Single-Transistor Driver Question

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Single-Transistor Driver Question

Postby Lawnboy » Wed Sep 26, 2012 8:08 am

I am currently building a full color adapter for my monitor using 3x Klaas' single transistor driver. Instead of using an array of leds, I am trying to use a single 3w RGB luxeon. Both the green and blue leds take about 3v at 100ma, and the red requires 2.2v. My thinking is, if I reduce the input voltage to the leds to ~13v, that should leave the 10v over the transistor. I would then add a resistor to the red to bring it down to the same as the blue/green. Would this work? What kind of resistor values would I need for the blue/green leds?
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Postby Klaas Robers » Sat Sep 29, 2012 9:17 pm

You don't need a resistor. The transistor is a CURRENT source, the current is as good as independent from the voltage. If you have 13 volt as a supply voltage, that is 10 volt over the transistor, then the current (the 100 mA) is not dependent from this voltage.

It is also not needed to stabilise that voltage. If it varies one or two volts, due to hum, you will not see that back in the current. And it is the current that defines the light output.
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Postby Lawnboy » Tue Oct 09, 2012 4:18 pm

Thanks Klaas. I actually built the RGB luxeon first, feeding it with the 20v as in the tradtional design along with power resistors calculated for 10v. It lit up, but would not modulate. The transistor got dangerously hot so I scrapped the luxeon for an array of leds, which has a severe color mixing problem due to the colors being in separate banks. I am going to try the luxeon again using the procedure above.
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Postby DrZarkov » Sun Jan 20, 2013 11:19 pm

For some new experiments I need two dirvers, so I ripped one of Klaas' simple drivers out of a previos made TV-set and made a new one. Both drivers are connected to an array of 8 LEDs (Nichia high Quality, 22000 MCD). The drivers are working well, but the LEDs are quite dark, much darker than I'm used from a "common" club driver. The 8 LEDs are connected like in the new NBTV handbook descripted. The input Voltage of the LEDs are 20 V (like in the handbook), I've measured the output (connected to the BUZ11 middle connector), giving 10 V, like in the handbook. Both drivers are practically identical, using the same kind of LEDs.

Is it just a mistake of this LEDs, not compatible with the driver, or could there be anything else wrong?
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Postby Klaas Robers » Sat Feb 16, 2013 11:22 pm

Volker,

what is the peak current through the LEDs Can you measure it? And can you measure the value of the resistor in the source of the BUZ11? That should be 5.6 ohm (and not 56 ohm)

I did this week some measurements on a BUZ 11 in order to accomodate the circuit for LumiLEDs that run at a white current of 350 mA. Then the source resistor should be 1.3 ohm (3 resistors of 3.9 ohm in parallel. This is for renewed activity on CCNC colour NBTV.
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Postby DrZarkov » Sun Feb 17, 2013 2:42 am

Klaas,
when I've read 5.6, I thought that is kOhm, not Ohm. So I think I know why the picture is so dark. :?

The driver is now working perfect. (Also does the whole televisor.)
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Postby Steve Anderson » Tue Feb 19, 2013 12:57 pm

An important point raised above, how to avoid errors in reading numbers in schematics and text.

Generally I avoid using decimal points if possible. So 5.6 Ohms I write as 5R6 (five point six), 5.6 kOhms as 5k6 (5600 Ohms) and 5.6 MegOhms as 5M6.

The same with capacitors, 100nF not 0.1uF, 4n7 not 0.0047uF. Of course this applies to inductors and other components too.

Ditto voltages, currents and power. 53mV not 0.053V, 150uA not 0.15mA and 350mW not 0.35W.

Often these documents get photo-copied and the 'points' get lost, even on a hi-res PC screen they can get lost when the text is small and the decimal point is just a single pixel.

There is a European standard for this, I've no idea what it's called but in general I believe the above is broadly based on it.

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Postby Klaas Robers » Sat Feb 23, 2013 8:18 am

Volker, a resistor of 5k6..... That you had any light output at all......

Eind goed, al goed.
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Postby DrZarkov » Sat Feb 23, 2013 5:19 pm

Well, that's what made the mistake not so very obvious...
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