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# How to cut down the Voltage??? Answered

Hello guys it's electricnewbie again,

I have another newbie question that I think you can help me out with.

OK so go easy with me since I'm a beginner. . .

So, I have a laser/light (from walmart) that takes three 1.5 volt batteries that gives it a total of 4.5 volts. So, I want to hook it up to USB cable. Now, a USB cable supplies about 5.1 volts. How do I drop the current from 5.1 volts down the 4.5? How could I use the same principle in my other projects? Say I have a 9v battery and a 3.4v buzzer. I know how to use resistors with an LED. [R=V1-V2/mAH] Since I know the mAH of the LED, then I know what resistor to use. But how use that equation if at all on other things??

Thanks guys,

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## 4 Replies

mpilchfamily (author)2012-11-05

The .5V difference between the laser's battery and the 5V from USB shouldn't make much of a difference so you can directly connect the laser to the USB port.

But when you want to drop your 9V to 3.4V you will need a voltage regulator. If your looking to use the 9V to power one or more LEDs then you use a LED calc like this one. LEDs are more concerned with the amount of current that is flowing and less about the input voltage. In this case if your LED has a forward voltage of 3.4V and a forward current of 20mA then all you need is a 330 Ohm resistor in line with the LED and you can run it directly off the 9V battery.

HavocRC (author)2012-11-05

I understand how to do it with an LED but how can I use it practical? Say a 12v power source and a 3 volt speaker or something else like that thats not just and LED

mpilchfamily (author)2012-11-05

Any other circuit that isn't a simple LED will need a voltage regulator to bring the voltage down to the appropriate level. BTW you won't be hooking up a 12V source directly to a speaker. You may need to adapt a 12V source to work on a 6V amp for a speaker.

HavocRC (author)2012-11-05

Haha ok thank you I'm just a little confused and trying to figure things out!

Thanks!