0
\$\begingroup\$

schematic_1

schematic_1

schematic_2

schematic_2

schematic_3

schematic_3

If I wired like schematic_1, L1 and L2 are lighting. But, in schematic_2, L1 is not lighting even though L2 is lighting. When I wired in series as schematic_3, L1 is lighting.

Why L1 is not lighting in schematic_2 but L1 in schematic_1 is lighting?

\$\endgroup\$
  • \$\begingroup\$ You need to get your hands on a cheap volt meter. Measure the voltage drops in those circuits and I think you will understand. (And then what Spehro says below) \$\endgroup\$ – George Herold Nov 14 '14 at 1:21
4
\$\begingroup\$

The LED in the optocoupler is an infrared type rather than a visible type. As such, it has a much lower forward drop than even a red LED, so when the two are in parallel, the IR LED in the optocoupler hogs all the current and has a forward voltage drop of perhaps 1.1 or 1.2V, which is not enough to allow the visible LED to light.

When two similar visible LEDs are in parallel, they share current better, and will typically light to (visually) similar brightness. When the two LEDs are in series (and there is enough voltage for both) then the same current flows through both LEDs and both will light.

Here is what typical curves look like (this one for a Lite-on 4N35 optocoupler)

enter image description here

This one for Cree LEDs of various colors :

enter image description here

\$\endgroup\$
  • \$\begingroup\$ thanks for your answer! I'd like to know if it is possible in schematic_2, share similar amount of current between L1 and U1 so to make L1 gets light. Can I? \$\endgroup\$ – Hongseok Yoon Nov 14 '14 at 1:23
  • \$\begingroup\$ Sure, just use two resistors rather than one (each LED gets its own resistor) and calculate the resistor values so that the currents are the same (the optocoupler will have a higher value resistor since it has less forward voltage). \$\endgroup\$ – Spehro Pefhany Nov 14 '14 at 1:25

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.