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Topic: Single transistor constant current power led driver. (Read 4021 times) previous topic - next topic

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?

Boffin1

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


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?

dhenry

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.

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

fungus


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.
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

dhenry

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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.

Erni

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


Grumpy_Mike


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

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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.

fungus


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.
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

Grumpy_Mike

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.

Erni

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

fungus


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.


No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

Erni

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One Watt?


Yes 350 mA  - 3V

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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

dhenry

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I use a BC337 and to LED's on basis


Not really sure what that means.

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Are these values as expected ?


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

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