I wanted to ask a quick question. When running (power) LEDs, they mostly require an adequate constant current source / LED-driver IC. Most of the time you connect multiple LEDs in series, so that the forward voltage adds up and the forward current remains the same as with only 1 LED:

source: https://d311uvhi8lkjbj.cloudfront.net/media/Datasheet/pdf/.fYUQjOlW/.t2a80a771bdbb0ef300f7/Datasheet-93/RCD-24.pdf

I can easily calculate the forward voltage of LEDs as this one is described in nearly every datasheet given the forward current and thus calculate the output power and the power loss on the driver IC:

\$P_{loss} = \frac{P_{out}}{Efficiency_{operating.point}}-P_{out}\$

I want to exactly control the light intensity of halogen lights and therefore also use a LED driver.

I want to use 3 halogen lights @5V/0.5A, each 2,5W, total 7,5W. Input voltage is 12V (lets consider dropout voltage as theoretically 0V).

My question: Can I apply the LED configuration for halogen lights like that:enter image description here Or does the light bulbs need to be all in series or parallel (like on a constant voltage source).

  • \$\begingroup\$ Is the second schematic using 2 LEDs and 1 halogen, or is it using 3 halogens? \$\endgroup\$
    – Oskar Skog
    Aug 11, 2021 at 11:29
  • \$\begingroup\$ Note that incandescent lights are designed to use constant voltage, so they can draw a lot higher power after turn-on and heat up fast. running them at constant current will make thier turn-on very slow. \$\endgroup\$
    – tobalt
    Aug 11, 2021 at 12:10
  • \$\begingroup\$ @Oskar Skoy First schematic is just for clarification of the typical application layout of LED driver ICs. Second one is the one applied for this special use case with 3 halogen light bulbs, none LEDs. But this one shouldn't work, because for all of them to be on the same light intensity, they need to be strictly on one string. But I still couldn't find a boost mode constant current converter 12V in to 15V/0.5A out with analog dimming function (strictly no PWM) \$\endgroup\$
    – Dakalaom
    Aug 12, 2021 at 13:40

2 Answers 2


If you are using a constant current source and you want the same current through all consuming devices, then the devices must be in series - whether LED, halogen bulb, or whatever.

  • \$\begingroup\$ That said, the voltage drop across one or several LEDs is quite predictable and so putting a halogen bulb in parallel will result in the bulb operating at a reasonably constant power. \$\endgroup\$
    – Frog
    Aug 11, 2021 at 11:09

When arranged in parallel to the LED string, the voltage accross the halogen bulb will equal the LED voltage, which is more or less constant. A change in current will result in a change of the led brightness while it doesn't change much for the halogen. If you put a halogen bulb rated for 5V in parallel with several leds which typically drop 5V each, you exceed the halogen's voltage rating.

To control the halogen bulb, they need to be in series (you can't have both voltage and current controlled).

  • \$\begingroup\$ I suspect "1st LED" and "Last LED" is some leftover text from the first schematic and that it's supposed to be three halogen bulbs. \$\endgroup\$
    – Oskar Skog
    Aug 12, 2021 at 11:15

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