In my audio project that is powered off dual +/-15v rails, I would like to connect a simple LED indicating power on. However, when I connect it with a 1.5k resistor in series (2W), the ~28V dissipated as heat are excessive and the resistor gets very hot (too hot for touching).

What is the best way of dealing with this? Should I use two larger value resistors in parallel with each other so they share the heat? Should I connect it just between + and GND instead of between + and -? Wouldn't that put a imbalance in my rails used mostly for op-amps?

  • \$\begingroup\$ What is the voltage and the current your LED is rated for? \$\endgroup\$ – Eugene Sh. Jan 11 '16 at 22:39
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    \$\begingroup\$ Try a 10K resistor... Unless you're trying to blind someone with a super bright blue LED. \$\endgroup\$ – Armandas Jan 11 '16 at 22:39
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    \$\begingroup\$ And yes, voltage is not "dissipated". Power is. 28V at, say 10mA , is 280mW. A bit too much I would say. Try finding a lower voltage around. for instance, do you have a ground/0V ? \$\endgroup\$ – Eugene Sh. Jan 11 '16 at 22:40
  • \$\begingroup\$ Even connected directly across 30 volts (15 to -15), a 1.5k resistor would only draw 20 milliamps, so 0.6 Watts. A 2 Watt resistor should be able to handle that without getting too hot too touch. \$\endgroup\$ – Passerby Jan 11 '16 at 22:53
  • \$\begingroup\$ For a dual-rail supply, I prefer an LED from each side to ground. And I'd agree with Passerby. \$\endgroup\$ – Ecnerwal Jan 11 '16 at 22:57

The first thing you should do is to increase the resistance. Using 20 mA for an indicator LED might have been necessary 30 years ago, but today you're perfectly fine with 2 mA. With an order of magnitude lower current, you won't have the same problem with heat dissipation.

To your second question, with regards to imbalanced rails, that is a non-issue unless your power supply is already so close to its maximum spec that the additional 2 or 20 mA will make it choke. Your op-amp already draws a very imbalanced current from the negative and positive rails, depending on the audio material.

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    \$\begingroup\$ You mean the power indication LED is not supposed to light up the whole room?? \$\endgroup\$ – Armandas Jan 11 '16 at 22:47
  • \$\begingroup\$ Alright, after using the same 1.5k series resistor between only one rail and ground (+the led), the heat problems disappeared while the LED is still bright enough. Thanks a lot for the clarification. I assumed 20mA was necessary for all kinds of LEDs. \$\endgroup\$ – jilski Jan 12 '16 at 12:08

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