Single sided AC LED circuit

[drtaylor]

I came across this circuit in a device.The circuit applies mains AC to an IR LED, but only the hot line. What I don't understand is how the LED is driven when only the AC hot line is connected to both sides. I was thinking that maybe there is enough phase lag caused by the resistors to put adequate current into the IR LED, but it still seems suspect to me.

Edit: New info, the LED is actually two IR LEDs in reverse parallel. So that takes care of one concern. However, I still don't see how this circuit can work.

[TimFox]

That circuit can't work as drawn.
If the bottom were connected to neutral instead of line, you could get current through the LEDs, but when the cycle swings to reverse-bias the diodes, the actual reverse voltage on each could be dangerous to the device.

[drtaylor]

Tim, I totally agree, but this circuit is in a production device that has been sold for years. So either the schematic is wrong, or this circuit in the device has never functioned.

[John B]

Is there some capacitive coupling to somewhere else in the circuit? Otherwise you don't have a circuit.

[drtaylor]

Is there some capacitive coupling to somewhere else in the circuit? Otherwise you don't have a circuit.

Not that I can see.

[TimFox]

With 50 k in series with the back-to-back diodes, you get a reasonable current for indication at either 120 or 240 VAC, so I assume the bottom of the string should go to neutral instead of how it's drawn.

[bdunham7]

Can you tell us what (specifically) this device is and perhaps a photo? 

[drtaylor]

With 50 k in series with the back-to-back diodes, you get a reasonable current for indication at either 120 or 240 VAC, so I assume the bottom of the string should go to neutral instead of how it's drawn.

I've looked at an image of the circuit board, and as far as I can tell, the circuit is as I've drawn it.

[drtaylor]

With 50 k in series with the back-to-back diodes, you get a reasonable current for indication at either 120 or 240 VAC, so I assume the bottom of the string should go to neutral instead of how it's drawn.

I've looked at an image of the circuit board, and as far as I can tell, the circuit is as I've drawn it.

I can't give any more details, sorry.

[bdunham7]

OK, are the LEDs active when the switch is open or when it is closed?

[RoGeorge]

As it is drawn in that small caption doesn't make sense.  What product is that from?

What does the SW1 do?  Is that a switch to connect/disconnect some other load that is not shown in the schematic?  If so, it makes sense, and the LEDs will emit IR only when the sw is open.  The phase will come from the load that is not shown in the drawing, but I assume it's connected at the node between the switch and the resistors.

[drtaylor]

As it is drawn in that small caption doesn't make sense.  What product is that from?

What does the SW1 do?  Is that a switch to connect/disconnect some other load that is not shown in the schematic?  If so, it makes sense, and the LEDs will emit IR only when the sw is open.  The phase will come from the load that is not shown in the drawing, but I assume it's connected at the node between the switch and the resistors.

I think you've hit on it. I just realized that SW1, which is really a relay contact does hook up elsewhere. The relays in the schematic have both PC pins and topside spade lugs. So I think the top side spade lugs go to a motor or something, which is not shown on the schematic. I'll consider the mystery solved, the LEDs are supposed to be off when the contact is closed. Thanks all!