LED pilot light connected to 120 vac

[sdowney717]

I bought one of those off ebay but it soon failed.
I rebuilt the lamp using a Radio Shack green LED, a 1/4 watt 10k resistor on one leg and reused the diode on the other, so they are in series with the LED pilot light. I have been running it for a day now as a test and it does heat up to about 120*F. No signs of melting the plastic, I slid it out of the holder and I can hold it with my fingers.

I think this was the one, came in a 2 pack.
**broken link removed**

30 ma max current it says.
It is either use this, or buy some neon 120v pilot lights, although I like the look of the lens I have now.
What is your opinion of these LED running on 120 vac with that resistor and diode in series with the LED?

 

[D.A.(Tony)Stewart]

consider a 1/2 W resistor

 

[Audioguru]

The average current in the LED is (120V - 2V)/10k ohms divided by 2= 6mA which is not bright. It will flicker at 30Hz due to the diode. The LED will heat with about 0.012W and the 10k resistor will heat with 0.7W (it is way overloaded).

 

[sdowney717]

Thanks for the comments
here it is, second picture I killed the flash so you can see it is on.
I slid it out unplugged and I can hold it between my fingers the resistor, I do have thick skin!

If it keeps working then I think I will use it as is. If it failed what would happen, the resistor burns, a burn out fail or would it ever catch fire?

This is how the ebay seller had originally configured the light. The resistor they used was 216k and the led bulb burnt out I think the LED's they use must be real cheap as I bought several of them AC and DC and only a couple still work, but the holders are nice.

Can you get a green neon bare bulb to slide into this holder? Or do the premade ones, can they be cracked open to use the bulb?

 

[Audioguru]

A cheap resistor might arc which will burn out the LED.
A neon bulb produces orange-ish light, not green. Inside your green holder it will appear very dim. It also needs a series resistor. You can probably still buy an NE-2 neon bulb or a smaller one.

 

[sdowney717]

A cheap resistor might arc which will burn out the LED.
A neon bulb produces orange-ish light, not green. Inside your green holder it will appear very dim. It also needs a series resistor. You can probably still buy an NE-2 neon bulb or a smaller one.

I had been looking at these here
https://www.ebay.com/itm/5Pcs-120V-...456?pt=LH_DefaultDomain_0&hash=item2a4a319778

Which say they are green. I think they would fit the existing panel hole.

I had some red neons in plastic holder from an old stove. I cracked one holder open and was able to slide the interior neon bulb into one of the ebay light holders I had with a red lens cover and it works very well and is quite bright. I think it uses a 33k or 37k resistor. I did set up some more with a 56k resistor to use as AC reverse polarity indicators for the boat electric panel. The boat has twin 30 amp AC shore power.

this AC green indicator light simply comes on if AC power is on at the switch and is mounted to a metal panel in the boat.

So i had been wondering if one of these ebay neon green lights I could do the same, or I would just use as is.

The LED I setup now in the pics, if it runs a few days, do you think it will just keep on running?

 

[D.A.(Tony)Stewart]

The average current in the LED is (120V - 2V)/10k ohms divided by 2= 6mA which is not bright. It will flicker at 30Hz due to the diode. The LED will heat with about 0.012W and the 10k resistor will heat with 0.7W (it is way overloaded).

Since it conducts current 50% of the time, , power is only 0.36W so a 1/2 W resistor is required. 120* 6mA * 50%.

But if the diode off state capacitance is less than 50x than the LED reverse bias capacitance the LED will see more than -5V and damage the LED. Hence a better design uses 2 diodes, one series and the other parallel to block reverse voltage or a zener protected LED. If the LED gets more than 50uA reverse leakage, it has either already been wounded or failed or will get damaged from reverse current.

It's hard to guess , but it seems like an unreliable source, that could be improved greatly with two 1/4W ~5k R's and a parallel diode across the LED.

 

[sdowney717]

I reconfigured the LED light for 12vdc.
I simply put a 1k resistor in series with the LED.
Since I switch the 4PDT 35 amp relay using 15 to 17vdc, it makes sense to let the green light show that the grid shore power is on or not on.
I already use a 12vdc yellow LED to indicate the other 4PDT 35 amp relay is selecting either gen or inverter power, yellow light is on when gen is on.
This just means running a multi wire control cable about 10 feet to the helm from the relay box.
I have such a multiwire cable I am not using for anything , it has many 24 gauge wires, maybe 12 even, so more than I need.

I wonder will the LED lamp resistor running at 1k get hot?
I will have to check that too.

(the 15 to 17vdc is sourced from a wallwart, not an engine alternator or charger, so the DC voltage level won't be jumping around)
It gets a little complex. the main relay is switching with a 15 vdc wall wart which actually outputs 17 vac, (the coil on the 4PDT will work with 15vdc to 17vdc without being warmed)

I then also have an auto cube relay SPDT cutout which cuts out the possibility of the other 35 amp 4PDT relay for the inverter or gen from ever being able to somehow interconnect another power source at the same time. That cube relay coil I drive with 12vdc derived using a bunch of 5 watt 100 ohm resistors in a voltage divider arrangement.

I have been thinking I could use a 15 vdc switching wallwart which would output 15vdc, but the heavier version is working. The one on there now is rated at 19watts outputs 500 milliamps.

 

[Audioguru]

The green LED uses about 2.2V if it is an old one or 3.2V if it is a bright new one in a clear case. Then with 12V the 1k resistor has 9.8V (or 8.8V) across it. Then the heating of the resistor is 9.8V squared/1k= 0.096W. A 1/4W resistor will get warm.
If the voltage is actually 17V then the resistor will have 14.8V across it and will heat with 14.8V squared/1k= 0.219W and a 1/4W resistor will get hot but not too hot.

 

[sdowney717]

I hooked it up to a 12 v battery and the resistor barely gets warm. This is a clear case LED.
Interesting that it will not burn out at 17vdc.

I originally hooked up a cube relay spdt (auto Potter & Brumfield) to the 17 volt output to be shared with the 4PDT 35 amp relay (which is really a 120 vac coil), but it got quite hot. so I made the voltage divider of those 5 watt resistors to drop the volts to 12. That keeps the cube relay cool. I measured the resistors with a probe at 125*F slid into the the sheath of heat shrink tube. I have them 4 large resistors in a heat shrink case. Doing this did put a greater load on the wallwart, it gets slightly warm.

My other option was somewhere I have a light weight wall wart switcher that outputs a steady 15 vdc. I think that would cool things a little more.
I have so much stuff, I don't know where I put it.