Constant Current Source again!

Yes, but you haven't told us what they are (speed, cost, stability, accuracy, or any other considerations you have).

James Arthur

Seems pretty simple to me. Then again you are posting from Google and have a Gmail account...... and also mention the Wiki......

Then you say....

"I've posted on this before and I've never really got it resolved."

So perhaps you are looking for something 'more' than a constant current source.

As you say.....

"I need to create an adjustable constant current drive that can supply between 150-300mA to drive an experimental LED. I only have a 5v supply to play with."

Again... seems quite simple to me..... so I am left thinking you are hiding something and perhaps you are doing it deliberately!!!!!! I mean.....

Can we expect that later on you will inform us that this has to be supplied from the auxilliary winding of a multiphase multisource power supply pack for use in an SLICBM and..... whoopsy, you meant to ask for one of those as well. Plus it turns out that the experimental LED is part of the on board high energy anti-anti-missile tracking system and you also want to pulse the laser whilst tracking on target with the facility for multi targeting as the MIRVS go down whilst the mother delivery system remains in orbit?

Or are you just going to ask for a programmable current source and change your mind as time goes by for a really nice long thread?

DNA

The simplest may be good enough:

A PNP transistor, emitter hooked to a 2 ohm resistor, other end of resistor to a 10 ohm pot, 2Watt, other end of pot and wiper hooked to

+5

base hooked to the cathode of a 1N4007 diode, anode hooked to another

1N4007 cathode. anode of second diode hooked to +5. Base also has a 330 ohm resistor to ground.

collector is your + side of the current source, ground is the - side. With the pot at maximum resistance, you'll get 100mA; when at lowest resistance, you'll get 300mA.

This should hold the current steady within 7.2% from zero to 3.14159 volts.

In message , dated Fri, 1 Sep 2006, GraemeC writes

How close to a constant current does it have to be? A 2N3055 will do it; put 3.3 ohms 2 W in series with the emitter and vary the base voltage with a 330 ohm resistor and a 100 ohm pot in series across your supply (330 ohms to +5 V). The current will stay pretty constant at the value you set, if you put a big heat sink on the transistor. The 3.3 ohms provides negative feedback to keep the current more constant than it would otherwise be.

An LED is a very low resistance device, so even though the 2N3055 isn't a brilliant choice for high output impedance, it makes a very simple circuit. You can go on to refinements later, if necessary. A power FET is probably the next step, but you only have 5 V to play with, so it needs care.

--
OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
2006 is YMMVI- Your mileage may vary immensely.

John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK

The OP hasn't posted what sort of LED he's using so we don't know what kind of compliance he needs.

Still, substitute an IR LED for the two 1n4007s and you'll get more headroom, plus temperature compensation.

Best, James Arthur

Ok, that was uncalled for.

In what way was it uncalled for?

DNA

In message , dated Fri, 1 Sep 2006, snipped-for-privacy@gmail.com writes

Almost all of what DNA contributes is uncalled-for, but when you get to SMPS and, as recently demonstrated, proximity effect, the slate is wiped clean.

--
OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
2006 is YMMVI- Your mileage may vary immensely.

John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK

What is the LED? Or, what are its simplified characteristics? A simple LED model uses a voltage and a resistance in an equation something like this:

V(diode) = 1.55V + 2 ohms * I(diode)

Or somesuch. Do you know what voltage is required for your LED, when driven at 150mA and at 300mA? Or can you at least say what voltage will be required at 300mA? How much less than 5V will cover it? (Stating the LED technology, at a minimum, would help.) Any precision/noise, drift over temperature and time, and accuracy specs? What about variation over supply voltage? How much supply voltage variation will there be to tolerate?

How will you vary the current? Manual knob? Computer control?

I don't think there is _any_ confusion about what a current source is.

It would help to know what you are trying to do, in detail. You had an opportunity to clarify these things earlier and didn't take that chance, as far as I'm aware.

Jon

What is the peak current the LED can handle and at what duty cycle?

Transcendentally compliant CS- too funny...

A simple prototype, made of simple and readily available parts, with maximum compliance, would look something like this: View in a fixed-width font such as Courier.

. . . +5V . | . .-------+------. . | | | . | [1K] | . | | |< . | +----| med pwr . | | |\\ PNP . | [330] | . | 100n | | . 50mV - 125mV .----|--||---+ | . | |\\| | | . Vin >-[10K]-+--|-\\ | o . | >------' | . .--|+/ ss OA V exp . | |/| - LED . [10K] --- | . | gnd o . | | . +-------------------' . | . [0.33]

Especially as it's supposed to be driving a load with a forward characteristic proportional to e ;)

Cheers

PeteS

Maybe juice that thing up a bit: View in a fixed-width font such as Courier.

. . . . . +5V . | . .-------+----+------. . | | | | . | | [1K] | . | | | |< . | [10K] +----| med pwr . | | | |\\ PNP . | | | | . | | [33] | . | | | | . | | | | . | 100n | |< | . 50mV - 125mV .----|--||---+--| 2N4403 | . | |\\| | |\\ | . Vin >-[10K]-+--|-\\ | | o . | >------' --- | . .--|+/ ss OA gnd V exp . | |/| - LED . [10K] --- | . | gnd o . | | . +------------------------' . | . [0.33]

The slate may be clean, but not Genome.

--
Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

In message , dated Sat, 2 Sep 2006, Michael A. Terrell writes

We only have his word for that, of course, and other things he writes aren't exactly credible.

--
OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
2006 is YMMVI- Your mileage may vary immensely.

John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK

And the pictures of his cooking don't look edible. ;-)

--
Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

the thermal behaviour is not very promising, since Hfe increases with T, as does Vbe decrease and also the Vf of the diode, all leading to an increase in Ic. I tried a simple compensation and it looked nice in the simulation:

.---------------+------- |470R...2k2 | .-. | | | | | |R1 LED V /_ '-' - \\ | Pot 1k | +-------. | | | | V | | - D1 .-. |/ | | | V D2 '-' | - | .-. | | | |2R2 | V D3 | | | - '-' | | | .-. .-. === | | | | GND | |100 | |100 '-' '-' | | === === GND GND (created by AACircuit v1.28 beta 10/06/04

R1 has to be chosen empirically according to the Hfe of the transistor. The range is from 20 to 330mA. The 3 diodes have to be mounted in thermal contact with the transistor.

--
ciao Ban
Apricale, Italy

Thanks for all help everyone

g