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Old Oct 30, 2004, 10:33 AM
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flyingflipper's Avatar
Joined Sep 2004
648 Posts
How can I reduce the voltage of AC/DC Converter?


I just bought a Hobbico Quick Field charger Mk2 and when I want to charge batteries in the house, I was going to hook it up to a power inverter (110V AC to 12V DC).

Here's my question. When I checked the output of the power inverter with a meter, it showed about 15.8 V. The Hobbico manual said that the unit can only be used with a power supply from 12V - 15V. I don't want to ruin a new charger, so does anyone know what type of resistor I can use to "step-down" the output voltage a few volts? I figured I could splice the resistor in-line to the output leads of the inverter.

Any other thoughts or work arounds would be appreciated.
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Old Oct 30, 2004, 10:38 AM
now that's a wattmeter...
simingx's Avatar
Joined May 2002
1,136 Posts
Is your power adaptor regulated?
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Old Oct 30, 2004, 11:13 AM
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pmackenzie's Avatar
Toronto (Don Mills), Canada
Joined Dec 2002
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110 AC to 12 volt DC would be a power supply. An inverter usually refers to a device to turn 12 volts DC into 110 volts AC.
Pat MacKenzie
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Old Oct 30, 2004, 11:39 AM
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Dan Baldwin's Avatar
United States, CA, Norwalk
Joined Apr 2004
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Putting a resistor in series with the power supply is not a good idea. The voltage drop would be proportional to the load. With no load applied (open circuit) the voltage would still measure 15.8 volts. The power supply is probably a linear-non regulated supply. If it is you could put a load across the supply to drop the voltage (like an automotive tail light bulb). If it is a regulated supply, you will need to find the adjustment pot and turn it down a bit. You could put a couple of big diodes (mouser #625-FES16AT for up to 16 amps) in series with the output. Each diode will have about .7 volts drop, so the voltage would end up about 1.4 volts lower, or about 14.4 volts. If it is a non regulated supply, the voltage will drop with load, so you could end up with the voltage too low if you are drawing a lot of current.

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Old Oct 30, 2004, 12:21 PM
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Joined Jun 2001
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While you are at it, make sure the AC/DC converter has enough current capacity to handle the charger you are using. It should be rated at xxamps or yyVA. This should compare to/be more than the input current/wattage of the charger.

Mel D.
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