Voltage reduction w/low heat...possible?

[MacGyverS2000]

I'm looking for ways to reduce (and regulate) a voltage source from +12.5-14.5 V down to an even +12.0 V, with a current range around 3-4 A. This is a small, inexpensive item, so regulator size and cost is a major concern. It will end up in a car and be handled from time to time, so heat is also a major concern.

Shunt regulators are out of the question due to guaranteed high heat from wasted energy in the resistors (not too concerned over the wasted energy, though). Linear regulators are the current candidate, but again, the high heat levels involved at dropping 10+ W doesn't make me a huge fan of those, either. The only other option I can think of is a separate switching regulator IC, but the cost on that would more than likely be prohibitive.

Am I overlooking any options, or am I stuck choosing between these?

 

[zeitghost]

'Scuse me for asking, but what colour are the leds?

4.6V forward voltage seems pretty high. Are they blue?

rgds
Zeit.

 

[MacGyverS2000]

zeit, Yep, blue.


pezas, I'm very hesitant to put little to no current limiting in the string itself. Should the user install the system in a vehicle that has a higher than average voltage with the alternator, my poor little LEDs would be toast in a very short time.

 

[Heydave]

I have an alternate simple circuit to regulate the current to LEDS instead of voltage on my web page:

http://www.inventgineering.com/LEDdriver.htm

The problem with trying to have a fixed voltage is that LEDS are current devices with a more or less constant voltage drop. This voltage changes with temperature so that a constant voltage source to the LEDs does not insure a constant current! The circuit I have uses the LEDs themselves to regulate and what is left are a few transistors and resistors. You can parallel strings of LEDS each with its own transistor and emitter resistor to insure constant current. Typical Automotive spikes and surges will not be a problem with this circuit. Heat dissipation is spread among several transistors and resistors.

 

[MacGyverS2000]

Dave,

I've seen this circuit before (or one similar to it), but unfortunately it will not work in my application for several reasons. I do not need absolute current control, and a fluctuation of a few mA here and there isn't a problem.

The voltage drop of an LED is significantly larger with changes in current when compared with current changes versus temperature, particularly near the max operating point of the LED. In at least one manner the LED is self-regulating (to an extent)...when connected in series with a resistor, the forward voltage drops as the current level drops, thereby allowing more current to flow. An equilibrium is reached that, while not a scheme one would want to use on an LED-based display board, makes the setup reasonably decent.

 

[Chuckles159]

Led brightness changes with current but not linearly. are you sure you need regulated power at all? Why not just run the LEDS with their resistor off the raw voltage and be done with it. The variation between 12.5V and 14.5V is not all that great per centage wise anyway. However dropping 12.5V to 12V is a stretch as you don't really have enough time between pulses to properly charge the inductor in a switching supply.

 

[MacGyverS2000]

Wow, this is an older thread...

14.5V to 12.5V is not a large percentage when your reference point is 0V, but when your reference point is 10V, you've just jumped from a 14% change to a 56% change. THAT'S a significant difference in brightness...