'floating' DC supply design needed

[sophiecentaur]

I am going to put a digital panel meter on my boat to measure the battery volts and a few other things. For this, I need a DC power supply for the meter with a negative which is below the boat ground.

I could easily use a 9V battery and a push button to keep it on just for taking readings.
But it would be better and more satisfying to provide a supply, using the boat's 12V power. I have looked for something on the web which can give me 7 to 12V, which is what the meter needs but I only seem to find unsuitable voltages.
Has anyone any ideas, please, about a suitable chip for the job (I only need a few mA!)? I am a bit out of touch with this stuff and there must be plenty of solutions if only I could get to them.

 

[jim hardy]

Digikey carries a huge varety of this sort of DC-DC converters.
http://www.murata-ps.com/data/power/ncl/kdc_nke.pdf

some will operate right off 12 volt battery, but they tend to need around 10.8 volts minimum.

"""something on the web which can give me 7 to 12V, """
is that what you want to hand the meter for power, or the range you want to track battery?

a lower volt isolator and 3 terminal regulator would let it follow the battery clear down into deep discharge..

 

[Studiot]

http://cpc.farnell.com/jsp/search/b...etricAttributeId=&prevNValues=411+2005+207658

 

[sophiecentaur]

Thanks for those posts, chaps.
There is a fair bit of choice there, altogether.
I'll see how I get on with the mini project. It's a bit more hi tech than most of the varnishing, cabling and drilling I seem to be doing all the time! Electronics is pretty predictable most of the time- unlike the wind (or even the tide, actually).

 

[jim hardy]

""It's a bit more hi tech than most of the varnishing, cabling and drilling I seem to be doing all the time! ""

in S Florida i grew to like white teak and green bronze...

 

[sophiecentaur]

I like to show her I love her.

 

[vk6kro]

Older LCD voltmeters needed a power source that had a floating ground. I have one of these that works OK but it has remained unused for 20 years because of this peculiar power requirement.

You can get meters now that do not require this.

Just as an example, I have some of these :
http://www.ebay.com.au/itm/4-Digit-Digital-Panel-Voltmeter-0-33V-Red-LED-Voltmeter-/120751739881?pt=AU_B_I_Electrical_Test_Equipment&hash=item1c1d5d53e9

They are about 4.2 pounds delivered.

They run off any power source from 7 to 30 volts (so they probably use 5 volt regulators internally), and they measure from zero to 33 volts with 2 decimal place precision. Seem pretty accurate.
The power source and the measured voltage have to have the same ground. There are 3 wires that come out of the device, ground, measured voltage and positive power.
The measured voltage can be greater than the supply voltage.

They have a bright red display and take about 22 mA supply current, depending on the displayed digits.

Possible disadvantages are that they do not come with a mounting box and they can't measure negative voltages.

The same company have lots of other meters as well as a lot of cheap power supply modules.

You can get other meters that would be OK for a boat where the voltage should never go to zero:
http://www.ebay.co.uk/itm/Blue-LCD-Digital-Voltmeter-Battery-Monitor-Panel-Meter-/270911120283?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item3f138fa39b
This one works from 7 volts upwards. Looks easy to mount.

 

[sophiecentaur]

I'm fixed up now, thanks.