2-PNP laser driver (easy!)

[wall]

Once again, sorry for asking so many questions, but, what is the power dissipation over the 1K and Rsense (2.2ohm in this case) from a 4.2V input?

 

[Benm]

Don't worry about the 1k resistor - it will only dissipate a small amount of power (20 mW perhaps). The sense resistor chosen to be 2.2 ohm will take the full output current of rougly 300 mA at 0.7 volts, about 200 mW. This is no problem for an ordinary 1/4 W resistor, though it will get noticably warm (thats what they look like to me from the picture).

 

[wall]

I think both resistor from that picture are 1W and the Rsense is 2.7 ohm. It was just for testing, I've built another driver with a 1W 2.2ohm Rsense and a ¼W 1K, it outputs 270~280mA.
The caps I used on both driver are 35V 10uF but I also have some 50V 22uF caps of the same size, would it make a difference?

 

[Benm]

The output capacitor will not matter, there is no need to make it larger.

The only reason its there is that i've seen this circuit produce a smalla current spike on turn-on without it. 10 uF is more then enough to arrest that spike without resulting in a voltage rise that is dangerous to the laser diode.

If you want, it is also possible to put a capacitor over the BC557's C-E pins to get a soft-start effect.

 

[wall]

I've done another driver for the third time, now with ¼W resistors and everything bent and trimmed to fit in a 14mm cylinder with less than 20mm of height, but I used a 50V 22uF capacitor.

10 uF is more then enough to arrest that spike without resulting in a voltage rise

Does that mean a capacitor with a larger value can result in a voltage rise?

Here is the third driver I built:

 

[Benm]

Using a larger capacitor will only delay the time it takes for the driver to reach full output, but with these values this delay is so short it feels like it is instant, doesnt matter if you use 10, 22 or 100uF.

Bigger capactiors don't do anything bad in this circuit, they are simply a waste of space.

Note that you must power off the driver before connecting the laser diode though, as with most drivers it will build up a large voltage (almost equal to supply) when operated with no load connected. You can safely discharge the output caps by just shorting the output leads.

 

[wall]

Thank you for all your help, I really appreciate it.
Both 35V/10uF and 50V/22uF caps are the same size.
Now I just need to wait for the module from dx, it has been 70 days since it was shipped.

 

[Benm]

It's odd that they are the same size, but then again, there are certain common form factors for these things, even though they could be made smaller if need be. If their size is bugging you, you can use a smaller one like 4.7uF/16v without any problem.