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Using Arduino => LEDs and Multiplexing => Topic started by: Inevitableavoidance on Nov 14, 2012, 03:38 pm

Title: Single transistor constant current power led driver.
Post by: Inevitableavoidance on Nov 14, 2012, 03:38 pm
I've seen a lot of constant current led driver circuits, however i've never seen anyone using just one transistor.

With the right resistor at the base of a transistor, you can determin the current flowing through it, right?
Why does it not seem possible to make a constant current power led driver this way?
Title: Re: Single transistor constant current power led driver.
Post by: John_Smith on Nov 14, 2012, 03:56 pm
heres a circuit which I have used, if you use an LED in place of the 2 diodes, the temperature coefficient of the transistor tends to cancel out.
You just adjust the value of the emitter res to set the current through the LEDs which can be run from an unregulated supply

(http://i.imgur.com/C3MTx.jpg)
Title: Re: Single transistor constant current power led driver.
Post by: Inevitableavoidance on Nov 14, 2012, 04:24 pm
Does the "33R" get the entire current from the led's through it?

If so, i'll need a high wattage resistor for that, right?
Title: Re: Single transistor constant current power led driver.
Post by: dhenry on Nov 14, 2012, 05:43 pm
All you need is a votlage reference - that can be diodes, other transistors, zeners, true voltage references, or even a resistive divider (not as good).

Sometimes people use leds as voltage references too.
Title: Re: Single transistor constant current power led driver.
Post by: Inevitableavoidance on Nov 14, 2012, 11:38 pm
To be honest, i don't quite get what the schematic, and the voltage reference does, could you explain it to me?
Title: Re: Single transistor constant current power led driver.
Post by: fungus on Nov 15, 2012, 12:17 pm

To be honest, i don't quite get what the schematic, and the voltage reference does, could you explain it to me?


Let's see if I get it (I'm not the best at electronics so I might be wrong...):

The transistor's resistance depends on the difference between base and emitter (Vbe). In this circuit the emitter isn't connected to ground so the voltage difference depends on the ratio of LED resistance to emitter resistor (they make a voltage divider). ie. Vbe isn't 1.4.

If I raise the supply voltage, the voltage at the transistor's emitter goes up, Vbe goes down. The transistor's resistance increases, the LED current goes down.

If I lower the supply voltage, the voltage at the transistor's emitter goes down, Vbe goes up. The transistor's resistance decreases, the LED current goes up.
Title: Re: Single transistor constant current power led driver.
Post by: dhenry on Nov 15, 2012, 12:51 pm
Quote
i don't quite get what the schematic,


The two diodes function as a voltage clamp. When drive goes h igh (sufficient for the diode to conduct), the base is held at approximately 1.4v (forward voltage drop for the diodes). So the voltage on the 33r resistor is 1.4v - .7v(the transistor's Vbe) = 0.7v. So the current goes through the transistor and the led must be 0.7v/33 = 20ma. Give or take a few.

Essentially it is using the diodes as a voltage reference. As such, you can replace the diodes with other voltage references, like a zener, or a led, etc.
Title: Re: Single transistor constant current power led driver.
Post by: Erni on Nov 15, 2012, 01:59 pm
That worked well.

I made a little experiment:
I used 2 LED's on the base and a 330Ohm resistor on the emitter.
With a LED on the colecctor I measured 0.9 mA
With a light bulb (4,8V 0,5A) I measured 1.0 mA

Title: Re: Single transistor constant current power led driver.
Post by: Grumpy_Mike on Nov 15, 2012, 02:52 pm

Does the "33R" get the entire current from the led's through it?

If so, i'll need a high wattage resistor for that, right?

Yes it does, you will need to work out how much power the resistor dissipates depending on the current, I squared R

Quote
i don't quite get what the schematic,

Think of it like this. The 33R resistor drops a voltage according to how much current is flowing. This in turn raises up the voltage on the emitter and throttles back the base current until an equilibrium is reached. If more current flows through the LEDs, because they heat up, then there will be more current through the 33R and the voltage across it would like to rise but this turns off the transistor a bit more and so in effect nothing happens.
The resistor is acting as a feed back mechanism for the transistor and thus supplying a constant current in the light of any changes in the load. The value of this constant current is controlled by the size of this resistor. You want more current, you make that resistor smaller so more current has to flow through it in order to develop sufficient voltage to throttle off the transistor.
Title: Re: Single transistor constant current power led driver.
Post by: fungus on Nov 15, 2012, 03:05 pm

The resistor is acting as a feed back mechanism for the transistor and thus supplying a constant current in the light of any changes in the load. The value of this constant current is controlled by the size of this resistor. You want more current, you make that resistor smaller so more current has to flow through it in order to develop sufficient voltage to throttle off the transistor.



So...if I know what the 'drive' voltage is (eg. 5V for an Arduino output pin) I don't need the diodes between the transistor base and ground. I can replace them with whatever resistor gives me 1.4V at the transistor base?

That would give me a constant current with three resistors and a transistor.
Title: Re: Single transistor constant current power led driver.
Post by: Grumpy_Mike on Nov 15, 2012, 06:47 pm
You could but it would not have the degree of stability that the diodes have with the voltage being independent (ish) of the current through them.
Title: Re: Single transistor constant current power led driver.
Post by: Erni on Nov 16, 2012, 03:32 pm
I have one more question to Boffin1's schematic.
I have tried with a emitter resistor at 5 Ohm and different supply voltages, and I get these results

Supply Voltage  Collector current mA
3.8      280
4.5      310
7.6      450

I use a BC337 and to LED's on basis

Are these values as expected ?
I was hoping for a constant current independent of voltage to drive my 1W LED's
Title: Re: Single transistor constant current power led driver.
Post by: fungus on Nov 16, 2012, 04:37 pm

I use a BC337 and to LED's on basis

Are these values as expected ?
I was hoping for a constant current independent of voltage to drive my 1W LED's


One Watt?

You probably need a lower value resistor between 'drive' and 'base' or the transistor can't turn fully on.


Title: Re: Single transistor constant current power led driver.
Post by: Erni on Nov 16, 2012, 05:29 pm
Quote
One Watt?


Yes 350 mA  - 3V

Quote
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
Title: Re: Single transistor constant current power led driver.
Post by: dhenry on Nov 16, 2012, 05:47 pm
Quote
I use a BC337 and to LED's on basis


Not really sure what that means.

Quote
Are these values as expected ?


No. If you have the circuit properly configured, you should see very little variance on the collector current.
Title: Re: Single transistor constant current power led driver.
Post by: Erni on Nov 16, 2012, 06:08 pm
Quote
Not really sure what that means.

a missing W

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

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


That was what I expected too
Title: Re: Single transistor constant current power led driver.
Post by: dhenry on Nov 16, 2012, 06:48 pm
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.
Title: Re: Single transistor constant current power led driver.
Post by: fungus on Nov 16, 2012, 07:14 pm
Quote
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.

Title: Re: Single transistor constant current power led driver.
Post by: fungus on Nov 16, 2012, 07:27 pm

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!


Title: Re: Single transistor constant current power led driver.
Post by: Erni on Nov 17, 2012, 04:36 pm
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
Title: Re: Single transistor constant current power led driver.
Post by: dhenry on Nov 17, 2012, 11:10 pm
This is what I got:

3.8v / 61.2ma
7.6v / 61.4ma
11.8v/ 61.6ma
Title: Re: Single transistor constant current power led driver.
Post by: themotorman on Nov 18, 2012, 02:56 am
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.
Title: Re: Single transistor constant current power led driver.
Post by: dhenry on Nov 18, 2012, 03:03 am
Quote
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.
Title: Re: Single transistor constant current power led driver.
Post by: Docedison on Nov 18, 2012, 06:20 am
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
Title: Re: Single transistor constant current power led driver.
Post by: John_Smith on Nov 18, 2012, 08:01 am
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.
Title: Re: Single transistor constant current power led driver.
Post by: Erni on Nov 18, 2012, 12:00 pm

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.
Title: Re: Single transistor constant current power led driver.
Post by: dhenry on Nov 18, 2012, 10:36 pm
Quote
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.
Title: Re: Single transistor constant current power led driver.
Post by: Docedison on Nov 18, 2012, 10:42 pm
@ dhenry...
Quote
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