Current Backflow through LED Circuit

[CorgiBrew]

Okay, this is for all of you EE's out there. I posted a couple of days ago about a wiring variation I'm considering for my panel based on Kal's design. To summarize, instead of powering the element indicator lights off of Hot A&B (downstream of a contactor between the power source and the heating element), I'm thinking of just using Hot A and the nuetral bus. Someone commented in response that I should use a diode to prevent current backflow throught the LED, presumably energizing the heating element circuit even when the contactor is "off". Sounds reasonable, but I have a couple of questions:

The LED indicator lamp has a circuit made up of a diode and a resistor. The resistor, I assume, drops the current to a manageable (<20 ma) level for the diode. The diode itself should prevent backflow from the neutral bus. So, do I really need another diode in the line? If there were backflow, wouldn't it be at a very low (reasonably "safe") level because of the resistor?

Since I'm unsure, I will likely take the easy route and just put the indicator lamp in line with the selector switch. Or do what Kal did and power the lamp with both hot leads, though I'd rather keep 220V off of the front of my panel. I'd still, from a learning perspective, be interested in any answers to the above.

 

[jeffmeh]

Since I'm unsure, I will likely take the easy route and just put the indicator lamp in line with the selector switch. Or do what Kal did and power the lamp with both hot leads, though I'd rather keep 220V off of the front of my panel. I'd still, from a learning perspective, be interested in any answers to the above.

I know I'm not addressing your specific question, but if you want to keep 220v off the front of the panel, have you considered wiring the 220v led to the gang box attaching the element to the kettle?

 

[AllanMar]

The problem here is not really backflow, or safety but the fact that your LED is likely to stay lit. See attached simulation images, may help explain more. While some Pilot LED's have bridge circuits/etc, most of the cheap Chinese ones I've seen are simply RC circuits with half the LED's reverse of the other half (they will alternate on/off).

If your concern is safety, 240V on the panel isn't really much more dangerous. You would have to touch both leads at once to get a 240V shock, otherwise each lead is still just 120V to ground. Although it is more wiring...

EDIT: this is assuming your only SSR switching one leg...

 

[CorgiBrew]

The problem here is not really backflow, or safety but the fact that your LED is likely to stay lit. See attached simulation images, may help explain more. While some Pilot LED's have bridge circuits/etc, most of the cheap Chinese ones I've seen are simply RC circuits with half the LED's reverse of the other half (they will alternate on/off).

If your concern is safety, 240V on the panel isn't really much more dangerous. You would have to touch both leads at once to get a 240V shock, otherwise each lead is still just 120V to ground. Although it is more wiring...

EDIT: this is assuming your only SSR switching one leg...

Thanks AllanMar. Interesting diagrams! In the end, I decided to go with Kal's solution, since it turns out I'm going to have to send 240v to the front panel anyway (understand the voltage isn't the real safety issue). I guess the whole diode thing is just a bit above my pay grade. : )