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Power Supply voltage with higher voltage than listed.

Barry365

New Member
I have a 7.5 VDC power supply that charges 4 aa batteries in a tool.

When I check the voltage I get 12 VDC coming out of the power supply. The label on it clearly states 7.5VDC.

I have never seen a power supply increase output like this. Is this possible. The meter (Fluke) is good and works correctly.
 

JimB

Super Moderator
Most Helpful Member
I have a 7.5 VDC power supply that charges 4 aa batteries in a tool.
Is the "Power Supply" a power supply or is it a battery charger?

That may seem a silly question, but a power supply and a battery charger have different requirements.
Also, the requirements of the battery charger will depend on the battery chemistry.

I am going to make some big assumptions here, they may be wrong...

The AA batteries are either NiCd or NiMH.
They are intended to be charged with a 100mA constant current.
The charger is "cheap", it is simply a resistor in series with a voltage source which is greater than that of a fully charged battery.
Four charged NiMH batteries will have a terminal voltage of about 4 x 1.4x = 5.6v
12v - 5.6v = 6.4v That excess 6.4v will be dropped by a current limiting resistor which is in series with the output to the batteries.
Resistance R = 6.4v/100mA = 64 Ohm.

So, my best (worst) guess is that your PSU is an unregulated 12v PSU with a 68 Ohm resistor in series with the output to the batteries.

JimB
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I have a 7.5 VDC power supply that charges 4 aa batteries in a tool.

When I check the voltage I get 12 VDC coming out of the power supply. The label on it clearly states 7.5VDC.

I have never seen a power supply increase output like this. Is this possible. The meter (Fluke) is good and works correctly.
It's a charger, NOT a power supply - the 7.5V rating refers to the batteries it's intended to charge.
 

Visitor

Active Member
Not to disagree but.....

I believe upand_at_them is correct. It's probably an unregulated DC supply that depends on a certain load to be at the correct voltage. The supply probably contains a line-frequency transformer, a bridge rectifier and possibly a small smoothing capacitor. Two physical characteristics will be that the power supply is relatively heavy (because of the line-frequency transformer), and will be close to a uniform cube in shape (to accommodate the large transformer).

Low cost "universal" power supplies like the ones you could purchase at Radio Shack were of this design. The unloaded voltage could be twice the rated voltage.

Check the voltage when connected across the batteries. It will be close to the specified voltage.

The linked Sparkfun tutorial explains this very thorouhly.
 

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