High power led array with BJT as switch

225

02:53:25

Hey, can someone help? For a projekt I am building a large lighting setup. (this is just a part that I need to wrap my head around) 
I am using 3W high power LED with 750ma forward current but I don't get why I can not get them to full utilization. 
Is the limiting factor the bjt ?

published 3 years ago

SuperFaradCapacitor

3 years ago

It's possible. The current that runs through the collector to emitter is equal to the beta (around 100) times the current running front the base to emitter. If too much current runs through either path, the transistor will blow. The transistor also has a 0.7 v voltage drop across the base-emitter and collector-emitter (when it's saturated) path. I don't know if I'm right about this, but the saturation current through the collector-emitter path might be lower than the current required to fully light the led. I would recommend using a mosfet. (although they are somewhat more complicated)

Jarmo

3 years ago

https://everycircuit.com/circuit/5630377902997504

Jarmo

3 years ago

I a newbie in electrical stuff but could the way i rearranged it work for you?

faceblast

3 years ago

yes. the transistor has a resistance of 2Ω. three 3.5V leds in series is 11.5V, so 0.5V remains at the top of the transistor. 0.5V / 2Ω = 250mA (ignoring the additional forward voltage across the transistor), so the max current you can pull through that particular transistor will be around 250mA. you can use better transistors or fets, or less diodes in series, but you'll need to limit the current to 750mA another way.

Robert_Kidd

3 years ago

Note that in the bottom row of LEDs the leftmost has not been set to 750mA, it’s the default 20mA.

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