, 4 and 5 digit versions, and they are all very good. I've compared them to a posh HP bench multimeter, and they are all surprisingly accurate. Should I fail to post the full review, the headline is "Excellent"!

[Mark Hennessy]

Hello again,

Apologies for resurrecting such an old thread, but I finally got around to documenting my findings. In short, they are astonishing for the money, but not without their faults. I was concerned about the safety aspects of the capacitor dropper (not using an X2 rated capacitor, for example), so I'm using one with my own power supply arrangements. I've worked out what all the option jumpers on the display board do, and I've adapted mine to read with 10mA resolution instead of 100mA. For anyone using (or thinking of using) these modules, then you might be interested to see how I got on.

Link is here: http://www.markhennessy.co.uk/mdu/index.htm

It's only just been uploaded, and no-doubt contains many typos and mistakes (I write web pages "raw" in a text editor, so there is no spell-checking).

While it is in use, the project is a "work in progress". Feedback welcome.

All the best,

Mark

[AlanBeckett]

I agree that the 100R Carbon Film Resistor is no good, but equally I wouldn't trust a Carbon Composition one. They can, and do, fail S/C is severely overloaded. Metal Film or WW will fail O/C.

Alan