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#1
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LED Driver delivers constant luminosity
I very much like the circuit by Israel Schleicher in EDN Jun 12 2003 for its
ability to drive a 3.6 volt Superbright LED off a 1.2 to 1.5 volt cell with very minimal and inexpensive components. Since it pulses the LED, greater perceived brightness can be acheived at great efficiency. See http://www.reed-electronics.com/ednm...e=6%2F12%2F200 3&spacedesc=designideas According to the formula, changes to R, L or C could thoretically provide more current to the bright white LED - I am looking for 30ma average, but in practice I have not been able to acheive this. Can anyone tell me where I am going wrong? I would be happy to use three cells if it helped. -- |
#2
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LED Driver delivers constant luminosity
On Wed, 16 Jun 2004 21:47:09 +1200, Simon Dyer wrote:
I very much like the circuit by Israel Schleicher in EDN Jun 12 2003 for its ability to drive a 3.6 volt Superbright LED off a 1.2 to 1.5 volt cell with very minimal and inexpensive components. Since it pulses the LED, greater perceived brightness can be acheived at great efficiency. See http://www.reed-electronics.com/ednm...e=6%2F12%2F200 3&spacedesc=designideas According to the formula, changes to R, L or C could thoretically provide more current to the bright white LED - I am looking for 30ma average, but in practice I have not been able to acheive this. Can anyone tell me where I am going wrong? I would be happy to use three cells if it helped. By believing that pulsing an LED produces greater perceived brightness. -- Best Regards, Mike |
#3
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LED Driver delivers constant luminosity
Simon Dyer wrote:
I very much like the circuit by Israel Schleicher in EDN Jun 12 2003 for its ability to drive a 3.6 volt Superbright LED off a 1.2 to 1.5 volt cell with very minimal and inexpensive components. Since it pulses the LED, greater perceived brightness can be acheived at great efficiency. See http://www.reed-electronics.com/ednm...e=6%2F12%2F200 3&spacedesc=designideas According to the formula, changes to R, L or C could thoretically provide more current to the bright white LED - I am looking for 30ma average, but in practice I have not been able to acheive this. Can anyone tell me where I am going wrong? I would be happy to use three cells if it helped. Simon, if the LED current should be 30mA average, then it has to be with 20% duty cycle(which is varying with battery voltage) 150mA. For this kind of current the right transistor should be something like a BC337 with more current gain at high currents. The inductor probably needs to be of bigger size and less inductivity too and you can try to solve the equation given, which IMHO is too simplified. The best would be to simulate the circuit with the free LT-spice. I think the first article gives a better circuit, in the pdf the connections to the driver transistor are not drawn or got lost in some pdf-conversion, but it is clear where they go. -- ciao Ban Bordighera, Italy |
#4
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LED Driver delivers constant luminosity
Simon Dyer wrote: Since it pulses the LED, greater perceived brightness can be acheived at great efficiency. This is not true of the high brightness white LED types. The original LEDs were more efficient when pulsed because the luminous efficiency peaked at high currents, therefore constant current pulses at this level produce an integrated light using less current than a DC current for that same light level. This is not the case for the white LEDs. The luminous efficiency peaks at low current levels like 5mA- so you actually make the circuit LESS EFFICIENT by pulsing at high current levels. The switching circuit referenced exists solely for the purpose of voltage boost from low cell voltage battery technology to the higher voltages required for the white LED. |
#5
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LED Driver delivers constant luminosity
Bonjorno
Thanks for the practical response Ban. I'm not up with Spice models, but will try swapping out Q2. Can you suggest values for the inductor? Simon "Ban" wrote in message ... Simon Dyer wrote: I very much like the circuit by Israel Schleicher in EDN Jun 12 2003 for its ability to drive a 3.6 volt Superbright LED off a 1.2 to 1.5 volt cell with very minimal and inexpensive components. Since it pulses the LED, greater perceived brightness can be acheived at great efficiency. See http://www.reed-electronics.com/ednm...e=6%2F12%2F200 3&spacedesc=designideas According to the formula, changes to R, L or C could thoretically provide more current to the bright white LED - I am looking for 30ma average, but in practice I have not been able to acheive this. Can anyone tell me where I am going wrong? I would be happy to use three cells if it helped. Simon, if the LED current should be 30mA average, then it has to be with 20% duty cycle(which is varying with battery voltage) 150mA. For this kind of current the right transistor should be something like a BC337 with more current gain at high currents. The inductor probably needs to be of bigger size and less inductivity too and you can try to solve the equation given, which IMHO is too simplified. The best would be to simulate the circuit with the free LT-spice. I think the first article gives a better circuit, in the pdf the connections to the driver transistor are not drawn or got lost in some pdf-conversion, but it is clear where they go. -- ciao Ban Bordighera, Italy |
#6
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LED Driver delivers constant luminosity
Simon,
If all else fails, try this: 3 AA batteries in series with 10 Ohm resistor (for current limiting) and 50 Ohm potentiometer (connect to 2 terminal only) to LED. 30 ma corresponds to the potentiometer at about the center of its range. Tam |
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