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# Need help on dropping voltage from 12vdc. to 4.5vdc?

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#### lt_1956

##### New Member
Hello,

I have a mini alarm that has a 12 vdc speaker alarm, what I want to do is add a light which is only 4.5vdc. The funny thing is the unit uses only 3 AA batteries, but the siren triggered uses 12vdc. So it would burn up the light.

I am new to the concept of electronics etc. I seen voltage regulators but they only go down to 5vdc, I thought of trimmers, but can you use them for dropping voltage? and I have no idea how to use the resistors or what type, there are so many types!

Pelase help. I just need a cheap, simple way to nock down 12vdc to 4.5vdc.

Thanks.

simplest way...use trimmer/variable resistor.....concept of votage devider....you can adjust the trimmer untill the output voltage is 4.5....

if the supply current is high...is better if you put the voltage ragulator...as protection....then put the trimmer as picture below

but if the current is not in concern....can simply put the trimmer only

#### Attachments

• 4.5v.jpg
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A resistor in series with the 12 volt supply would work. The value is determined by the current drawn by the light. You'd need to look up the design value for the bulb or measure the current the do some Ohm's Law work (we'll help you) to find the correct value of the resistor.

The 7805 will work as shown but the current draw of the bulb might be more than the variable resistor can handle. You could use an LM317 (variable) using a variable resistor to get the 4.5 volts or fixed values. National Semiconductor datasheets for LM317 will show what you need to do.

Note that whether you use a resistor or linear regulator you will be wasting about 63% (7.5/12)of the power in the light circuit. That might or might not matter to you.

Are the three AA batteries you mentioned hooked up in series?

If so, just add a few wires to the battery compartment and grab the 4.5v from the batteries, bypassing the 12v power supply for the siren altogether.

use a 12 volt lamp.

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