Size of Resistors

Hi, Graeme. Try small wirewound resistors -- they're commercially available down to 0.1%. They cost a little more, but they're remarkably durable, and also have a lower temperature coefficient of resistance.

Good luck Chris

I've only just begun to look at the arrangement. You have 14 BJTs in order to provide a constant current into an LED!?? Can you explain a little more about why?

Jon

I ditto this reponse.

If you are trying to provide 250mA into an LED as a constant current (which seems awfully high), it can be done with only a single transistor and a few other parts.

Apart from that, the part you specify (QTLP690C) is listed as obsolete

Please tell us the application requirements :)

Cheers

PeteS

I can confirm that the 250ma is correct

The QTLP690C is not the device that will be used. I just added it to the design for illustration. I need to drive the LED at a range of of constant currents to check the beam profile.

My design is two bootstrapped current sources, hence having to use

14BJT.

I accept that I might be over complicating the issue but I can't seem to find anything in the literature.

thanks for the help so far.

g PeteS wrote:

Not that much help, so far. I already could see the two current sources, in parallel. What I didn't understand are some things you still haven't discussed. If you want 250mA into an LED, how important is the accuracy, precision, and drift over temperature and time? What about information about the power supply? You are using a test voltage that ranges and it's not possible to tell what you intend, here. How critical is the voltage threshhold? Over what voltage may the transition from 0mA to 250mA take place? How 0mA must it be below this point?

It's hard to know if you are over-complicating. But it would help to know why you put the pieces in that you did and what your design thinking is. My own take, from the fact that you include a SINE voltage source, is that you are looking at power supply variations against diode current. On that score, I don't consider it all that good. The current source impedance is only some 700 ohms with typical

3904's and 3906's substituted and working into a simple LED model and requires some three times as much current to operate it as the LED requires. I can only imagine the heating going on and I think better can be done with a lot less. Transition spans about 400mV. That may be just fine. Or not.

You are asking for others to suggest some thoughts about a particular design you have in mind. I'm not familiar with the details, as I haven't tried to work through them in each half-section and don't really want to try with so little information. You haven't disclosed your work and thinking on it, either. If someone else has already worked through this particular design, you are lucky indeed and may get some excellent comments. But with no clear information from you about what you are trying to achieve, and otherwise in unfamiliar territory with your design and no information from you about why you did what you did to help me think about it, it's hard for me to either recommend something else or find the time to try and fathom your own thinking when you won't just write it out on your own.

You also haven't said anything about cost or using parts that are designed to be current sources for driving LEDs or LED displays. Some may be a good fit without design time, but I'm just a hobbyist so I probably cannot recommend them. I'm sure others can.

I wish you'd written more so I could have written less. But there it is.

Jon