# Trying to understand this circuit

#### scratchy123

Joined Jun 21, 2019
7
Hello everyone, im pretty new at electronics so if this is a dumb question please explain it to me.

Im trying to understand this power source circuit, my only problem is that I dont really understand it. Why are there 2 transistors and where are those 2 resistors for? All I know is that its supposed to be a power source for an individual LED.
I hope anyone can help me, would be appreciated!

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#### ericgibbs

Joined Jan 29, 2010
9,138
hi 123,
It is a circuit that provides a CC [Constant Current] thru the LED.
Can you describe how you think it achieves that.?
As this is Homework have a go at explaining the action.
E

#### scratchy123

Joined Jun 21, 2019
7
hi 123,
It is a circuit that provides a CC [Constant Current] thru the LED.
Can you describe how you think it achieves that.?
As this is Homework have a go at explaining the action.
E
I think it's if an amount of voltage increases, a current will flow through the collector of Q4A which makes it to the base of Q4B? And then something will happen but I dont really know what which is why im here

#### ericgibbs

Joined Jan 29, 2010
9,138
hi,
As the current thru R46 & the LED D3 increases, so does the voltage drop across R46.
When the voltage across R46 reaches the turn on Base voltage of QB4, it conducts.
As Q4B conducts it pulls down the voltage on Q4A Base, this tends to turn off Q4A and so the current thru Q4A and the LED reduces.

What do you think happens if the current thru the LED start to decrease.??

E

#### Jony130

Joined Feb 17, 2009
5,094

#### MrAl

Joined Jun 17, 2014
6,838
Hello everyone, im pretty new at electronics so if this is a dumb question please explain it to me.

Im trying to understand this power source circuit, my only problem is that I dont really understand it. Why are there 2 transistors and where are those 2 resistors for? All I know is that its supposed to be a power source for an individual LED.
I hope anyone can help me, would be appreciated!
Hi,

Also, as you may already know, you can build a constant current circuit with just one transistor. So why two transistors? There's a good reason for it see if you can figure it out.
I actually independently designed this very circuit for a commercial flashlight several years ago. I dont know if the owner took out a patent or not though it's been years now.

#### scratchy123

Joined Jun 21, 2019
7
hi,
As the current thru R46 & the LED D3 increases, so does the voltage drop across R46.
When the voltage across R46 reaches the turn on Base voltage of QB4, it conducts.
As Q4B conducts it pulls down the voltage on Q4A Base, this tends to turn off Q4A and so the current thru Q4A and the LED reduces.

What do you think happens if the current thru the LED start to decrease.??

E
Hello,

thanks for the help, i understand mostly everything but I still dont know what R46 is for exactly? would you mind explaining this

#### ericgibbs

Joined Jan 29, 2010
9,138
hi,
R46 the 47R resistor generates an Emitter voltage.
Example:
I guess you know that the Base to Emitter turn on voltage for a NPN transistor, is approx 0.65v, lets say 0.7V .

So the current thru the 47R to generate 0.7V across it would be: I = V/R ...... I = 0.7v/47R = 14.89mA ,, lets say 15mA,
that's how much current is flowing thru the transistor and LED.

If the current thru the 47R increases then Q4B Vbe will increase and start to turn it On and reduce the Vbe on Q4A, so the circuit is self balancing, due to the 'negative' feedback from the 47R current.
So the LED current will be steady around 15mA.

OK.?
E

#### MrAl

Joined Jun 17, 2014
6,838
Hello,

The 47 Ohm resistor senses the current so the circuit can regulate it to a somewhat constant value.

I may have misquoted before though this may not be the circuit i thought it was at first. Thus the reason for the two transistors rather than one may be and probably is different than the circuit i did. For this circuit, just guessing it looks like the reason for the second transistor here is to keep the sense voltage lower than a one transistor constant current circuit, and it is very unlikely that there was any attention to current changes due to temperature fluctuations. Is the temperature profile worse than the one transistor circuit? We could investigate.