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#1
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Current Regulator
I have a requirement for a constant current supply to drive 7 high power
LEDs wired in series. The voltage applied to the LED series string must be adjustable between 25 to 33 volts and the current regulated at 300mA. Any circuit ideas, chips etc? |
#2
Posted to alt.electronics
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f Current Regulator
On Mon, 18 Sep 2006 05:02:20 GMT, "Farticus"
wrote: I have a requirement for a constant current supply to drive 7 high power LEDs wired in series. The voltage applied to the LED series string must be adjustable between 25 to 33 volts and the current regulated at 300mA. Any circuit ideas, chips etc? --- Try this: (view in Courier) LM317 +-------+ VIN--+--|IN OUT|---+ | +---+---+ | | | [3R9] | | | [0.1µF] +-------+ | |A | [LED] | | GND--+--------------+ If your LEDs have a Vf of 3.5V, that'll be a total of 24.5V, so with a Vin of 25V you'll only have 0.5V across the regulator, which will cause it to drop out of regulation. It needs about 2V across it to work at room temp with a 300mA load, so your lowest supply would have to be 26.5V. If you absolutely _have_ to have compliance down to 25V it can be done with an opamp and a MOSFET and I'll give you a schematic if you need one. With a 33V input, the drop across the regulator would be 8.5V and the current through it would be 300mA, so it would be dissipating about 2.6 watts and you'll need a heat sink. -- John Fields Professional Circuit Designer |
#3
Posted to alt.electronics
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Current Regulator
"John Fields" wrote in message ... On Mon, 18 Sep 2006 05:02:20 GMT, "Farticus" wrote: I have a requirement for a constant current supply to drive 7 high power LEDs wired in series. The voltage applied to the LED series string must be adjustable between 25 to 33 volts and the current regulated at 300mA. Any circuit ideas, chips etc? --- Try this: (view in Courier) LM317 +-------+ VIN--+--|IN OUT|---+ | +---+---+ | | | [3R9] | | | [0.1µF] +-------+ | |A | [LED] | | GND--+--------------+ If your LEDs have a Vf of 3.5V, that'll be a total of 24.5V, so with a Vin of 25V you'll only have 0.5V across the regulator, which will cause it to drop out of regulation. It needs about 2V across it to work at room temp with a 300mA load, so your lowest supply would have to be 26.5V. If you absolutely _have_ to have compliance down to 25V it can be done with an opamp and a MOSFET and I'll give you a schematic if you need one. With a 33V input, the drop across the regulator would be 8.5V and the current through it would be 300mA, so it would be dissipating about 2.6 watts and you'll need a heat sink. -- John Fields Professional Circuit Designer Thanks for that info John. I can't quite see how you derive the constant 300mA with that LM317 cct above. The LEDs are 4.1 volt 1W devices. I made up a PSU using 6 x LM317T regs, one for each bank of 7 LEDs. This drove the LEDs "sort of" OK, not quite getting to their required 300mA, a bit short on voltage. The customer now insists that the LEDs be "current driven" and produced some chips suggesting I build a PSU using them. They are DD311 - I don't know if you know them. Quite interesting devices, except I can't seem to get the buggers to work as I would expect. (Data sheet available at http://www.tenrod.com.au/htdocs/prod...ad.asp?pid3302 ). The way I see it is (refer to page 11 of the .pdf using the diagram at the top of the page) if the cct was set to provide a constant current of 300mA, then if load ILED was increased, no additional current should flow, it would be limited to 300mA. However when I hook it up using a stiff voltage souce for VLED/REXT and a REXT resistor as specified using the chart on page 6 it doesn't work out. If I increase the load then the current in the path VLED (pin 2) also increases? I would expect it to stay at 300mA? I use load resistors in place of LEDs for the test - could this be the incorrect way of testing and be the cause of this result? Your comments would be appreciated. |
#4
Posted to alt.electronics
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Current Regulator
Should be http://www.tenrod.com.au/htdocs/prod...d.asp?pid=3302
.... I missed out the *=* before 3302 "Farticus" wrote in message ... "John Fields" wrote in message ... On Mon, 18 Sep 2006 05:02:20 GMT, "Farticus" wrote: I have a requirement for a constant current supply to drive 7 high power LEDs wired in series. The voltage applied to the LED series string must be adjustable between 25 to 33 volts and the current regulated at 300mA. Any circuit ideas, chips etc? --- Try this: (view in Courier) LM317 +-------+ VIN--+--|IN OUT|---+ | +---+---+ | | | [3R9] | | | [0.1µF] +-------+ | |A | [LED] | | GND--+--------------+ If your LEDs have a Vf of 3.5V, that'll be a total of 24.5V, so with a Vin of 25V you'll only have 0.5V across the regulator, which will cause it to drop out of regulation. It needs about 2V across it to work at room temp with a 300mA load, so your lowest supply would have to be 26.5V. If you absolutely _have_ to have compliance down to 25V it can be done with an opamp and a MOSFET and I'll give you a schematic if you need one. With a 33V input, the drop across the regulator would be 8.5V and the current through it would be 300mA, so it would be dissipating about 2.6 watts and you'll need a heat sink. -- John Fields Professional Circuit Designer Thanks for that info John. I can't quite see how you derive the constant 300mA with that LM317 cct above. The LEDs are 4.1 volt 1W devices. I made up a PSU using 6 x LM317T regs, one for each bank of 7 LEDs. This drove the LEDs "sort of" OK, not quite getting to their required 300mA, a bit short on voltage. The customer now insists that the LEDs be "current driven" and produced some chips suggesting I build a PSU using them. They are DD311 - I don't know if you know them. Quite interesting devices, except I can't seem to get the buggers to work as I would expect. (Data sheet available at http://www.tenrod.com.au/htdocs/prod...ad.asp?pid3302 ). The way I see it is (refer to page 11 of the .pdf using the diagram at the top of the page) if the cct was set to provide a constant current of 300mA, then if load ILED was increased, no additional current should flow, it would be limited to 300mA. However when I hook it up using a stiff voltage souce for VLED/REXT and a REXT resistor as specified using the chart on page 6 it doesn't work out. If I increase the load then the current in the path VLED (pin 2) also increases? I would expect it to stay at 300mA? I use load resistors in place of LEDs for the test - could this be the incorrect way of testing and be the cause of this result? Your comments would be appreciated. |
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