How can you tell if Curent is Flowing?

[jdurand]

Depending on the LED used, it may not like being reverse biased every cycle. You could use two LEDs wired in parallel but reversed so one lights at a time or put a diode across the LED to clamp the reversed half cycle.

The old, dim, green LEDs are really rugged so are often used in a circuit like this to avoid the cost of the diode. More modern LEDs like bright green and others don't like reverse voltage.

[BuffaloJohn]

Depending on the LED used, it may not like being reverse biased every cycle. You could use two LEDs wired in parallel but reversed so one lights at a time or put a diode across the LED to clamp the reversed half cycle.

The old, dim, green LEDs are really rugged so are often used in a circuit like this to avoid the cost of the diode. More modern LEDs like bright green and others don't like reverse voltage.

As I mentioned in a comment about the original circuit, the voltage generated was only around 1.5v, even when I pushed the current way higher than the little current transformer was rated for. The burden resistor was higher than the rated burden resistor. I believe the current transformer was saturated.

So - even if input current was 3x higher than the rating of the 5A/5mA current transformer (block heater is a 1600w device so current is about 15A), I didn't get much more than about the forward voltage of the diode and the that value as a reverse voltage was not enough to stress the red led I used.

It's been working for about a decade, so I'm, going to leave it as is.