# Size of Resistors

Discussion in 'Electronic Basics' started by GraemeC, Aug 25, 2006.

1. ### GraemeCGuest

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

2. ### ChrisGuest

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

3. ### Jonathan KirwanGuest

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

Jon

4. ### PeteSGuest

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
http://www.fairchildsemi.com/sitese...temap+id&ia=1&text=QTLP690C+&as=1&render=1&w=

Please tell us the application requirements

Cheers

PeteS

5. ### GraemeCGuest

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.

6. ### Jonathan KirwanGuest

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
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