AC Powered LED Pilot Light

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Hi All,

Does anybody have a specific circuit and/or parts recommendation for a simple AC Pilot LED?

I merely want to power an LED directly from the U.S. 120 volt 60 Hz mains to act as a pilot light - a job traditionally filled by a small neon bulb and high-R dropping resistor. There is no other equipment associated with the requirement, so no DC voltage available, no low AC voltage, etc.

Is there already a packaged solution out there?

If I must build it, I'm thinking to use a capacitor as the current limiter and some kind of high voltage rectifier diode (like a 1N400x) to protect the reverse voltage of the LED. LED current for the visibility requirements would be around 10-20 mA.

The smaller the better, I'm hoping to gut an existing pilot light housing and stuff this LED implementation into the housing. SMD parts are preferred to thru-hole.

Low parts cost is an issue too, and there may ultimately be a UL approval involved.

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How the hell is a capacitor going to limit ac current?
and a diode in series?

I don't recommend you run 110 to the front panel to indicate ac present. Do you have a power transformer in the design? YOu could use one of the secondaries to drive the LED far more safely than direct off of the mains comming in.

Jim

P.S. the answer is YES, it can be done the question is, why would you want to?

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Atmel Studio6.2/AS7, DipTrace, Quartus, MPLAB, RSLogix user

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I doubt you'll find the required 270nF/400V capacitor in a SMD version, besides this I don't see any problem.

You can find finished products starting at a couple of $$, most are expensive, though:
Sample LED panel light (Farnell)
Digikey Panel - mount lights selection

Markus

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jgmdesign wrote:
How the hell is a capacitor going to limit ac current?
and a diode in series?

Reactance?

1N4007 in series as LEDs don't like 100+ volts reverse voltage.

You may want to add a resistor to limit inrush current though (adjust the capacitor to compensate)

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A diode in series with a capacitor just will not work (guess why).

Pick up a superbright (= low current) LED, connect any diode in antiparallel with it, calculate the required capacitor's reactance to provide a 2..5 mA short current @60 Hz and connect the capacitor in series with your LED||diode. The capacitor must be a high voltage one (~3*120=>360V) and a film type (= with "soft" breakdown, unlike a "hard" breakdown of the ceramic caps).

A small resistor (~1k) in series will additionally protect the whole circuit from inrush turn-on current, and a large (~1M) resistor in parallel to a capacitor will protect you from being bitten by a cap discharge in case you accidentally touch it after turning off the circuit.

Warning: Grumpy Old Chuff. Reading this post may severely damage your mental health.

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A cap will work just fine. Choose a value that will flow the RMS current needed for the LED. Put the LED in series. It will be small compared to the capacitor and the current will be defined by the cap. You do may need to put a reverse-shunt diode (almost anything) across the LED. Otherwise, the reverse voltage will be very high and the cap will not flow its expected current in the other half cycle.

Example: 100nf cap has reactance of 26 Kohms at 60Hz. Connected across 115VAC, a reactive current of 4.3mA will flow. In series, connect SuperBright LED with a reverse-shunt diode and you have a pilot light. Cap MUST be rated for well more than 160V, say at least 250V.

Power losses are quite low since current is 90 degrees out of phase with voltage. The cap dissipates (almost) no power.

This circuit is quite old - I saw it at least 50 years ago, when I was a boy. It was used to run low voltage pilot lights from mains without the huge power dissipation of a resistor.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

Last Edited: Fri. Dec 30, 2011 - 04:37 PM
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I repeat: a cap with a series diode will not work - a cap will get charged to Vpeak- and diode will just close, feeding no current to output.

Warning: Grumpy Old Chuff. Reading this post may severely damage your mental health.

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But if the "diode" consists of a pair of anti-parallel diodes, it will, just fine.

The diodes do not have to be the same. They just have to insure that current flows both directions.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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MBender,
Please exp
lain the soft vs hard breakdown of film vs ceramic. What are the consequences in an application such as this of a hard breakdown? Thanks.

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Get a bicolour LED and a capacitor-resistor combination

I use a bicolour LED to indicate current flowing through a load ( via a small current transformer designed to provide a safe peak LED current. No other components used.

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Hard breakdown of capacitors can involve large currents and permanent damage to the capacitor (and likely permanent damage to other components in series with the capacitor).

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Quote:
and a film type (= with "soft" breakdown, unlike a "hard" breakdown of the ceramic caps).

More than just a "film" type, you should use "metallized film". Plain "film" caps can fail hard shorted, metallized film caps do not.

In addition, the series resistor in a capacitive ballast circuit should be "metal oxide film". These do not catch fire when things go awry.

Tom Pappano
Tulsa, Oklahoma

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Also note the antiparallel diode is not there to balancethe current, but to protect the led. Most leds have a reverse breakdown of around 5V. Things will work without the diode, but not for long!

The capacitor in question is X2 rated.

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And the final schematic will show up in the next message hopefully....

Imagecraft compiler user

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As you wish Bob:

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P ~ --\/\/\--||-+-|>|-+--- N
                |-|<|-|
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Cool. One of the diodes is an led, and the other is a 1N4007?

Imagecraft compiler user

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Quote:
Also note the antiparallel diode is not there to balance the current,

Having an unbalanced current is actually rectification. Thus you will have DC flowing in your mains. And with the passgae of sufficient current & time may/wil cause corrosion of water & gas pipes, downpipes, metal roofs etc.
In addition, it can cause radio frequency especially in the AM broadcast band due to harmonics & a the very good antenna system distributed around the walls & roof.
So I am with Kartman for that reason as well.
IIRC either ATMEL or Microchip have a application note on running complete MCU systems directly from the mains (if you are happy to flirt with such things).

Charles Darwin, Lord Kelvin & Murphy are always lurking about!
Lee -.-
Riddle me this...How did the serpent move around before the fall?

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Quote:
Thus you will have DC flowing in your mains.

Quote:
due to harmonics

It takes a really good spectrum analyzer to see the DC harmonics :wink:

JC

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bobgardner wrote:
Cool. One of the diodes is an led, and the other is a 1N4007?

Correct. (or a single two-color led (red/green) which will light up yellow)

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Quote:
It takes a really good spectrum analyzer to see the DC harmonics

Yes it takes a very good specan to see DC harmonics. :lol: Oddly enough(not), there are plenty of 50(60) Hz. harmonics that can be heard on various receivers when you have DC in your mains!

Charles Darwin, Lord Kelvin & Murphy are always lurking about!
Lee -.-
Riddle me this...How did the serpent move around before the fall?

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Only 3 components are required. A rectifier, a resistor and an LED. the rectifier blocks half of the sine wave so the LED is only on for half of the ac cycle and no current for the other half of the cycle. The resistor value depends on the brightness you want from the LED. Since the LED will be pulsed it will require a current pulse that is considerably more than the unpulsed DC current specified to provide the same brightness to the eye. For example a 20mA LED supplied by a 28 volts would require a 1300 Ohm limiting resistor. A 20mA LED supplied by 120Vrms through a rectifier would require a 2500 Ohm limiting resistor to produce the same apparent brightness. The power dissipation of the components in the ac circuit are providing current less than half of the ac cycle.  "to the eye" is a key principle for optoelectronic devices that are for displays. This confuses beginner circuit designers and causes such displays to be very dim as a result.   See the attached image.

Attachment(s): 

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The leakage of the diode will allow reverse voltage on the led. It will eventually die. Not a problem at lower voltages, but with 110V and transients, the led is going to have a hard time.
Best to put the diode across the led and size the resistor accordingly.
Note- you are three years too late to the party!

Last Edited: Fri. Sep 18, 2015 - 09:25 PM
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Note UL approval requires a large copper-free gap between high and low voltage traces on a printed board. The high frequency transformers in wall-warts bridge the gap by having primary and secondary pins on opposite sides.  Such a gap would be hard to bridge with surface mount resistors or capacitors.

 

Not sure if you could use a smaller gap if the whole board were encapsulated or otherwise insulated, with only the high-voltage wires coming out.

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This method has been used for many years with no problems. Rectifier leakage in picoAmps is not going to bother a pulsed LED in this circuit. smiley

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1n400x diode spec says 5-50uA reverse leakage depending on temperature. Not what I'd call low leakage.

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Don't use a 1N400X diode. Use a low leakage one like a 1N4148. And consider that the Light Emitting Diode and the Rectifier are both part of the same series circuit and combine their characteristics to make this application feasible.  Pulsed optoelectronic devices.

 

Last Edited: Sat. Sep 19, 2015 - 11:29 PM
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The very first reply to this post given four years ago is IMO the only reasonable one:

Quote:
I don't recommend you run 110 to the front panel to indicate ac present.

If you must have a mains-driven indicator on the panel, use one designed for the task, like those linked to in the second reply also given four years ago.

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

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I was going with the line voltage that would be going to the front panel for the neon indicator. The line voltage would still be going there and the neat little package with a 1N4148 rectifier and a resistor and LED would take the place of the Neon indicator. The design is in compliance with rules for pulsed optoelectronic devices.

 

One person suggested a resistance of 10,000 Ohms but the LED would be very dim. That person was treating it as a DC circuit instead of a pulsed circuit that is only on for a short time. The brightness and duration has to be such that it tricks the eyes. That requires a 2500 Ohm current limit resistor. LEDs can be pulsed at currents considerably higher than their average dc rating to provide the same brightness as perceived by the eyes.

 

A good optoelectronics application manual is a good addition to the library. That is what I used. 

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Jon, the 1n4148 has low leakage but does not have the voltage rating! Whilst i get the point re brightness, the issues of reliability and electrical safety have a greater importance.

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Right, it seems odd that the peak reverse voltage rating never caused a problem in the design.

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Jon B wrote:
The line voltage would still be going there and the neat little package
Which is precisely the product linked to by @markus_b in post #3.

 

Still unclear why you brought this three-year-old thread back to life.

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

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Jon B wrote:

Right, it seems odd that the peak reverse voltage rating never caused a problem in the design.

It may not be that odd. Related thinking ("normalization of deviance") led NASA to lose not one, but two space shuttles.

- John

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Couldn't you just use a resistor and the classic neon lamp -- NE2 or similar. Takes about 1.2 ma @ 95 v. If I recall, they last a long time.  Surprisingly, they are still available and cheap.

 

edit -- I just read your whole post, and I see that you don't want to do that -- not sure why not.

 

hj

 

 

 

 

 

Last Edited: Sun. Sep 20, 2015 - 05:50 AM
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Not odd. It's a numbers game. Recently i had to debug a product problem. The product had sold in large quantities over a 15 year period with supposedly no bad reports. I had a hard time convincing the engineer that designed it that there was a design issue. Once we started asking questions, we found that it was common for the distributors to just replace the failed item and throw away the failed unit. Thus we had a significant percentage of failures that never got reported or investigated. Millions of units over 15 years! It took one service tech to do the wrong thing and cause a major problem that brought it to light.

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I did this many times, for high power white LED's on linear bulb replacements. One good example is linear retrofits, in which the balast is used to limit the string current, just as it does with a standard flourescent tube, but since there is no inductive kick requirement to start the bulb a capacitor works just as fine (XL and XC).

 

A Single antiparalel diode is not the best aproach. The best way to achieve this is a bridge rectifier with the LED on the output (and perhaps a series capacitor to smooth the output) and the series limiting capacitor in series with the AC input. This ensures there's no DC Biasing and both half-cycles are simetrical. The bridge has to be rated for 400V, at least.

Last Edited: Sun. Sep 20, 2015 - 04:53 PM
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Jon B wrote:

LEDs can be pulsed at currents considerably higher than their average dc rating to provide the same brightness as perceived by the eyes.

 

True at high frequencies, nevertheless, at low frequency AC, the flickering gets very noticeable, which drives you nuts after a while.

Many PFC corrected LED bulbs that use a single stage driver have this issue. Once the AC sags a cycle because the fridge or other high inrush appliance kicks in you can see the light changing brightness for a split second. Incandescents and non PFC corrected bulbs (with storage capacitor after the bridge) dont suffer from this problem.