Mains adapter (SPN 4079A) and voltage with no load

[Joe Smith]

I have a mains adaptor (marked as "model SPN 4079A") which is made in
the UK and which is marked ... "OUTPUT 12.5v 400mA".

I plugged it in the mains here in the UK and measured the voltage at
the end with the round concentric applicance plug. It was about 17.3
volts!

This seems rather a too high. I am worried this will damage a unit I
have got which needs 12 volts and which I was going to use this
adaptor for.

Could this high 17.3 volt reading be entirely due to the lack of load
on the adapter or is 17.3 volts too high even for that?

Unfortunately I forget where this SPN 4079A adaptor came from.
Google is not my friend either! :-(

 

[Andrew Gabriel]

I have a mains adaptor (marked as "model SPN 4079A") which is made in
the UK and which is marked ... "OUTPUT 12.5v 400mA".

I plugged it in the mains here in the UK and measured the voltage at
the end with the round concentric applicance plug. It was about 17.3
volts!

This seems rather a too high. I am worried this will damage a unit I
have got which needs 12 volts and which I was going to use this
adaptor for.

If the appliance requires a regulated adaptor, then it may well damage
the appliance. What is the appliance? Someone might be able to say
if it needs a regulated supply or if it includes it's own internal
regulator, or doesn't care about having an unregulated supply.

Could this high 17.3 volt reading be entirely due to the lack of load
on the adapter or is 17.3 volts too high even for that?

Yes, it's an unregulated adaptor. It probably approximates rather more
to a 17.3V power supply in series with a 12ohm resistor (which would
drop the voltage to 12.5V at 400mA), but that's a rather simplistic
model of an unregulated PSU.

 

[Dave Plowman (News)]

I have a mains adaptor (marked as "model SPN 4079A") which is made in
the UK and which is marked ... "OUTPUT 12.5v 400mA".

I plugged it in the mains here in the UK and measured the voltage at
the end with the round concentric applicance plug. It was about 17.3
volts!

Very likely with no load and not regulated.

This seems rather a too high. I am worried this will damage a unit I
have got which needs 12 volts and which I was going to use this
adaptor for.

You need to load it with a resistor so it draws the same sort of current
as the unit you wish to run it with. And check again. If it's a very low
current device, it may well still be too high. But a regulated 400mA 12
volt wall wart costs less than a tenner from Maplin, etc.

 

[alitonto]

Once I was in a similar situation.
If you can, connet the item to a 12 v car battery and measure the
current draw.( the battery will have 12 to 13.5 v and provide whatever
current is needed)
Then if it is about 400ma then it would be ok to use the adaptor
because anything drawing about that current will cause the voltage to
drop to about 12-13 volts.( as per the specs of the adaptor).
Ali Tonto

 

[mc]

I have a mains adaptor (marked as "model SPN 4079A") which is made in
the UK and which is marked ... "OUTPUT 12.5v 400mA".

I plugged it in the mains here in the UK and measured the voltage at
the end with the round concentric applicance plug. It was about 17.3
volts!

What you are seeing is quite normal for an unregulated adapter. Use it with
a load of 300 to 400 mA and you'll get the right voltage.

 

[Dimitrios Tzortzakakis]

Yes, but most devices have fluctuating amperage demands, thus max 400mA and
when idle less.Only a pocket light will absorb a constant current.That's why
regulated power supplies were invented.

 

[Dimitrios Tzortzakakis]

I had an adjustable power supply, from 1.5 to 12 V, which in 1.5 V outputed
7.2 V!Definitely get a regulated power supply they are not that expensive.I
got an 6 V for a radio for less than 12 euro, or you can construct one, if
you have got a soldering iron and can do a little soldering.email me for
schematics.Devices like cell phones must be having their own regulation,
because their power supply has only rectifiers.

 

[Watson A.Name - \Watt Sun, the Dark Remover\]

Yes, but most devices have fluctuating amperage demands, thus max 400mA and
when idle less.Only a pocket light will absorb a constant current.That's why
regulated power supplies were invented.

That's why circuitry that is very tolerant of wide voltage changes was
invented. Especially considering that a battery operated radio might
start out at 9V and have to operate clear down to 7 or 6V before it
quits. It's just the nature of batteries.

 

[Robert Baer]

What you are seeing is quite normal for an unregulated adapter. Use it with
a load of 300 to 400 mA and you'll get the right voltage.

Not true!
*All* unregulated "wallwarts" have an output voltage higher than the
"spec" even at full load.
Measure some and find out for yourself.

 

[Watson A.Name - \Watt Sun, the Dark Remover\]

Not true!
*All* unregulated "wallwarts" have an output voltage higher than the
"spec" even at full load.
Measure some and find out for yourself.

And just exactly _what_ axiom do you base your unequivocal statement on?

 

[mc]

*All* unregulated "wallwarts" have an output voltage higher than the

"spec" even at full load.
Measure some and find out for yourself.

All generalizations are false, including that one. I've found some that run
low.

 

[mc]

*All* unregulated "wallwarts" have an output voltage higher than the

"spec" even at full load.
Measure some and find out for yourself.

.... However, there is a kernel of truth to what you say... the rated voltage
tends to be treated as a minimum, so they're probably more likely to be high
than low. But the ratings are very inexact.

 

[Robert Baer]

it with



And just exactly _what_ axiom do you base your unequivocal statement on?

Fact, supported by more measurements than you would want to do.

 

[Robert Baer]

All generalizations are false, including that one. I've found some that run
low.

Plwase specify brand, make and model and/or where i could buy one of
those wierdos.

 

[Robert Baer]

... However, there is a kernel of truth to what you say... the rated voltage
tends to be treated as a minimum, so they're probably more likely to be high
than low. But the ratings are very inexact.

One of the reasons they are so high, appears to be that they aer
"designed" for 110VAC input, and power here in the US seems to be
semi-universal 123VAC.

 

[mc]

One of the reasons they are so high, appears to be that they aer
"designed" for 110VAC input, and power here in the US seems to be
semi-universal 123VAC.

But Japan, as I understand it, is still 100V. I wonder if they are intended
to work OK there even though nominally designed for America.