# Auto voltage reduction

Discussion in 'The Projects Forum' started by Andygadget, Jan 9, 2011.

Jan 9, 2011
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Anyone got the circuit details to build an auto voltage reduction from 12 volts down to 8 volts, Iam fitting some DRLs (L.e.d.s) to my car and want them on during the day and dimmed when i turn my car head lights on so we are looking at a voltage reduction circuit but also a wire linked to the head lights to detect 12v and then automatically switch over to 8v supplying my L.E.D.S hope i have explained this

2. ### Kermit2 AAC Fanatic!

Feb 5, 2010
4,166
1,125
LED's don't dim that way.

The most common way to dim an LED without affecting its output characteristics and color temp. is with Pulse width modulation.

A simple 555 timer ckt can be used along with some mosfets. You will also need that voltage regulator/converter you mentioned as well for a power supply.

Jan 9, 2011
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Do you have any circuit diagrams please, i did connect a varible power supply to them and when i reduced it from 12v to 8v they dimmed

4. ### #12 Expert

Nov 30, 2010
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9,679
I'd say you must know the voltage and current requirements of the LEDs because you didn't tell us, so...Put 2 resistors in series with the LED, wire a relay with the normally closed contacts powering the LEDs from the battery through one resistor, and when the headlights come on, have them activate the relay and power the LED through the 2 resistors in series.

Jul 7, 2009
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LED's don't have the notion of a color temperature, as they're not incandescent sources. Excluding the white LEDs that use fluorescence, they are narrow-band devices (typically 5-10 nm FWHM) whose spectrum doesn't change as a function of current. And a pot to adjust the current through them is the simplest method of changing their brightness. PWM is a good choice too, but more complicated for a beginner.

I think #12's solution is a good suggestion for a practical, low cost solution.

6. ### Kermit2 AAC Fanatic!

Feb 5, 2010
4,166
1,125
In the spec sheets for the few high power white LED's I've worked with, there are graphs with color coordinates vs. Iav,

AND GUESS WHAT? It changes color temp when you change voltage and current levels.

But do what you want to do anyway. I seldom let the facts stop me from doing something I really want to do.

7. ### Audioguru Expert

Dec 20, 2007
10,609
1,183
Maybe the OP has "12V" LEDs that have a built-in current-limiting resistor?
Then they dim when their supply voltage is reduced.

8. ### wayneh Expert

Sep 9, 2010
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5,454
If the OP is still here, please specify the current these LEDs need, or the wattage. That has a big effect on the available options. Most pots won't take the current I suspect is needed for a DRL. A LM317-based solution might not even work if we're up over an amp or so. Burning off excess volts with a power resistor might even be a challenge.

If we're down at <100mA, there are all kinds of easy options.