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[on:]

Hi all.

I am trying to control some LED downlighters (3w). I had the circuit working from arduino PWM pins with BC337's nicely, but I have now got hold of some TLC5940 IC's so want to use them. Because they sink current I had to get some BC327's instead.

Not sure why but the BC327's are getting very hot? I'm switching about 350mA through them (same load as BC337's) and have the base connected through a 1k resistor to one of the pins on the TLC5940  (the TLC is set as constant current drivers 20mA on all pins). I have also tried other values to current limit from the base, 1K does not seem to be fully saturating the transistor, and lower than 1k the things gets super hot!

I hooked up the same circuit using both the BC337 & 327, obviously driving the base high or low according to the transistor, and same behavior, the 327 getting very hot and the 337 staying cool?

Really confused about this at the moment!

[Reply #1 on:]

Whereas you were using NPN (337), you're using PNP (327) instead?

Schematic.

[Reply #2 on:]

Yes that's right, I was doing digital and analog writes from the arduino pins using the NPN's and it was working OK, the TLC5940 works fine too when sinking 20mA from LED's directly.

Attached is a schematic of how I have connected it

[Reply #3 on:]

That's not the way you hook up a PNP
This is

[Reply #4 on:]

Right.
And.. does the TLC output ever get up to 12V?

[Reply #5 on:]

He needs to rethink this after reading the data sheet The TLC5940 sinks power only can use pwm for fading to look at this

and think about flow of current

[Reply #6 on:]

Thanks, just corrected the wiring error, still the same issues though

getting red hot. Measuring the TLC output  between the 12v rail, when set to 4095 its nearly at 12v - about 11.5v, set to 2000 the pin is about 5v?

[Reply #7 on:]

The TLC outputs are open-collector, open-drain? 

(Elaborate.)

[Reply #8 on:]

You hooked them up backward, they may be bad now and two the TLC5940 should not be hooked to the base your led should be hooked to the Tlc pins to sink current and the collector of the PNP

The TLC can only float it's pins which is going to heat up them PNP. There not push pull  there open-collector

[Reply #9 on:]

LED downlighter (3W).

Are those supposed to run from a current regulated source?

[Reply #10 on:]

The hole setup is wrong  really.

[Reply #11 on:]

Hole thing?  PNP, majority carrier - holes?

Still, these LED downlights, they need a regulated current, not a straight shot between Gnd and 12V ?

[Reply #12 on:]

Sorry but I'm a little lost now. How should I be connecting it? I thought that instead of the TLC5940 sinking current through an LED it could sink through the base of the PNP?

[Reply #13 on:]

Can you post a link  to your leds and are you wanting to use PWM for dimming?

[Reply #14 on:]

I would connect them like this and set the TLCs to 25mA or greater to make sure they saturate. But we really need to known the details of the LEDs that you are trying to control.