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blokemanFlag for Australia asked on

How do I determine the output of an unmarked AC adapter?

I have an AC adapter that is missing the usual sticker which would mention the AC or DC volts plus amps that it outputs.

I have used a multimeter to measure the volts and I get :
Multimeter set to DC = 16.96v
Multimeter set to AC = 7.86v

What is the best way to determine whether the output is AC or DC and at what current (amps)?
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blokeman

8/22/2022 - Mon
2PiFL


You can use an oscilliscope to actually see the output.
http://en.wikipedia.org/wiki/Oscilloscope#Features_and_uses
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tigin44

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ASKER
blokeman

Hi Tigin44
All I have is a multimeter, so your idea sounds great!
I tested your idea on a working DC adapter and on an AC one and you are right.
I therefore conclude that the adapter I have basically converts to 17v DC.  

But what about amps?  When I set my multimeter to AMPs and then measure between the postive and negative parts of the output lead from the AC adapter, I received a reading of 1.8A.  So I tried measuring a known adapter that outputs 1 amp but it showed 5.6 amps and slowly reduced to 4.7amps before.  Am I measuring the Amps the correct way?
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ASKER
blokeman

FYI - This adapter is not for a notebook, and the wires are very light guage moulded two strand ones that usually come with other peripherals or household applicances.

Arthurjb:  My testing with known AC-DC adapters shows that the voltage measurement between the positive and negative tip is quite accurate.  For example a 5v adapter measure 5.1v, so I am confident of the 17v DC.

The amps is the difficult one though, and I take on board what has been said.   I think from my amps measurement of known adapters (without load) I can deduce that my measurement is around 4-5 times the real current output.  This is a very rough estimate I know, but for small current devices that I can guess would be within the current range of my rough amp output calculation, then the adapter could still be useful.

What do people think of this idea?  From messing with electronics in high school, a potentiometer (aka volume dial) has the ability to increase the current.  Could I use a potentiometer with a 17volt 2A bulb, being fed by my 17v 'unkown' amp AC adapter.  Then increase the 'volume' until...?   What can I measure, if anything, that would tell me when I am drawing more than the maximum rated amp output?  
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ASKER
blokeman

Thanks all for the insight.
At least I am sure of the voltage.
Amps not so sure -  a bit of an educated guess, but most likely under 1 amp.  Based on rough calcs, probably around (1.8 / 5)  360 mA.
So I will use it with caution and ensure that I don't overload it, (if it ever gets used!)

Thanks again.
Blokeman