How to reduce output current of a constant current LED driver?

Andy_C_

Joined Sep 24, 2016
11
Hi all,

I recently bought a solar lamp circuit from AliExpress. The board requires three peripheral components - a li-ion battery, a solar cell, and an LED. The board has a constant current output to the LED of 300mA (output voltage of 2V - 3.3V).

I'd like to use it to drive about 60mA to an LED, and so I need to reduce the board's 300mA output. I know that the simple option is to use a current limiting resistor, but this would be inefficient (power would be wasted in the resistor). So, I'd like to try to modify the circuit board so that it produces a lower output current.

I've created a circuit diagram of the board (although many of the components don't have identifying markings), and I've taken photos of the board and attached them.
Can anyone figure out which components supply the output current, and how to change the current? (I'm assuming that one of the board's resistors is used to set the current).

Andrew.

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Audioguru again

Joined Oct 21, 2019
6,615
R6 limits the LED current because it is in series with it.

Alec_t

Joined Sep 17, 2013
14,227
If R6 is 30mΩ (?) it is the current-sense resistor, so increasing its value by a factor of 5 would reduce the constant current by that same factor (to give you ~60mA).

Andy_C_

Joined Sep 24, 2016
11
Hi Audioguru and Alec_t,
Yes I see what you've each said about R6 - it limits the current to the LED. I reckon its value is 30Ω (component has "030" printed on it), which is a relatively low resistance. It seems inefficient for Q5 to product lots of current and then waste a lot of that current by dissipating it in a beefy series resistor? (R6 is physically larger than the others on the board).
I'm not particularly familiar with current limiting circuits, so my understanding may be a little off, but I'd rather reduce the LED current by reducing the base current to Q5. However, I don't want to go playing around with Q5 without understanding the how that would affect the rest of the circuit.

What are your thoughts on how to reduce the output current as efficiently as possible?

Many thanks!

crutschow

Joined Mar 14, 2008
34,055
What are your thoughts on how to reduce the output current as efficiently as possible?
Whether you use a series resistor or transistor to limit the current, the power loss (and efficiency) is the same.
To increase the efficiency you would need to use PMW with an inductor.

Andy_C_

Joined Sep 24, 2016
11
R6, I reckon its value is 30Ω (component has "030" printed on it)
I meant to say 3Ω for 030.

Andy_C_

Joined Sep 24, 2016
11
Whether you use a series resistor or transistor to limit the current, the power loss (and efficiency) is the same.
To increase the efficiency you would need to use PMW with an inductor.
OK, thanks.
So make R6 larger, or add another series resistor, is the way to do it.

MisterBill2

Joined Jan 23, 2018
17,848
I suggest experimenting with the value of R5, since it looks like the setpoint resistor for U1, the current regulator. Q5 seems to be the power switch, not the regulator. And it seems that something is missing because I see no way for the solar cell to charge the battery in that circuit.

Andy_C_

Joined Sep 24, 2016
11
... it seems that something is missing because I see no way for the solar cell to charge the battery in that circuit.
Hi MisterBill2,
My circuit diagram diagram may not be entirely correct, I drew it simply by looking at the circuit board. So please let me know if anything doesn't look right to you. Having said that, I had reckoned that U1 is a li-ion battery charger (something similar to a TP4506, but U1 has no marking on it), and it sinks current from the battery while the solar cell supplies current to the battery. Does this make sense?

I also reckoned that the network of transistors and resistors (everything to the right of U1 in the circuit diagram) form a constant current source, with some kind of switch to turn on and off the LED when it's nighttime or daytime (Q5, as you said). Does this make sense?

Many thanks!

Andy_C_

Joined Sep 24, 2016
11
It looks like I have the LED the wrong way around in my circuit diagram...

MisterBill2

Joined Jan 23, 2018
17,848
Examine that circuit and you will see no way for that collection of transtors to sense the LED current. It has to be from the IC device.