Vbe of 2n3904 in led driver circuit

[Kenneth Tan]

ok. Perhaps just file that away as best practice for next time.

Another alternative is to test the circuit using a power supply capable of current limiting the output. Setting the current limit to (say) 20% more than the expected current will also provide you with a safety net.

Steve, now I understand. Of course. That how I test my leds everytime.
I've build this mobile PSU using 4x 18650 Lipo's in series connected to a digital PSU (ebay) and a fixed 3.3, 5V and 12V buck. With the digital PSU I can control voltage and current. That how I tested the COB leds for their best-max current/heat for this project.

A few connectors, a SPST switch, a 18650 tray and a lot of hot glue and I was done!

http://www.ebay.nl/itm/LM2596S-DC-D...674?pt=LH_DefaultDomain_0&hash=item1c5419516a

http://www.ebay.nl/itm/DC-To-DC-Con...153?pt=LH_DefaultDomain_3&hash=item3377dad4e9

 

[(*steve*)]

Excellent, you're one step (or more) ahead of me

 

[AnalogKid]

LOVE the 2596 module. Great find!

ak

 

[Kenneth Tan]

LOVE the 2596 module. Great find!

ak

It's great for testing with more (digital) voltage/current control.
It has it's downsides where the voltage often collapses at the higher end as it cannot supply enough current.

The unit has 10 memory locations where you can save/load voltage/current settings.
Als you can monitor power usage.
And it's cheap!

Excellent review here from Julian Illet:

 

[Kenneth Tan]

Just finished testing the perfboard version. Works like a charm.
Vbe is 0.65V with PWM @100%, and it drops below 0,1V at <10% duty cycle.

Made a schematic with diptrace and PCB.
Before anyone asks, why I use 3 pin connectors, I just bought them at a very low price on ebay.

 

[Kenneth Tan]

Today tested the flexibility of the schematic and changed the leds for a 9V 100mA.
Calculated the current limiting resistor
0.68V/100 mA =6.8Ohm.
Replaced it and it worked perfect.
Pwm does the rest.