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I need to take 11.1v to 4.5v

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New Member
Hi there guys title says it all really I have a lithium polymer battery which is 11.1 volts and I need to bring it down to the same power as three AA batteries.

I was things of doing it with a few resistors, but big-bada-boom I have a feeling they will burn up after a long use of about 4 hours is there another way if not, what resistors would I need? How would I keep it from over heating?
Thanks In advance:p


New Member
Use ohms law to calculate dropping resistor to get correct voltage. Probably need to know amperage draw of what you're powering. Then calculate power to determine what watt resistor(s) to use. Over-rate resistors to keep them cool.


Well-Known Member
Most Helpful Member
A voltage regulator is probably needed unless the load is just an incandescent light bulb then a resistor can be used.
A linear voltage regulator wastes power getting hot. A switching regulator does not waste as much power so it does not get as hot.


Well-Known Member
If you can scrounge a cell phone cigarette lighter adaptor it will get you to 5vdc with an efficient switching regulator. Add the 10 ohm series resistor from the 5vdc. Most adaptors will work from 11.1 vdc input.

With resistor drop from 11.1 v to 4.5v at say about 50 mA for a AA nicad charge will take 130 ohm series resistor. The resistor will dissipate 0.4 watts so use a one watt rated resistor.
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If it's an incandescent light, I would recommend PWM to save power.

If it's an LED then you need constant current so I would recommend a simple open loop switched mode regulator which can be made with a 555 timer.
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