Led running on AC

Thread Starter

kenzo42

Joined Feb 26, 2014
42
I am making a smart dehydrator based off of this design.

He put an indicator led in the circuit (circled in red). So my noob self was wondering how will the led respond to running on AC current? Does it just need an appropriate limiting resistor?

Thanks
 

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bertus

Joined Apr 5, 2008
21,861
Hello,

Yes, you still need a current limiting resistor.
Leds can not handle reverse voltages of more than 5 volts.
Put a diode or a second led anti parallel.

Bertus
 

Dodgydave

Joined Jun 22, 2012
10,246
That's a Neon , leds need a diode in anti-parallel and a series resistor also, i would put a 220nF 400V capacitor in series with the led to lower the voltage drop for mains.
 

MisterBill2

Joined Jan 23, 2018
11,624
Certainly the symbol in the drawing is not right for whatever was used , that much is certain! It is a labeled "neon"it would be an assembly that already has the resistor and diode embedded, a suitable resistor is required no matter what. The symbol is wrong is the problem.
Also, the drawing shows the contact of a "fan relay" but we do not see a coil of such a device. So I am suspecting that there is a switch that is closed by the pressure of the air flow so that the heater will not be powered unless the air is flowing.
 

Thread Starter

kenzo42

Joined Feb 26, 2014
42
That's a Neon , leds need a diode in anti-parallel and a series resistor also, i would put a 220nF 400V capacitor in series with the led to lower the voltage drop for mains.
Interesting. I've never heard of neon bulbs other than store signs. After a Google search, apparently they are not very easy to find.

I'm on 110v if that's any difference. Does the 200nF 400v cap still apply? Thank you.
 

Thread Starter

kenzo42

Joined Feb 26, 2014
42
Certainly the symbol in the drawing is not right for whatever was used , that much is certain! It is a labeled "neon"it would be an assembly that already has the resistor and diode embedded, a suitable resistor is required no matter what. The symbol is wrong is the problem.
Also, the drawing shows the contact of a "fan relay" but we do not see a coil of such a device. So I am suspecting that there is a switch that is closed by the pressure of the air flow so that the heater will not be powered unless the air is flowing.
You're right. The symbol is wrong.

The relay is an octocoupler relay and is controlled by an arduino.

This is the project :
https://github.com/truglodite/Dehydrator
 

dl324

Joined Mar 30, 2015
14,332
I've never heard of neon bulbs other than store signs. After a Google search, apparently they are not very easy to find.
All kinds of listings on Amazon. Digikey has 11 items, but they all seem to be bayonet.

1638910684968.png

I bought several bags of 50 or 100 for $1/bag from someone on Craigslist a month or so ago. I think he was selling off parts he acquired at an estate sale for a longtime Tektronix employee.
 

Dodgydave

Joined Jun 22, 2012
10,246
Interesting. I've never heard of neon bulbs other than store signs. After a Google search, apparently they are not very easy to find.

I'm on 110v if that's any difference. Does the 200nF 400v cap still apply? Thank you.
It's just to lower the voltage drop for the series resistor, if you're on 110V it's just the same.
 

MisterBill2

Joined Jan 23, 2018
11,624
One last question. If I use a neon with appropriate resistor, no caps or diodes are needed? Thank you.
The resistor can be as low as 47K ohms, but 68K ohms will be better. The forward drop on a NE2 bulb is about 62 volts, with more current it gets brighter but does not last as long.
And for fun, use an adjustable resistor, say 1 megohm, and set it just above the resistance where the neon bulb lights. Then shine a light on the neon bulb and it will light up. That is because the external light will provide some of the energy towards making the neon gas conduct.
 

Tonyr1084

Joined Sep 24, 2015
6,861
I recall an interesting thing I did with a relaxation oscillator (one full wave rectifier, one resistor [don't recall the value], one fairly large capacitor and one neon bulb parallel to the capacitor). It's been a very very long time since building this thing, but the capacitor would charge at a given rate depending on the resistor chosen. When the voltage was high enough the neon bulb would flash and drain the cap to some point where the neon didn't conduct. The flash rate would be fairly constant. However, and here's the interesting part - when I would put my finger near the neon bulb the flash rate would increase quite noticeably. The reason was from the external static charge I was carrying on my body. The presence of a higher voltage field would cause the bulb to excite at a lower capacitor voltage level. I went on to use that circuit as a part of an ESD training device. The electronics were all encased in a clear plastic box, including the neon bulb. An observer could walk up and put their finger near the bulb and observe the quicker flash rate. This was a teaching tool to demonstrate that ESD sensitive components could be damaged even without touching them. It was an effective training tool.

[edit] Rub a balloon on your hair (not mine - it's pretty much gone now) then hold the balloon next to the neon bulb and watch its flash rate increase dramatically. [end edit]
 

Tonyr1084

Joined Sep 24, 2015
6,861
No diodes or caps needed with neons
Very true. But for the relaxation oscillator you DO need the diode. Otherwise the capacitor can not charge up to the flash point of the neon bulb.

Geez! Thinking I need to build one of these again just for the sake of building something. I've been way too involved with building my woodshop, so • • • .
 

Tonyr1084

Joined Sep 24, 2015
6,861
IF that's an LED then it would need a resistor AND another diode to protect it from reverse currents. Do you have (or can you take) a picture of the light? That would settle a lot of questioning that's going on regarding what we're discussing. IF it's an LED then it needs another diode and a resistor. IF it's a Neon then it only needs a resistor.
 

MisterBill2

Joined Jan 23, 2018
11,624
Inverse parallel LEDs do protect each other and do not need additional diodes for protection.The forward voltage is always less than the maximum allowable reverse voltage.
 

ThePanMan

Joined Mar 13, 2020
359
(I'm) wondering how will the led respond to running on AC current? Does it just need an appropriate limiting resistor?
To answer this question, which has already been answered, it depend on the AC voltage. If it's too high then when reversed current flows the LED will not be able to block all that current and it will break down. A proper resistance will mitigate the danger, but it is still seeing significant reverse current. That's why people are suggesting an additional diode in series with the LED. The diode (for instance, a 1N4001) will block reverse current at much higher voltages than the LED will. The diode will protect the LED.
 

ThePanMan

Joined Mar 13, 2020
359
With D1 being a diode - 1N4003 if you're using 110VAC (see the chart) your circuit will work just fine either way - a single LED or double two LED's anti-parallel will work. Choose R1 based on the amount of current you want going through your LED(s). Recommending 20mA (0.02A).
1639086621554.png1639086653623.png
 
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