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]

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.

[John Rumm]

Is the transformer a very light weight unit?

If so it might be one of the ever more common switched mode types.

This can be true of many PSUs. In the case of the older "linear" designs of PSU, the voltage would often rise well beyond the nominal setting when unloaded. With modern switching supplies the voltage can be very low (or the PSU even turn off) if there is no load.

However it is also possible that you have a regulated supply that is supposed to give the marked voltage under all conditions, but it has just been poorly calibrated.

Start lower and work up.

[Scott M]

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.

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]

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.

[Gareth Magennis]

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.

[The Natural Philosopher]

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

[Jules Richardson]

I've had a system built largely around LS-TTL survive 9V before. I was rather surprised that nothing fried.

[David Nebenzahl]

On 3/5/2011 8:29 AM Skipweasel spake thus:

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]

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.

[Nick Leverton]

These both look very sheddy newsgroups to me ...

Nick

[Tabby]

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.

NT

[Skipweasel]

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

[David Nebenzahl]

On 3/6/2011 2:41 PM Tabby spake thus:

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

Just checking.

[geoff]

In message , David Nebenzahl writes

Oi - don't take my name in vain

[George Herold]

I like the switch mode power supply drooping under no-load idea. Can you put a bit of load on it and test again? 1 k ohm might be enough.

George H.