resisting AC


Old 03-06-05, 08:58 PM
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resisting AC

so, i have a new project, and im trying to run a pump that is made for 12V DC, and rated at 8.3W, and .69A.

My question is, can i use a 140Ohm resistor to make it run correctly?

since I'm trying to do: 98V/.70A(i cant find a resistor at .69A) that is the resistor i am looking at.

thanks a bunch,
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Old 03-07-05, 01:36 AM
Join Date: Oct 2004
Location: USA
Posts: 719
Your title states AC.
If your trying to run a DC motor, and feeding AC into the motor its not going to work.

This calculation should be close for what your doing.
If you want to create a voltage drop of 98 volts with a 0.70 amp load.
98 volts / 0.70 amps = 140 ohms
98 volts x 0.70 amps = 68.6 watts

You would need a 140 ohm resistor good for over 68.6 watts. (Not a 10 watt resistor)
That (resistor) 68 watts will generate enough heat to boil water.
that's 68 watts of wasted power.

If any part of the circuit opened up, you will measure 110 volts across the break without the 0.70 amps loading down the circuit.
Assuming a water pump, you can end-up having 110 volts electrifying the water.
Don't do it.

I would not use 110 volts if I don't need to. 110 volts can be a shock danger and fire hazard.

A safer way.
Get a 12 volt DC power supply rated at 1 amp.
Old 03-07-05, 03:43 AM
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aye, im new to this, i usually only work with DC,

sorry, i thought the 10W rating was the LOAD not the power resisted.

my problem: limited space, how big is a 12V DC powersupply rated at 1 amp? i am going to be powering a fan also rated a 2W or .16A

and cost? If i need to, ill use a computer power supply rated at 240W, but it's pretty big for what im doing.

thanks again, those where some quick replys.

Old 03-07-05, 05:45 AM
Join Date: Jan 2004
Location: Oregon
Posts: 1,219
Using a resistor for your application is beyond a bad idea. Not only will it be unsafe as all get-out, but it also won't work very well, and will very likely destroy the pump. I you connect a resistor to a 120V supply, then the output voltage from the resistor will be anything between 0V and 120V depending upon the current flow (remember E=I*R; if you don't know Ohm's Law, then you should be reading an electronics text book, _not_ building stuff at lethal voltages). On top of this, the power out of the wall is 120V AC; you would need a rectifier to provide DC.

What you should do is buy a power supply, something UL listed to take 120V in and provide 12V out. 12V can certainly kill you if mis-applied; but you would actually have to make significant effort trying intentionally to hurt yourself with 12V. Use the 12V to run your project.

Don't use a PC power supply. They are made for use inside a PC enclosure, require external signals to properly turn on, and finally, are generally made to regulate the 5V output, with the 12V output as an afterthought. Get a _12V_ power supply, for example:

Old 03-07-05, 06:15 AM
Join Date: Dec 2000
Posts: 510
Here's a 12VDC 1A power adapter:
Old 03-07-05, 12:19 PM
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hehe, well the project that im doing is for a computer, and i know how to turn one on... green cable to any ground.

i think i will use the adapter winnie linked to, it seems to be more of what i'm looking for.

thanks for the links, im gonna order everything today!


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