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Can I Limit Current Without Dropping The Voltage? Answered

This kind of a noob question. I have a power source that supply 12V, 500mA. And the device I'm powering takes 12V, 140mA. How can I drop just the current (from 500mA to 140mA) without dropping the voltage? Since using a resistor would cause the voltage to drop as well. (Right?) Any help is greatly appreciated. Thanks.

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westfw (author)2008-02-08

If your power supply is "unregulated", it may end up providing HIGHER than 12V if you only draw 140mA from it. That might be a problem, depending on the device. Have you got a meter? What exactly IS the device?

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gyromild (author)westfw2008-02-09

Thanks for the reply westfw,
It's a 12v, 0.14A computer casing fan.

The problem I have is, when the fan is running from the computer PSU, it runs at its normal speed, no problem there.
But when I take it out, and attach the fan to a universal AC-DC adapter/charger that is set to supply 12V, 500mA, the fan spins faster that it should, start to vibrate, heats up.

Adding a resistor simply caused the fan to stop running, as the voltage drops below 12V.

The AC/DC adapter I'm using.
http://www.110220volts.com/Sub1/prods/Model23_SS104.html

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westfw (author)gyromild2008-02-09

Yeah, that sort of supply is almost certainly unregulated, and will put out higher than 12V if the current draw is lower than the rated current. Fortunately, fans are relatively insensitive to a certain amount of under or over voltage. If you had a meter, I would say to change the output voltage setting (lower) until the voltage actually reads 12V WHILE POWERING THE FAN. Without a meter, I'd say you should just lower the voltage setting of the power supply until it SOUNDS like the fan is running the same speed it does when running from the computer PSU. (fans run OK off a certain amount of under voltage as well, a trick sometimes used to get them to run more quietly...)

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gmoon (author)westfw2008-02-09

If the voltage supply is variable, absolutely. Using a small valued power resistor will work, too. Something on the order of 5 to 10 ohms, with a high power rating (10 watts.)

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tech-king (author)westfw2008-02-09

you could also install a zener diode voltage regulator.

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gmoon (author)2008-02-08

You don't need to--your power source is the correct voltage for the device.

-- Supply amperage is the amount of power that can be supplied

-- Device amperage is the amount of power the device actually draws (uses) at it's rated voltage.

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