Basic Electricity Question

[W1RKW]

Stupid question:  If one wants to use an LED as an indicator light on an AC line (for example 12VAC) is it really necessary to use a diode in series to convert the AC to DC in series with a dropping resistor?  Why not just use an adequate dropping resistor in series with the LED only.  After all an LED is a diode. 

[steve_qix]

Stupid question:  If one wants to use an LED as an indicator light on an AC line (for example 12VAC) is it really necessary to use a diode in series to convert the AC to DC in series with a dropping resistor?  Why not just use an adequate dropping resistor in series with the LED only.  After all an LED is a diode. 

Just use the LED with the resistor !  No diode needed.  I do this all the time - works GREAT !

Regards,

Steve

[W1RKW]

Thanks Steve, That's what I thought, why waste a diode.

[K1JJ]

That's actually a very good question, Bob. To use a diode for a diode for AC? Bet many of us had to think about that unless we did it before.

T

[W1RKW]

Tom,
It's funny, I've always wondered why convert the voltage to DC unless PIV voltage of the LED is an issue but I've never seen that to be an issue when looking at the specs of some LED's.  I've always used a diode in series with the LED but I got to thinking while I've been working on the 813 rig and tie points being limited for components, do I reallly need a diode for the LED's I'm installing.   It's just one of those little things that occurred to me.  Maybe there's something I'm not seeing like the possibility of pulsating indicator.

[kf6pqt]

Ok, an even MORE basic question, what value resistor would you use with that LED to drop 120vac down to 3.3v?

[Tom WA3KLR]

LEDs have a very low PIV around 3 to 6 Volts.  Ideally, even with the 12.6 Volts a.c. you should be using a rectifier diode across the LED.  The PIV will be 18 Volts without the additional diode, 0.7 V with the additional diode. 

Maybe you get by as Steve said, but I wouldn't do that with a 120 Vac circuit.

For a 120 Volt a.c. application the resistor should be around 12 k and 2 Watts, reverse clamping diode on the LED for sure.

In these applications, the LED is emitting half-wave only.  If you need some more brilliance from the LED, it might be bettter to first half-wave rectify the a.c. to a capacitor, then a series resistor to the LED for 100 % on time.  No additional reverse clamp diode needed in this case.

[kf6pqt]

What about using a second LED in paralell, but reversed poliarity, as the clamping diode?

[Art]

i think i remember a bipolar led. . . . that would be interesting. . . red. . . white. . . red . ..  prolly pink

[Bacon, WA3WDR]

Back in the 70s I put an LED in backwards in a 12V circuit, and then I turned it around, but it still didn't light.  I replaced it, and bingo, the replacement LED lit.

From that experience, I assumed that LEDs can not tolerate reverse voltage more than maybe 5V, or they can be destroyed.

I never tested this assumption, I was just more careful not to reverse-connect LEDs after that.  Over the years, I believe that I connected an LED backwards in 12V circuits and powered them on briefly a few times, but the LEDs have worked when I turned them around.  I scratched my head and figured that either I was wrong about reverse voltage destruction of LEDs, or that maybe that reverse 12V doesn't always destroy them.  Maybe that 1970's LED was bad to begin with.  Maybe it depends on the exact chemistry of the LED.

If you put a two-lead dual-parallel LED on an AC circuit, the lit LED will protect the reverse (unlit) one, assuming that you have an appropriate series resistor in line.

[WA1GFZ]

(120 - 3.3) / diode current in amps=dropping resistor
The current is usually around 10 ma= .01 amps.
120 - 3.3  = 116.7
116.7 - .7 (diode) = 116

116 / .01 = 11600 ohms  12 k a good value

I would not reverse bias a led unless  you are sure of the piv rating.
Diodes are chaep fixing J.S. stuff sucks. 






 

[Rob K2CU]

You don't want to use just a series diode, unless you add a cap across the LED. When you have two non identical diodes in reverse bias, the voltage across each will be inversely proportional to each diode's junction capacitance when reverse biased, i.e. unpredictable in this instance. 

For 12 VAC you might get away with just a resistor, but, line transisents could easily frap the part. Best bet for 120 VAC, as stated above is series resistor around 12K to 18K at 2W and reverse connected clamping diode across LED.

If heat is a problem, Max suggests using a 0.22 uF cap instead of the 12K resistor... has this thing about dogbone caps.

Also, as with any rectifying circuit, check for EMI generated by the circuit.

[WA1GFZ]

Yea and the 60 HZ flashing may send you into a coma.

[NE4AM]

Ok, here's a stupid ham trick - take an LED, and solder a 3" dia loop of wire to the terminals.  Suspend it off the end of your PA tank coil, and it will light up as the RF is coupled into the coil.  FUN!

[K1JJ]

Speaking of tricks:

A few weeks ago I presented the following question to a group of hams...

If you took a standard 60 watt light bulb, put it in the microwave oven, pushed the button - would it light up?

Everyone in the group basically said, "no". An internet article said, "yes".  I tried it and sure enough the bulb started dark, then after a second started to pulsate in brightness until it looked like it would burn out from brillance!

Try it.  I assume the small filament picks up enough microwave energy, just like an antenna, to induce serious current. As we know, RF can light a bulb.  But in this case I was really surprised to see it happen without wire connections.

T

[WU2D]

With a neon bulb like an NE-2, when AC is applied much above 60 VRMS, both electrodes ionize; with DC only one will. That is how you use a neon bulb to tell AC from DC. We learned this back in high school when our first electrical shop project was a neon handi tester. I seem to remember that we put a 100K resistor in each leg to the test prods.

Mike WU2D

[WD8BIL]

If you use a bipolar red/green without rectification beforehand it will glow orange !

[John K5PRO]

Dave

In the late 1980s, I was riding motorcycle near Red Lion, PA, and stumbled upon WINBs wire rhombic and their nearby television station. The feedlines from the rhombic antenna ran into a pair of bowl insulators to what looked like a chicken coop/barn. As i curiously rode up, the operator running the transmitter invited me in for a tour. First thing I noted was a  string of LEDs hanging in the studio, with wires on them to pick up stray RF. They were flickering with the transmitted signal. I wondered what sort of RFI they were getting into their audio there, as they were feeding programming from what looked to be a consumer unbalanced cassette deck. The TX was only running about 50 kW or less that day.

Your idea reminded me of that.

[W3FJJ]

Speaking of tricks:

A few weeks ago I presented the following question to a group of hams...

If you took a standard 60 watt light bulb, put it in the microwave oven, pushed the button - would it light up?


T

Another trick is to put a CD shiny side up in microwave, it's quite the light show and leaves some
neat lightning effects on the cd, just don't leave it in there for more than a sec or
two....

[WA1GFZ]

and if the ne2 glows purple you need to adjust your plate suppressor.