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120v A/C Indicator help

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Jerry D

New Member
:?: I'm installing a roof vent fan for my stove hood and am having trouble figuring out the best way to attach an indicator to indicate that the fan is turned on.

I am using an old lamp dimmer (Triac?) for fan motor speed control. I am playing now with leds and resistors to use as an indicator. I've found I can connect a 20MA LED and a 9100 ohm resistor directly across the fan terminals and not burn the LED out. (How can a DC LED work on A/C? I don't know but it does. Have used one for years on my 18vac doorbell circuit!)

My problem is that the resistor (1/4W) gets too hot. Guess I could find a 10K 10W resistor somewhere, but maybe there's a better way?

Guess I could use a night-lite bulb (4Watt) under the hood and fiber optic it to the old fan switch hole. This one's in my back pocket in case I can't figure how to use a led.

Anyone know of a cheap way to connect a LED to 120 VAC?

Thanks, Jerry
 

crust

Member
The LED is working on the AC only during 1/2 of the AC cycle. If you look at it on a scope, you will see that it is blinking at 60Hz. You can somewhat see this when you have the two lead bi-color LEDs. On one polarity of DC they are green, the other red, on AC, each of the LEDs conducts 1/2 the time making it kind of yellow.

The reason the resistor gets hot is that it is underrated for your application. You are dissipating about 1.5W in that resistor (P=V*V/R). The voltage across your resistor is about 118Vrms or so. I dont know what the codes are in your area, but I would suggest using a 120V indicator. In any case, a larger resistor (2W would work) is needed if you go this route. At 9.1K you are supplying about 13mA to the LED for 1/2 of each AC cycle.
 

Jerry D

New Member
Thanks for the reply Crust. My AC theory never was any good. I looked in Radio Shack for a 120vac indicator and all I could find used neon bulbs. My fan speed control varies the voltage from 120 down to 70 VAC. I'm not sure where a neon would start cutting out. If I remember right it has a resistor required to get it to operate at 90vac or so. The LED stays lit throughout this range.

Code? Yes, that's part of my concern with the led. That and safety are two of the reasons I posted and shopped for an alternative.

Is there an available 120 vac indicator that is not neon?

Jerry
 

crust

Member
Ok, I did not realize that you are varying the voltage across your LED as well. The neon would cut-out early. On the other hand, the LED might not work well either. As you lower the voltage (lets say the rms gets to 60), the current through the LED will be about 1/2, it will be very dim.
 

k7elp60

Active Member
I would add a diode to the circuit to prevent the opposite polarity of the AC exceeding the rating of the LED. If you put a diode in series with the LED and the resistor it would have to be 200 PIV diode. If you put the diode in parallel with the LED. That is cathode of the protection diode to the anode of the LED and the anode of the protection diode to the cathode of the LED a 50 volt diode will do.
 

Roff

Well-Known Member
k7elp60 said:
I would add a diode to the circuit to prevent the opposite polarity of the AC exceeding the rating of the LED. If you put a diode in series with the LED and the resistor it would have to be 200 PIV diode. If you put the diode in parallel with the LED. That is cathode of the protection diode to the anode of the LED and the anode of the protection diode to the cathode of the LED a 50 volt diode will do.
Use the anti-parallel diode, like a 1N4148 or another LED, but instead of using just a resistor, use a 1k 1/4 watt resistor in series with a capacitor. Try about 0.33uF, which has an impedance of about 8k at 60Hz. The 1k resistor, which will only dissipate about 150mw, is to protect against transients, which could cause very high peak currents if the capacitor were the only thing in series. The capacitor will limit current, but will not dissipate any power.
 

Jerry D

New Member
K7elp60, you got me by 5 years. I was an Electronics Tech 1st class in the Navy for 8 years, then 25 yrs with IBM as a large computer service technician. I know circuits but am lame in AC knowledge. Now mostly PCs. Thanks for the info.

Ron, I was wondering about a series Cap. Very interesting. I'll try that one. Now for another trip back to Radio Shack. (I'm really pissed that Layfayette had to close down. Parts are hard to come by without having to order them.)

:lol: Guys, I really appreciate the comeback. Most forums I've found are old and nobody answers you. Have to bookmark this one.

Jerry
 

Jerry D

New Member
All I could find at Radio Shack anywhere near a .33 cap was a .22 Polyester film rated at 50wvdc. Hooked it up to two parallel reversed leds and a 1K resistor. Seems to work OK.

My concern is the cap rating and type. Do you guys think I am OK with what I have or should I order one with a higher working voltage? Would also need to know what type to order.

Thanks, Jerry.
 

Roff

Well-Known Member
I wouldn't count on a 50 volt cap working for very long, and you might not be happy with the failure mode. You should pick a cap with at least 200 volt rating - considering power line transients, 400v would be safer. The type doesn't matter.
 

Jerry D

New Member
Thanks Ron, I'll head off to DigiKey and see what they have. I'll sure be glad to get the hood and motor off my workbench. (Oh darn, I need to poke a hole in my roof to finish up! And this was supposed to be a simple project!)
 
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