AC-LED circuit problem 1.6V instead of 3V

[smilem]

Hi, I'm trying to power a LED for a switch as indicator.

Yes I know about the dangers.

The switch is 230AC 3 pins, and indicator must be mounted away from the switch.

I have simulated this circuit I found elsewhere and in simulation it works fine, except in real world I get only 1.6V and LED is very dim.

Any ideas? all parts are like in simulator resistors are 1/2W

Well this is for use as indicator light that an appliance is on (LaserJet printer, YES HP forgot to put ON/OFF switch), I could use neon light but I want to use LED. The LED is better because low voltage at the LED side, and it looks better instead of flickering neon. Besides I already have all parts.

If my tread would no be locked at "allaboutcircuits" I would have said that the printer suspends itself after a while and there is not low voltage to connect the led to.

 

[Boncuk]

Hi,

you forgot to mention mains frequency.

For a frequency of 50Hz 330nF will be alright allowing a maximum current flow of 23mA.

If frequency is 50Hz decrease the value of the 1.5K resistor, since the 330nF capacitor will be a the lower end of the tolerance. (20 to 40%)

Boncuk

 

[ericgibbs]

Hi, I'm trying to power a LED for a switch as indicator.

Yes I know about the dangers.

The switch is 230AC 3 pins, and indicator must be mounted away from the switch.

I have simulated this circuit I found elsewhere and in simulation it works fine, except in real world I get only 1.6V and LED is very dim.

Any ideas? all parts are like in simulator resistors are 1/2W

Well this is for use as indicator light that an appliance is on (LaserJet printer, YES HP forgot to put ON/OFF switch), I could use neon light but I want to use LED. The LED is better because low voltage at the LED side, and it looks better instead of flickering neon. Besides I already have all parts.

If my tread would no be locked at "allaboutcircuits" I would have said that the printer suspends itself after a while and there is not low voltage to connect the led to.

hi,
The forward voltage drop of a RED led is approx 1.65v to 2.1v, not 3.4V as shown.

Also the led is being driven with half wave rectified AC, so without any DC smoothing capacitor, the 'DC' voltage you measure with a DVM will be not accurate.

As the led is being driven by half wave cycles you will not get the 17mA stated.

A quick fix would be to connect a 47uF 16V capacitor across the D4 diode, positive to the top of the led.

NOTE:
If you add a smoothing capacitor, NEVER power the project with the led disconnected.!!

 

[audioguru]

Since the LED is turned on only half the time by the half-wave rectifier then its average current is half and its brightness is half.

 

[smilem]

Thanks for pointing out that lef forward voltage will be lower (I been working with high power leds so much I forgot that low power ones have only 1.7V-2.2V, except High intensity blue).

I Nice site here with different led spec Light Emitting Diodes (LEDs)

I tested a green led too, both were about 1.65V, 17mA. It seems from the table above the voltage with green led should be about 2V volts, but it's the same for red and green leds so audioguru can be right. Leds are turned only half the time by the half-wave rectifier that makes current only half and brightness only half. I think if I want full power then I have to use some cheap SMD bridge rectifier.

I found this schematic with full-wave rectifier, however since simulator will lie to me perhaps somebody can look at component values, are they fine for 230V 50Hz AC, green or red led?

**broken link removed**

Thank you.

 

[audioguru]

Modern high intensity green LEDs are made from the same materials as white and blue LEDs so their forward voltage is also the same at about 3.5V.

Instead of using a full-wave-bridge rectifier you can connect two LEDs back-to-back then eliminate the rectifier. One LED lights on one half-cycle and the other LED lights on the other half cycle.

 

[ericgibbs]

hi smilem,
Have you tried a cap as suggested in my post #3.???

 

[smilem]

hi smilem,
Have you tried a cap as suggested in my post #3.???

You stated stated that:
"If you add a smoothing capacitor, NEVER power the project with the led disconnected.!! "

If the led would fail the cap would blow? So I was under impression that it's safer to use full bridge rectifier (SMD 1A 1000V just 5mm x 5mm).

 

[colin55]

A 330n capacitor will deliver a peak of over 16mA, which is adequate for an indicator.

 

[smilem]

A 330n capacitor will deliver a peak of over 16mA, which is adequate for an indicator.

Yes the indicator glows but I think it can do a bit better

 

[smilem]

Did anyone took a look at **broken link removed**

I tried to simulate it and capacitor of 220nf is too big? In the simulator I had to change it to 115nf to get 2.16V out for LED.

 

[colin55]

Drive the LED via a resistor and battery with 10mA 15mA and 20mA and see what you need

 

[audioguru]

The brightness of an LED is determined by its average current, not its voltage. It sets its own voltage. The capacitor limits the average current.
The voltage at the LED pulses on and off so you cannot measure the LED voltage anyway.

 

[DemonicSandwich]

If the LED is not physically part of the switch and is being used a nothing more than an mains powered indicator, why now just use a neon lamp instead?

They are available in panel mount types and require only a resistor to work off mains power. Some come with a resistor built in.
You can probably get them from a local hardware store.

 

[Boncuk]

"The LED is better because low voltage at the LED side, and it looks better instead of flickering neon."

any more suggestions?

Boncuk

 

[ben7]

Check the circuit here: https://www.electro-tech-online.com/threads/120volt-4-led-night-light.112759/

Note: You can use 1 led instead of the 4 leds shown in the circuit that is on the linked page above.

I find that a lot of people come and ask these kinds of questions when those questions are already answered in other fourms!
Would that be okay for you?