Confusing output voltage from transformer

[john reves]

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.

 

[Skipweasel]

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.

Try the lower - if that leads to distorted sound, use the higher.
There's very little chance of damaging it on the higher, and nil on the
lower.

 

[Scott M]

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

With an unregulated, low current power supply you will find the voltage
sags as the current draw increases. And, normally, with these cheapy
wall-warts the actual voltage is *higher* than labelled as a) they're
cheap and innacurate and b) they expect the voltage to sag down towards
the indicated value. I've not come across any before where the voltage
starts off lower but it falls in with a) above.

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.

Like as not 10v will be fine (but as said, 7.5 is a safe starting
point.) It's rare for any device to require an exact voltage unless it
uses 5v TTL ICs and doesn't have any sort of on board regulator.

 

[Skipweasel]

Like as not 10v will be fine (but as said, 7.5 is a safe starting
point.) It's rare for any device to require an exact voltage unless it
uses 5v TTL ICs and doesn't have any sort of on board regulator.

Much 74LS series is OK up to around 7V. Most data sheets say 5.5V, but a
few actually admit to more - but experience suggests that above 7V
you're /really/ pushing your luck.

 

[Lady Veteran]

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.

I would put it on 9 volts. Isn't that what the portable speakers take.
Don't always believe what your multimeter is telling you. You can your
multimeter on the pud pounder setting and probe Sitre Magana while he's
at the Post Office and it could tell you "not a probability."

But it would be wrong, wouldn't it?

LV
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[The Natural Philosopher]

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?

 

[David Nebenzahl]

Much 74LS series is OK up to around 7V. Most data sheets say 5.5V, but a
few actually admit to more - but experience suggests that above 7V
you're /really/ pushing your luck.

Where do you get 74LS chips in this situation? The O.P. posted a
question about a portable Sony audio amplifier: it's not going to
contain *any* logic chips, more than likely.

Sheesh.

 

[Skipweasel]

Where do you get 74LS chips in this situation? The O.P. posted a
question about a portable Sony audio amplifier: it's not going to
contain *any* logic chips, more than likely.

I didn't - it got mentioned along the way in the post I replied to.

Sheesh.

Bah - thread-drift. Some of the best stuff comes out of drifted thread.
May not be relevant to the OP, but then we're not a public service, we
don't have any duty to stay on topic if we don't want to.

 

[Nick Leverton]

I didn't - it got mentioned along the way in the post I replied to.


Bah - thread-drift. Some of the best stuff comes out of drifted thread.
May not be relevant to the OP, but then we're not a public service, we
don't have any duty to stay on topic if we don't want to.

These both look very sheddy newsgroups to me ...

Nick

 

[Skipweasel]

These both look very sheddy newsgroups to me ...

And proud of it. Some people take it all rather too seriously.

 

[David Nebenzahl]

MMs are accurate on dc, and ac 50Hz sinewaves. Its only when you
depart from those that things /can/ get inaccurate. There is one other
known cause: a low quality digital meter with a dying battery, some
can read low.

You do realize that you replied to a total troll, don't you?

Just checking.