Suppose I have a regulated power supply that has +20V and -20V outputs, and an amplifier circuit that uses them. I also have a push-button (not momentary) power switch with an integrated LED which requires 12V to run. What is a simple way for me to adapt the existing PSU rails to also light the LED?

  • \$\begingroup\$ What current does the LED need? Is the LED wired independantly of the switch contacts? What are the switch contacts wired to? \$\endgroup\$ – Andy aka Nov 26 '14 at 19:57
  • \$\begingroup\$ Does the PSU also have a center reference (0V) between +20V and -20V? \$\endgroup\$ – Dan Laks Nov 26 '14 at 19:58
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    \$\begingroup\$ For something that requires as little current and isn't picky about regulation, a zener diode and resistor is a good choice. \$\endgroup\$ – Mr. Mascaro Nov 26 '14 at 21:45

You can simply add a resistor in series with the LED. Either measure its current consumption, or assume that it's about 20mA. Then use Ohm's Law to work out a resistor to drop 8V (assuming it's connected from +20V to GND). Connect that resistor in series with the LED terminal of the switch.

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  • \$\begingroup\$ Thanks, that sounds pretty good. How do you suggest tapping into the PSU lines? A three-way splicer like delcity.net/store/Heat-Shrink-Y-Butt-Connectors/… ? \$\endgroup\$ – gcv Nov 26 '14 at 19:24
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    \$\begingroup\$ I don't like those connectors, as the terminal is exposed in the center of the "Y", which may cause a short circuit if it touches another connector or the chassis. There are tap connectors that can be installed without cutting the main connector. These don't leave any exposed conductor. \$\endgroup\$ – Peter Bennett Nov 26 '14 at 19:58
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    \$\begingroup\$ @gcv I agree with Peter, don't use those Y-connectors. They leave an exposed conductor at the centre. How about a simple screw-terminal block (or "chocolate block" as we call it in the UK)? It really depends on how the wires are connected from PSU to board, and how thick the wires are. You can, of course, ask another question (with photo) if you're still unsure. \$\endgroup\$ – John Honniball Nov 26 '14 at 20:14
  • \$\begingroup\$ I understand, thanks! @jbarker2160 mentioned using a zener diode here along with the resistor — what is the purpose of this? \$\endgroup\$ – gcv Dec 1 '14 at 0:27
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    \$\begingroup\$ @gcv The zener diode acts as a voltage regulator. If you needed precisely 12.0V for the LED in the switch, it'd be great, but I think it's over-complicated in this case. \$\endgroup\$ – John Honniball Dec 1 '14 at 10:42

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