# Size of Resistors

• posted

I'm a digital disginer who is having to some some pesky analog stuff.

I've been desiging some circuits using pen and paper and a little with spice. As far as I can tell the designs theoretically work.

I've designed a a constant current source using npn and pnp transistors. My only concern is that im using resistors in the 1-10ohm range.

I need to do this because I need a high output (around 250mA) and only have a small supply (+/- 5v)

If I build this am i likely to encounter problems. Will the tolerance of the resistors make it difficult to control. If so does anyone have any solutions.

G

The spice netlist is below if anybody is interested

This circuit provided a constant current through the load LED (D1) when Vi1 is greater than approx 3v The value of the current is dependent on R2 and R5

V1 V+ 0 SINE(0 5 100K) R1 V+ N005 1k Q2 N003 N001 V+ 0 PNP Q3 N001 N001 V+ 0 PNP Q4 i N001 V+ 0 PNP Q1 V+ N005 N003 0 NPN Q5 N005 N005 0 0 NPN Q6 N003 N007 0 0 NPN Q7 N001 N003 N007 0 NPN R2 N007 0 10 R4 V+ N006 1k Q8 N004 N002 V+ 0 PNP Q9 N002 N002 V+ 0 PNP Q10 i N002 V+ 0 PNP Q11 V+ N006 N004 0 NPN Q12 N006 N006 0 0 NPN Q13 N004 N008 0 0 NPN Q14 N002 N004 N008 0 NPN R5 N008 0 10 D1 i 0 QTLP690C

• posted

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

• posted

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

• posted

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

• posted

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:

• posted

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.