panel meter needs independant supply ...

Yes, many of those panel meters do not like being ground referenced on the supply side. Some have a link or can be easily moded for a common supply, but others can't. You didn't supply any details on the meter, nor your power supply configuration, so it's kinda hard to help... If you have a spare isolated tap on your transformer you can use that.

There has been talk about this recently on the group, search the archives.

Dave.

What David said is correct. I went through that experience a couple of years back. Ever since I have been asking specifically if the meter can be used with a common negative, and they all eventually admit shyly that the answer is NO.

Luke Tubnor at Jaycar's tech support went through this with me at some length, having already been there in his own experimentation. You can sort of fudge it if you are only using it as a voltmeter, but if you want to switch it between voltage and current then that fudge isn't a goer.

If you havent got an isolated uncommitted secondary, the easiest ways - seriously - are either to get a small transformer and connect that to provide the separate (microamp) supply, or build a small isolated DC-DC switcher off your bridge oputput.

It's a real bummer.

Many years ago I built a simple inverter type circuit for a LCD display power supply. It used a nand logic chip and a few other components. Have a look around.

Been there, done that, use a SRS-1209 miniature DC-DC converter module.

Google for it.

JD

Some panel meters do not have that limitation:

For instance.

which is a LED type. They don't have any problem sharing a neg rail. In fact it is almost a requirement on most common units.

which is more interesting (although from USA).

Mind you, many of the cheaper ones claim to be able to do it, but can't deliver. Having been thusly disappointed before, I wouldn't get overly excited until I had seen it perform.

with

it

I can't speak for every place that sells them, but many of the LED meters at

do not. I don't believe it has anything to do with whether they are LED or LCD, but rather the A/D converter or input amps.

This is what I used the last time I needed an isolated supply for a cheap little LCD meter (a month ago)

I have used in the past a 4011 quad NAND gate to do exactly the same thing. Scroll right to the bottom of the page for "floating supply..."

These are good for a couple of mA and work fine with an LCD meter. Not sure how much current an LED meter draws.

PH

hi, mine is a led panel meter , and draws up to 60 ma , the schematic you mentioned only has a 1ma ?

mark k

you'll need a bigger converter for that (scrounge an 0509 from an old thin ethernet card? they're good for 222ma )

or use an op-amp to shift the input voltages:

Yep, a milliamp or two which doesn't help you if your LED panel meter draws fifty times that.

Other posters mentioned the possibility of a tiny mains transformer,perhaps salvaged from some small mains powered junkbox device like a radio or even the guts of a low current plugpack. Depends on how good your junkbox is. That SRS-1209 DC-DC converter mentioned by another poster is available in Australia from

56mA out for $7.10 plus of course whatever they want to bite you for postage. They look very handy in a DIP sized package. I may even get some myself to save me reinventing the wheel next time around.

PH

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As a lateral thinking idea for your next project, use a $20 DMM from jaycar etc instead of a panel meter. Make it visible via a cutout with connections made internally, and provide a 9v supply from your main supply. Will be cheaper and easier.

Huh? Same problem exactly that the O/P raised.

Hi Rebel, Jaycar have a small PCB kit which uses radio IF coils and and an oscillator circuit creating an isolated supply to drive an LCD panel meter.

cheers, Dex

rebel wrote:

Hi Rebel, Oatley Electronics may have it also can't remember exactly, so many websites so little time.

cheers, Dex

Dex wrote:

I was wrong - actually 56mA, same as the 1209 mentioned previously

4mA to few for you, (maybe enough if you leave "half a digit" dark)