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Author Topic: Single transistor constant current power led driver.  (Read 3169 times)
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Denmark
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Not really sure what that means.
a missing W

I use a BC337 and two LED's on basis (from base to ground)

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No. If you have the circuit properly configured, you should see very little variance on the collector current.

That was what I expected too
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Not sure what leds those are: they can render the circuit not working, depending what they are.

If you look into how the circuit works, it requires that those diodes (or leds) are sufficiently conducting when the drive goes high. That means that combined, those diodes cannot drop more than 5v (from a 5v arduino), and the resistor should be sufficiently small to allow the conduction.

It also requires that the collector sits higher than the base so the supply voltage - voltage on your load is higher than the base.

You will find that lower voltage drop over the diodes works better. I would have used just one led - two leds add no value in their circuit.
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You probably need a lower value resistor between 'drive' and 'base' or the transistor can't turn fully on.

Ok, I wil try. In the above example my base current is 1.6, 2, 5 mA, with a 1kOhm resistor, so I am in OutputPin safe area

Thanks for your answer
[/quote]

Also...grab a multimeter and check that the voltages on collector, base and emitter all make sense.

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It also requires that the collector sits higher than the base so the supply voltage - voltage on your load is higher than the base.

Doesn't that make it useless for 5V supplies and most LEDs?

eg. If a LED needs 3.6V (blue/green/white) then there's only 1.4V left over for the transistor/resistor...not enough!


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OK I tried:
Diodes on base: 2 x 1N4148
Resistor on base: 330 Ohm
Resistor on emitter: 10 Ohm

This setup give a collector current almost independent of the suply voltage:

Supply Voltage /Collector current

3.8V            78 mA
7,6V            89 mA
11,4V           91 mA

The base current is 9,5 mA
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This is what I got:

3.8v / 61.2ma
7.6v / 61.4ma
11.8v/ 61.6ma
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There are chips that are constant current drive for LEDS, usually for long strings of LEDS or those running on a 12 volt supply, usually they are switching types and so more efficient. Look up LED drivers.
Using a transistor as a constant current device works well if the voltage is high enough compared to the load. Also the higher gain device the better. BTW You could use the Arduino to provide a constant current by including it in the control loop. This would allow you to try different currents without rewiring. It will be a PWM output with a analog input from a resistor in series with the LED (S). You will need to put a filter ( resistor /cap ) at the analog input.
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Using a transistor as a constant current device works well if the voltage is high enough compared to the load.

That depends on your definition of "well": the higher Vcc vs. forward voltage drop on the leds, the lower the efficiency of such a circuit.
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What?? Are you Gibbering about now? I see your "Idea" about a voltage source to control current worked well.
It can't work... Unless your Voltage source... can sink current too...
Btw the current source using a single transistor was a novel means to control current, Exactly as drawn.

Bob
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Sorry I didnt have this thread marked to notify me.

The circuit I drew assumed 20 mA  LEDs for the resistor values I gave as an example.  With high current ones you will want an emitter resistor of 0.6 / the LED current,  as you found out.  Say  1.8 ohms

The base resistor will need to be able to supply the LED current divided by the gain ( hfe ) of the transistor, which for a BC337C is typically 400, so would require only 1mA base current for your 350 Ma LEDs, say 5mA from the arduino to leave some flowing through the reference LED.  ( one red LED at 2v is fine, and the temperature change will tend to cancel out the transistors change of gain with heat.)

So the base resistor should be no more than  3v / 0.005  =  560 ohms.
« Last Edit: November 18, 2012, 02:03:34 am by Boffin1 » Logged

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Thank you for the clarification Boffin1

I tried with a red LED and 12 Ohm emitter resistor which give me 350 mA collector current. I am not sure how to calculate it.

I think you always have to do some experimentation regarding the base resistor as the Hfe has a large spread, and depends on the collector current

From the datasheet:
 
hFE           
VCE=1V, IC=100mA  100 - 630
VCE=1V, IC=300mA  60

It is a very nice circuit. Earlier I have used LM317 for this kind, but I have a lot of BC337's so this is a cheep solution.
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I think you always have to do some experimentation regarding the base resistor as the Hfe has a large spread,

You don't have to: the only essential assumption here is that Vbe is constant. Once you have that, the base resistor only needs to be sufficient for the two diodes (or your voltage reference) to be conducting.

Most of the times, something like 330ohm is sufficient.
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@ dhenry...
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This is what I got:

3.8v / 61.2ma
7.6v / 61.4ma
11.8v/ 61.6ma
That must be one phenominimal transistor you tested? with... to vary 400 uA Ic over a 3 to 1 load range...
thats .0004/.062 = .645%... or is it bad data...
Because it is Much better than Many Commercial CC drivers.
The Data posted by Erni is good and fairly representative of a high beta transistor driven within it's limits however yours is an order of magnitude better.
I think you are on to something there with great commercial promise.
Or you are just "On" something which seems more likely.

Bob
« Last Edit: November 18, 2012, 04:48:55 pm by Docedison » Logged

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