Electronic – Vce too high. Help

lednpntransistors

I have been working on a high-powered LED light bar.
The design powers them almost directly from the mains, but limits current using a 555 and beefier NPN. Below is a simplified schematic.

Obviously the LED resistance changes with several factors, and I have blown out a few, but that was due to soldering issues on my part. Essentially I know the circuit works. The only hold-up is that I have tried two different NPN transistors (salvaged from old CRTs) and both end up with Vce voltages around 35V causing them to heat up rather quickly since the current is around 1 amp. I will link the datasheets for the two in below. I have measured the output of the timer at 2V RMS and set the resistor values for each transistor according to their Ib vs Vce graphs (around 0.2A) since neither include a Beta value. My question is: how do I bring the Vce down to around 5V or lower?

Fuji C3866 | Toshiba 2SC5386
https://drive.google.com/folderview?id=0B4n242xWd2TBOUVBZF9uQ1BDc2M&usp=sharing

EDIT: Removed terrible CAD, added hand drawn circuit
These are 10W, 12V LEDs

enter image description here

Best Answer

You can't bring Vce down - you are operating it with pulse width modulation therefore the transistor is either fully-on of fully-off. What you may measure is the average Vce.

And now to the main issue - you are not controlling the current it seems from your circuit diagram - there is nothing to stabilize the PWM duty cycle - there is no current feedback hence it is open-loop voltage control.

The next issue is you need an inductor and capacitor to filter the "strong" square wave delivered by your switching transistor - the peak voltage across the LEDs will be basically rectified AC and this could kill them unless there is internal current limiting but, if there is then why both with this ornate solution?

Bad, bad circuit it seems. Take a look at this: -

enter image description here

Note the inductor L1 in series with the LED string and note Rsense to detect current and control it.