isolated power for digital meter

I have a small LCD panel meter which I'd like to use in an application in which the power supply and the voltage being measured share the same ground.
I know I've seen circuits before to generate an isolated voltage of 9V or so at a few mA for this application but can't find one at the moment.
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On Sat, 22 Mar 2008 23:00:24 GMT, "James Sweet"

http://tinyurl.com/2pwntw
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wrote:

That's not an acceptable solution, this needs to be externally powered, no battery to replace.
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James Sweet wrote:

Something like this? Cheap enough for you not to have to get PCBs made, transformers wound & EMC test. http://uk.farnell.com/jsp/Electrical/Power+Supplies+&+Converters/C+&++D+TECHNOLOGIES/NMA1209SC/displayProduct.jsp?sku 21437 Or perhaps some photovoltaic cells lit by LEDS?
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wrote:

http://uk.farnell.com/jsp/Electrical/Power+Supplies+&+Converters/C+&++D+TECHNOLOGIES/NMA1209SC/displayProduct.jsp?sku 21437
That link doesn't seem to be working.
At any rate I think you guys are making this out to be more complex than it really is. I saw a few circuits at one time that used a little home made transformer, a couple of transistors, diodes and a capacitor to form a little self oscillating inverter. I think I saw another one that used a 555 and a few capacitors. It's the sort of thing I can hack together on a scrap of perfboard using junkbox parts. I just figured I'd ask around for a tested schematic rather than reinvent the wheel and design one myself. I only need one, this isn't something I'd have a pcb made.
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James Sweet wrote:

A few years back I made one of those with a 2 transistor multivibrator with a small center tapped 600 ohm modem transformer as the load. A zener, diode resistor and cap on the output side provided the 9V.
+---->|-----+----[R]---+--- +9 | | | | [22uF] [Zd]9v | | | +-uuuuuuuuu-+----------+--- 0V ========== +---+---------uuuu-+-uuuu-------+---+ | | | | | | +--------------------+--||--+ | | | | .1uF | | | | | | | | +----------- | -----+ | c\ | | | /c |--[10K]--+ [60R] +--[10K--| e/ | \e | o + Vin | | | +------------------+----------------+ | Gnd
Vin was tested from 6 to 16 volts and the circuit maintained 9 volts out at 10 mA. The transformer provided the R to limit the current through the zener.
Ed
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If your DC voltage is at least that required to operate the LCD meter and you can accept a pust-to-test 'feature', then you can use a double-pole/2-position push button switch, resistor, a zener diode, and a capacitor.
The capacitor goes across the common poles of the switch (watch polarity), the normally closed poles of the switch go to a charging circuit, and the normally open poles of the switch go to the LCD meter power inputs. If the voltage to be measured is higher than the allowed limit for the LCD power, use a series resistor and a zener diode across the capacitor to limit the voltage.
The capacitor is charged when the push button switch is not pushed. When the push button switch is pushed, the capacitor is briefly isolated, then connected to the LCD meter. Years ago I used with a 250 MF 16V capacitor and each push would operate the meter for 10-15 seconds. Meters these days may require less power.

Bill Kaszeta Photovoltaic Resources Int'l Tempe Arizona USA snipped-for-privacy@pvri-removethis.biz
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James Sweet wrote:

Do you have any old ethernet cards, with the 5 VDC to isolated 9 VDC coverter modules?
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Ooh, I do! I hadn't even thought of that, I've got a whole pile of them.
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