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

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! :-(
Reply to
Joe Smith
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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.
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.
Reply to
Andrew Gabriel
Very likely with no load and not regulated.
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.
Reply to
Dave Plowman (News)
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
Reply to
alitonto
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.
Reply to
mc
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.
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Reply to
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.
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Reply to
Dimitrios Tzortzakakis
current.That's why
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.
whatever
Reply to
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.
Reply to
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.
Reply to
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.
Reply to
Robert Baer
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.
Reply to
mc

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