# Electronics Gurus: How do i lower DC voltage?

Discussion in 'Archived Threads 2001-2004' started by NickSo, May 29, 2002.

1. ### NickSo Producer

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Hi, I'm trying to lower a 12V DC voltage source to around 3-4.5 volts (tapping power from my car's cigarette lighter to light up some LEDs), but i have very little to no technical knowledge of electronics whatsover (i know the very basic BASICS), but i can solder and stuff okay.

My friends say i can hookup a few resistors in parallel to do it, but i dunno what resistances i need to get.

Is there a formula or something?

Thanks

2. ### Tony_P Stunt Coordinator

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You could use a voltage divider if you want something cheap and simple. It is not an ideal solution, since it wastes a lot of energy, though. I found this page on Google that will calculate the values for you:
http://ourworld.compuserve.com/homep..._Bowden/r2.htm
It would probably be easier just to put a current limiting resistor in series with your LEDs. However, you should know that these two circuit elements will be connected across your battery and will continuously drain charge unless the circuit is broken (with a physical switch).
You should really understand how simple electronic circuits work before you go fiddling with this or hooking it up to your car.
(Edited because I thought of a simpler idea about five seconds after I posted the first time)

3. ### Danny R Supporting Actor

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Here is a simple circuit to build that does the task (your circuit connects where he places the battery pack).

4. ### NickSo Producer

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Awesome, thanks...

but how do i set it to the voltage i want? Do i use the pot to set it?

5. ### Grant B Producer

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easiest way hook it up it a voltmeter and twist the pot till you get the desired reading.
If you don't have one, pick up a cheap one at RS. They are vital for a host of reasons. There is rarely a week that goes by that I don't use mine.

6. ### PatrickM Screenwriter

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You can also use a DC-DC converter that's all in one silicon package. You should be able to find these at ITT in Burnaby or Active electronics in Vancouver.

Patrick

7. ### Kevin P Screenwriter

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How much current does the LED unit you're trying to power draw? If it draws a fixed current (e.g. it's not a flashing LED) you can figure out the resistor value by using Ohm's Law: I = E / R (I = current in amps, E = voltage in volts, R = resistance in ohms).

A single LED typically runs at about 20 mA (.02 amps). To run such a LED from a 12 volt source, you'd need .02 = 12 / R, or R = 12 / .02 = 600 ohms. If it's multiple LEDs it'll probably draw more power.

If it's a high current device (say, more than .25 amp) or a variable load (flashing), you'll probably want to use a voltage regulator instead of a series resistor.

If you're not sure, err on the high side for a resistor value (low side for current/voltage). LEDs can burn out if overdriven.

KJP

8. ### NickSo Producer

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If i have more LEDs, do i just multiply the amperage by the number of LEDs i have? So if a LED needs 20mA, then does 2 LEDs need 40mA?
Also, im gonna use those bright hi-intensity with the clear plastic (white or blue) LEDs, not those cheap green/red/yellow ones with colored plastic.
Ill try experimenting around

9. ### Kevin P Screenwriter

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Are you building this LED display yourself, out of discrete LEDs? Ideally, use one resistor per LED, and go by the LED's actual current rating (I used 20 mA as an example). Stay below the MAXIMUM rating though.

Note that different color LEDs may have different voltage/current requirements, or may display at different brightnesses. You might have to experiment with resistor values to get the appropriate brightness for the LEDs in your array.

What is it that you're making? How many LEDs will it contain?

KJP

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