Gareth Magennis wrote:


"john reves" wrote in message
...
I have a very small sony amplifier (the portable type such as you
would travel with and attach to an Ipod). I dont have the 9 Volt DC
transformer that came with it, so decided to use one of those
'variable voltage' transformers that I bought from Lidl some time ago.

It's not clear from the transformer which of the polaritys the
polarity switch is indicating. So to check the polarity i used a
voltmeter.

The choice of output voltages on this transformer is 3, 4.5, 6, 7.5,
9, and 12 volts. The output I require for this particular amplifier
is 9 volts. But when taking a reading from the voltmeter it only reads
7.5 volts on the 9 volt setting.

Checking the others, 3volts is really 2.5volts, 4.5 is really 4, 6 is
5, 7.5 is 6.5 and 12 is 10v.

I read somewhere that a voltage reading from a voltmeter is different
from the reading taken when something is 'under- load'.

Would you use the 9 volt setting (which reads 7.5v) or use the 12v
voltage setting which reads 10volt, for use with this 9volt
amplifier. Thanks for advice.




Well if your voltmeter isn't broken, the power supply certainly is, it
is miles away from what it should be.
Setting it to a higher voltage may well be fine for a while, until
whatever is causing the drop suddenly falls off and you get full voltage
to your Ipod and fry it.

Why risk it? Spend a few quid/dollars on a proper working power supply.


Or is it simply that he is reading RMS AC, not rectified smoothed DC,
which is about 1.3 times higher?