9v DC from 12v DC??

you could use an LM7809 voltage regulator IC.
Datasheet and application notes:
http://www.datasheetcatalog.com/datasheets_pdf/L/M/7/8/LM7809.shtml

Mik

Ta very much!....as you were!

Only reason I could think of for that is if the voltmeter needed a reference voltage and you wanted it to be different from the voltage rail that was supplying it power. Say you want to monitor a 12V battery voltage and that was also the supply to the meter, as the 12V voltage dropped your measurement reference would shift and your reading would be wrong.

So to avoid this you could use another battery for the meter power that would keep a charge for much longer than your main battery because it was supplying a very small current to the meter displays only. Another would be to use a regulator to drop your main supply to about half it's value and then use a double or triple voltage ladder circuit to bump it high enough to power the meter. it more work involved in this one but as the battery voltage drops from it's initial value the regulator will keep it's output constant. You'd design so at the regulator output + its dropped voltage would be lower then the minimum supply for your boombox.

There may be other ways too, I might be overthinking this!

Edit - Ammeters often need a separate supply to avoid shorting depending on their design and if you use a shunt resistor or a current transformer.


Edited by odc04r - 09 August 2014 at 11:10am

I bought a cheap meter module off eBay and it can work from either a 9v battery or the measured supply if it's under 15v, however the instructions are pretty Engrish and not very clear, so given the possibility of letting all the magic smoke out of the thing/ actually setting it on fire I opted for the 9v battery. Seems to work fine even off a partially used 9v that measures 7-8v and stays as accurate as ever it is.