using a single LED on a dual rail supply?

[scratch]

I've checked books, I've Googled, but can't seem to find an answer to this ....

On a bipolar/dual rail power supply is there an issue with connecting an LED from rail to rail (observing for polarity, and correct resistor value for voltage drop and sized for power dissipation)? I'm using 7815/7915 for voltage regulators.

All of the schematics I've seen connect either a single LED from one rail to ground, or two LED's, one per rail to ground.

[amptramp]

You can put a LED across the plus to minus supply and ignore ground, but if one of the power connections goes missing, you will have power of the wrong polarity being sent through the LED.  So use series diode protection ahead of the regulators and put the LED ahead of the protection diodes as well.  Otherwise, you could end up blowing the regulator connected to the missing supply.

[merlinb]

On a bipolar/dual rail power supply is there an issue with connecting an LED from rail to rail (observing for polarity, and correct resistor value for voltage drop and sized for power dissipation)?

Yes, that's perfectly fine.

You can put a LED across the plus to minus supply and ignore ground, but if one of the power connections goes missing, you will have power of the wrong polarity being sent through the LED.

No you won't, you will just get half the current throught the LED, or no current at all because the regulator is dead.

[petemoore]

  Ignoring the current limiting resistor value and starting with say 5k6 so as not to blow the LED @ 30v supply...
   but why?...just use a 15v 'side' to light the led [invert the leads of the LED/Current Limiting Resistor circuit until it lights] to ground.
   Once the LEd lights note the polarity, run the ground through a switch pole/throw...this IS an indicator LED ?
   Other LEDs can be put in in addition...pretty much to anywhere that lights the LED/CLResistor circuit, switched on/off through the sw'd common ground of the first LED. 
  It's separate from the audio supply, so as long a current isn't an issue, any number of LED's could be strung across 1/2 supply or supply [with appropriate current limiting resistor for the voltage].
   See AMZ calculator for LED currents/resistor values/voltages.

[scratch]

yep, power indicator LED's ...

It's a rack mount PSU I built and it had a separate LED for each rail (+/- 15V).  I've added a second 'regulator' board for +/- 12V but reworking the front plate is a PAIN and was thinking to simply use the one LED 'hole' per module, wiring each across the rails of each module.

Oddly, I've blown LED's in the past by accidental shorts across the rails (have to make sure my power connector pins don't stick out of the shell), but never blew the regulators

So it sounds doable, just use a 'bright' LED so I can minimize the current through the resistor, so that I can keep the power dissipation down and not have to go to a 1/2 watt resistor.

[R.G.]

The power dissipation is the biggest issue. The more voltage, the more power waste where the LED voltage and current are the same.

[scratch]

yep  ... so I'm looking to use 'bright' LEDs so I can cut the required current