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Designed a 10watt LED driver...

cjdelphi

Oct 26, 2011
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with a bit of tweaking, this should drive a 10watt LED of 12v, a couple of pots might be a better idea in real life.

But this circuit has current limiting, and will limit the LED to less than 800ma.
 

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(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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And do you have a question?

I do!

Why is the circuit so complex? Wouldn't the standard 2 transistors constant current circuit work just as well?

Are there other design criteria you have?

How does one calculate the current limit here?

What does that middle transistor do. It seems to me that if I just removed it, the circuit would operate as I expect (and I could lose a couple of resistors as well).

I guess this all boils down to a request for an explanation of your circuit.
 

cjdelphi

Oct 26, 2011
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Under the 2 version transistor circuit, the sense resistor will trigger, which turns on the first pulling the 2nd (that drives the LED) to gnd reducing current.

That works great for led's less than a watt, problem arises when applying to high powered LEDs, as the voltage rises the current will, the 2nd transistor and sense resistor is unable to pull the LED transistor to gnd while supply high enough current for a high powered LED ..

(I can show you that in a simulation it's easier lol)

So the PNP transistor (3rd transistor middle of the 3) why use it... the base of the pnp sits on the collector, higher the voltage the less saturated the middle (pnp) transistor is...

That means the base of transistor 1 is solely dependent on the sense resistor to limit the current to the 1st transistor (as normal in the 2 transistor combo)

So then lets turn up the voltage, 30v.

The sense resistor now shows over .7v and transistor 1 fires bringing the collector voltage down, because the emitter now has a higher voltage on the middle pnp transistor, it's now starting to saturate...

The pnp middle transistor then dumps its load out to it's collector, this in turn saturates transistor 1 bringing the base of the 3rd npn firmly down to gnd turning off the LED.

In a 2 version current limiter it can't reduce a high powered led's current but will fine for less than a couple of hundred milliamps...
 

cjdelphi

Oct 26, 2011
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Take picture 1.

To get the LED bright a much lower resistor value is required to drive the base (a darlington combo may work?)

This prevents the 2nd transistor to be fully turned off...
 

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(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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For high current applications, try replacing the pass transistor with a mosfet. The current required to tun the mosfet on is tiny so the problem essentially disappears. You may be advised to connect a zener between the mosfet's gate and source though.
 

cjdelphi

Oct 26, 2011
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Hope you understand the circuit now....

(Not just asking for the hell of it)
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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No, I was not asking for the fun of it.

I've not reviewed the circuit again, but I will in the light of your comments. I'm still of the opinion that it's overly complex, but I'll have a look at your simulations as well. I'm also not 100% convinced by your argument. But I have an open mind.

As you can see, I'm a little undecided...
 
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