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AC adapter - higher than expected output voltage

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giftiger_wunsch

New Member
Hi,

I'm new to electronics, but I've converted a mains AC/DC adapter plug (230VAC input, 6VDC output) from an old set of speakers into a power supply for a microcontroller, but after checking the voltage when using it for another project recently, I noted that when it was plugged in and switched on, but the output circuit was not connected, my voltmeter read 25V and was slowly climbing.

Is it possible that the transformer is producing excess charge in the output circuit since it's not being discharged into a circuit, or should I buy a new voltmeter? :rolleyes:

Thanks in advance for any responses.
 

smanches

New Member
Put a small load on it and see what it says.

If it's a standard transformer type wall-wart, it will show the correct voltage when loaded at it's maximum load. When it is unloaded, or less than max load, it will show a higher voltage, although it shouldn't be 24V out of a 6V adapter. It's usually ~1.41x the rated voltage, unloaded.

If you're getting 25V out, either it's not a 6V adapter, it's not a transformer based wall-wart, or something is wrong.
 
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giftiger_wunsch

New Member
I tried again with the adapter connected to the infra-red receiver test circuit I'm working with (phototransistor, 100Ω resistor, and standard LED in series) and read 10.4V, which dropped to 9.8V when the IR transmitter was used to activate the phototransistor. It still seems high given that the plug is marked as 6VDC output, but it appears that the much higher reading of 24V was mainly caused by the voltmeter being too insensitive for the task.

As a side-note, when I tested the voltage with the more sensitive multimeter I'm now using while no load was being applied, it peaked at 10.4V, and then I unplugged the adapter and shorted the wires to remove any residual charge. But once the contacts were separated again, the voltage started to build up (albeit slowly). Could you explain what might cause that? :confused:
 
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Speakerguy

Active Member
You're observing an effect known as dielectric absorption. It has to do with the filter capacitors in the supply that smooth the rectified AC. Don't worry about it.

The higher than 6v readings you are seeing are caused by "regulation". Your wall wart is an unregulated supply - it has a small transformer, some silicon diodes and then some capacitors to turn wall voltage into a lower voltage. At light loads it will read much higher than rated voltage. At full current it should be a lot closer to rated voltage (6v). If the circuit requires a good solid known voltage, then I would recommend using a 7805 regulator to get a good +5V that doesn't change with current.
 

giftiger_wunsch

New Member
Thanks for the detailed explanation. I believe the acceptable range of voltages for the module I'm powering is 6-14VDC, so there shouldn't be a problem. I was just concerned when I read 25V from a device I was using to power a sensitive (and expensive) device with an upper limit of 14V.

Anyway, question answered. Thanks a lot.
 

audioguru

Well-Known Member
Most Helpful Member
If it's a standard transformer type wall-wart, it will show the correct voltage when loaded at it's maximum load. When it is unloaded, or less than max load, it will show a higher voltage, although it shouldn't be 24V out of a 6V adapter. It's usually ~1.41x the rated voltage, unloaded.
No.
The 1.414 times is the peak voltage to the RMS voltage of a transformer. If the filter capacitor is big enough then it charges to the 1.414 times peak voltage with or without a load.

This voltage regulation problem is caused by the resistance of the windings of a low power cheap transformer. The transformer's resistance causes the voltage to drop when it is loaded. It could have an unloaded voltage that is double or triple its loaded voltage.
 

Willbe

New Member
I've never seen an adaptor that, when unloaded, was over 1.2x nominal loaded output voltage.
 
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Speakerguy

Active Member
I've seen tons of them with bad regulation, like 20-50%. Right now I have a 12VDC 400ma supply hooked up to a custom board, it hits some additional filter caps and a 7805 regulator supplying a PIC running at 48MHz. Measured load is 39.3ma and the output of the wall wart itself is 15.03VDC. This is typical in my experience. Maybe you have been using regulated or switching wall warts?

Edited to add: Unloaded the same supply is 16.00VDC on the dot. All I/V measurements done with a Fluke 177.

Edited to add more: Tested a Jameco ReliaPro 12VDC 200ma wall wart. Unloaded voltage was 16.04VDC. I've got more odds and ends in the junk box to run through if anyone wants.
 
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giftiger_wunsch

New Member
This voltage regulation problem is caused by the resistance of the windings of a low power cheap transformer.
I don't know what "windings" means, but that sounds likely; it's a transformer plug which was attached to some cheap computer speakers; its quality probably isn't fantastic. As long as I don't have to worry about the voltage exceeding the limits of the board it's powering, that shouldn't be an issue. It was approximately 10.4V unloaded, which is still within the limits, so it'll be fine for my needs.
 

audioguru

Well-Known Member
Most Helpful Member
Cheap low current wall-warts that are made in China have horrible voltage regulation and a very low max allowed output current.

I have a little one rated at 12V/100mA and its is actually 12.3V at 100mA. But its voltage is 27V without a load and it gets pretty hot with or without a load.
 

Speakerguy

Active Member
All the ones I have I've pulled off of equipment. I don't think I've ever bought a wall-wart. Most are pretty decent like the two I posted about above. But probably because they came with semi-decent consumer electronics.
 
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Willbe

New Member
is actually 12.3V at 100mA.
But its voltage is 27V without a load
This could almost count as a current source. The highest internal imp. of anything I've seen advertised as a voltage source was 40 Ω.

And I still don't get e-mails notifying me of responses to threads I replied to.:confused:
 
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