low-loss voltage regulator

I've got a bike helmet light (40 LED's 11,000 mcd's each, 7 AAA NiMH
batteries in series) that works great in terms of photonic spray. But
because the LED's need a precise 3.6 volts supply, I have added an
LM317 variable voltage regulator. This works fine, almost. The 317
works so inefficiently that the 2 square inch heat sink screwed onto
its tab gets almost too hot to touch. I'm looking for a variable 5-10
volts input, constant 3.6 volts output power supply that has very low
heat loss.
Bruce Ratcliffe,
Fresno

 

[mike]

ehsratcliffe@sbcglobal.net wrote:

I've got a bike helmet light (40 LED's 11,000 mcd's each, 7 AAA NiMH
batteries in series) that works great in terms of photonic spray. But
because the LED's need a precise 3.6 volts supply, I have added an
LM317 variable voltage regulator. This works fine, almost. The 317
works so inefficiently that the 2 square inch heat sink screwed onto
its tab gets almost too hot to touch. I'm looking for a variable 5-10
volts input, constant 3.6 volts output power supply that has very low
heat loss.
Bruce Ratcliffe,
Fresno

Look at some of the maxim and linear technology parts.
mike

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[John Larkin]

On 7 Feb 2005 19:27:01 -0800, ehsratcliffe@sbcglobal.net wrote:

I've got a bike helmet light (40 LED's 11,000 mcd's each, 7 AAA NiMH
batteries in series) that works great in terms of photonic spray. But
because the LED's need a precise 3.6 volts supply, I have added an
LM317 variable voltage regulator. This works fine, almost. The 317
works so inefficiently that the 2 square inch heat sink screwed onto
its tab gets almost too hot to touch. I'm looking for a variable 5-10
volts input, constant 3.6 volts output power supply that has very low
heat loss.
Bruce Ratcliffe,
Fresno

Switching regulator is the only way. National, Maxim, LTC.

What's the total LED current?

John

 

[Paul Burke]

ehsratcliffe@sbcglobal.net wrote:

The 317
works so inefficiently that the 2 square inch heat sink screwed onto
its tab gets almost too hot to touch. I'm looking for a variable 5-10
volts input, constant 3.6 volts output power supply that has very low
heat loss.

Apart from fairly small differences in quiescent current, ANY linear
voltage regulator will dissipate the same power, and consequently
produce the same heat. The only way to reduce the heat waste is to use a
switching regulator - google for "simple switcher". What current are you
running at?

Paul Burke

 

[Anders F]

"Paul Burke" <paul@scazon.com> wrote in message
news:36rbegF55g7f0U1@individual.net...

ehsratcliffe@sbcglobal.net wrote:
The 317
works so inefficiently that the 2 square inch heat sink screwed onto
its tab gets almost too hot to touch. I'm looking for a variable 5-10
volts input, constant 3.6 volts output power supply that has very low
heat loss.

Apart from fairly small differences in quiescent current, ANY linear
voltage regulator will dissipate the same power, and consequently
produce the same heat. The only way to reduce the heat waste is to use a
switching regulator

Well - in the current design just loosing some of the batteries would
improve the efficiency with close to no effect to the battery life... 3.6V +
LDO drop (say 200mV) should not take more than maximum 5 cells!

Besides that I second using a switcher dedicated for LED-driving. Meaning:
Outputting constant current so the power isn't wasted in series resistors...

Cheers,
Anders

- google for "simple switcher". What current are you
running at?

Paul Burke

 

[Robert Baer]

ehsratcliffe@sbcglobal.net wrote:

I've got a bike helmet light (40 LED's 11,000 mcd's each, 7 AAA NiMH
batteries in series) that works great in terms of photonic spray. But
because the LED's need a precise 3.6 volts supply, I have added an
LM317 variable voltage regulator. This works fine, almost. The 317
works so inefficiently that the 2 square inch heat sink screwed onto
its tab gets almost too hot to touch. I'm looking for a variable 5-10
volts input, constant 3.6 volts output power supply that has very low
heat loss.
Bruce Ratcliffe,
Fresno

You cannot get around P = I * E in a linear regulator.
So, at 10V in, the regulator must drop 6.4V; multiply by the current
to the load (guess 20mA per LED times 40 = 800mA) --> about 5 watts
dissipated ===> need goodly heatsink.
Solution: a switching buck regulator.

 

[Leon Heller]

<ehsratcliffe@sbcglobal.net> wrote in message
news:1107833221.220498.10390@g14g2000cwa.googlegroups.com...

I've got a bike helmet light (40 LED's 11,000 mcd's each, 7 AAA NiMH
batteries in series) that works great in terms of photonic spray. But
because the LED's need a precise 3.6 volts supply, I have added an
LM317 variable voltage regulator. This works fine, almost. The 317
works so inefficiently that the 2 square inch heat sink screwed onto
its tab gets almost too hot to touch. I'm looking for a variable 5-10
volts input, constant 3.6 volts output power supply that has very low
heat loss.

You'd be better off redesigning the thing and using a switching technique to
drive the LEDs, like this:

<http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1824&appnote=en012040>

Leon

 

[Mark]

Do you have all the LEDs in parallel?

Connect them in groups of two in series and with a small dropping
resistor in series with each pair.

Mark


Mark

 

No, the LED's are not regulated. I'm not current limiting them at this
point. My (simplistic) thinking is that any resistor is going to cause
I^2R power loss. Also, you mention "really low dropout". Pardon my
ignorance, but what is "dropout?"
Thanks,
Bruce

 

Thanks for the note, Mac. I looked at the LM2734Y you mention. It
says it is "configured to convert 5Vinput to 1.8V output at 1 Amp".
Since the spec sheet on the LED's says their optimum voltage is 3.6, it
sounds as if the LM2734Y will come in shy of the mark.
Bruce

 

HI, Paul,
I'm running at about 1.2 A when the battery pack (7 cells) is fully
charged. i.e. when the voltage regulator is delivering its nominal 3.6
V.
Bruce

 

I third the switcher solution. Do you have a vendor/product number to
recommend?
Thanks,
Bruce

 

Dear John, I've got several questions, if you've got the time.
I've contacted Maxim, and thbey have an evaluation kit that appears to
do the job for. . . $60 (ouch!) Am I going to be able to get what I
want for less? (The kit contains a PC board, SMT, a MAX1685 switching
buck regulator and associated components to give me output
characteristics I need). I've thought about contacting other ventors.
Will anyone have the equivalent of the Maxim 1685 in a 14-pin DIP
package? You suggest LTC. What company is that?
Thanks, Bruce

John Larkin wrote:

On 7 Feb 2005 19:27:01 -0800, ehsratcliffe@sbcglobal.net wrote:

I've got a bike helmet light (40 LED's 11,000 mcd's each, 7 AAA NiMH
batteries in series) that works great in terms of photonic spray.
But
because the LED's need a precise 3.6 volts supply, I have added an
LM317 variable voltage regulator. This works fine, almost. The 317
works so inefficiently that the 2 square inch heat sink screwed
onto
its tab gets almost too hot to touch. I'm looking for a variable
5-10
volts input, constant 3.6 volts output power supply that has very
low
heat loss.
Bruce Ratcliffe,
Fresno


Switching regulator is the only way. National, Maxim, LTC.

What's the total LED current?

John