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I'm modifying a light that lights multiple LEDs at the same time, but with unequal current to each LED. Assume the LEDs are identical and that the power supply gives 35VDC and 0.5A.

Circuits

The first circuit in the picture is one of the wiring configurations of the unmodified circuit. From what I can tell, all three diodes are in parallel and each LED receives 1/3 of the total current from the power supply, and full voltage.

How can I get 1/2I to LED 1, 1/4I to LED 2, and 1/4I to Led 3?

I'm also curious if there's any advantage to the positive side "loop" in the first circuit?

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  • \$\begingroup\$ What type of LEDs are these? How do they manage to drop 35 V? Aren't there any resistors or current sources in the unmodified circuit to set the LEDs' currents? What currents do you want to flow through the LEDs? \$\endgroup\$
    – ocrdu
    Commented Jan 3, 2022 at 0:45
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    \$\begingroup\$ The two circuits are electrically identical - all three diodes are in parallel. If the diodes are identical, they will each carry 1/3 of the total current supplied by the battery. \$\endgroup\$
    – Peter Bennett
    Commented Jan 3, 2022 at 0:50
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    \$\begingroup\$ electrically, there is no positive side loop per se ... there would be be a difference if the top wire was heavy gauge and the bottom wire was very thin \$\endgroup\$
    – jsotola
    Commented Jan 3, 2022 at 0:51
  • \$\begingroup\$ They are 6W 35V SOL LEDs: sunlikelamp.com/index.php?route=product/… The power supply I drew in my circuit is actually a transformer from 120VAC to 35VDC, but there aren't any other components in the circuit between the transformer and LEDs. I had a feeling the circuits were identical. I want to send 1/4 of the total current to LED 1, and 1/4 of the total current each to LEDs 2 and 3. \$\endgroup\$
    – Headlamp
    Commented Jan 3, 2022 at 1:04
  • \$\begingroup\$ that makes 3/4 of the current .... please ask about the problem you are trying to solve, not about your solution to the problem ... update your post with the new question \$\endgroup\$
    – jsotola
    Commented Jan 3, 2022 at 1:27

2 Answers 2

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Your leds are series modules that take .36A at 35V. Of you connect them directly to 35V without any current limiting, they will likely blow out if the voltage or temperature changes.

If you connect 3 leds modules that take .36A at 35V without any current control, they will try to pull .36A * 3 or 1.08A. Your supply supposedly provides up to 0.5A. We don't know of it has current control so it could provide more. This is bad. Can lead to shutting down or burning out, worst case a fire.

If you want to only provide 1/4 to 2 of them and 1/2 to the other, you will need to use an led driver. You could try to use a resistor but without knowing what the led IF curve is, the voltage may drop too much. Driving 12W leds at a 3rd of the current .125) will mean significantly less light as well.

Google 12W led drivers that have an input voltage of 35V.

As to the diagrams you draw: the "loop" in 1 does nothing at all electrically. In practice it can help if your wires are too small and you need to carry more current with a lower voltage drop. But you can just use a bigger wire. The setup in 2 is the same, makes no difference electrically.

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  • \$\begingroup\$ If I found an appropriate driver, would it need to have a different adjustable output for each LED in order to split the current like I want, or would having a current controlled driver with one output somehow make it possible to make a circuit that would split the current how I want? \$\endgroup\$
    – Headlamp
    Commented Jan 8, 2022 at 3:00
  • \$\begingroup\$ @headlamp running parallel leds for a single constant current driver output is not recommended AFAIK. You need individual drivers unless their datasheets specifically allow you to put multiple led modules in parallel. \$\endgroup\$
    – Passerby
    Commented Jan 8, 2022 at 3:30
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  1. Never connect LEDs directly to a voltage source. Always add a current limiting resistor in series with each LED.
  2. Never connect LEDs directly in parallel. Always add a current limiting resistor in series with each LED.
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