Hello everyone,

(USING VARIAC TRANSFORMER - PLEASE DO NOT BLOCK THIS THREAD - LEDS ARE NOT CONNECTED DIRECTLY TO MAINS)

I'm making a solar simulator with 13 white LEDs in series, they operate in 20V/450mA in full brightness (YUJI VTC Series LEDs 135L).

As a power supply I'm connecting 220V mains to a Variac, then to a full bridge rectifier and to 2x 400V/680uF capacitors in parallel.
Even with so much capacitance, when I increase the voltage on the variac, the LEDs start to shine around 210V, but when I reach 238V, about 70mA, the LEDs start to flicker and I can't reach maximum brightness.

I saw the ripple on the scope and is about 600mV peak-peak.

What am I doing wrong?

Thanks for any help.

Best,

Marcelo.

 

Please post a full circuit.
If you do not have an external transformer on your Variac, it most likely IS directly connected to the mains. A Variac usually is an auto transformer so does not have isolation.
That may be why a previous thread has been blocked.
As I said at the start, please post a full circuit!
Without a circuit, we cannot help you as we have no clue as to how you have it wired up.
Include ALL components.

 

I have a Variac that acts that way. I bought it surplus and did not at the time realize that past 100% the coil was burned where the brush was to make contact.

Do your self a favor and measure the resistance from the slider (brush) to the coil as you adjust it in the range in which you see the flickering. Make sure the Variac is not connected to the mains while taking this measurement.

After that, measure the resistance from the output to the Line input.

Good luck!

 

Please post a full circuit.
If you do not have an external transformer on your Variac, it most likely IS directly connected to the mains. A Variac usually is an auto transformer so does not have isolation.
That may be why a previous thread has been blocked.
As I said at the start, please post a full circuit!
Without a circuit, we cannot help you as we have no clue as to how you have it wired up.
Include ALL components.

Dendad,

Thanks for your reply. Here is the circuit. Sorry for the bad drawing.
As you pointed about the variac, I included a transformer.

 

I have a Variac that acts that way. I bought it surplus and did not at the time realize that past 100% the coil was burned where the brush was to make contact.

Do your self a favor and measure the resistance from the slider (brush) to the coil as you adjust it in the range in which you see the flickering. Make sure the Variac is not connected to the mains while taking this measurement.

After that, measure the resistance from the output to the Line input.

Good luck!

DickCappels,

Thank you for the help. I tested the variac as you suggested, and you are right! No isolation, so I included the transformer on the left of my drawing above.

I also tried taking the variac out of the circuit, connecting the bridge directly to the transformer, but in this case using 5x 100 ohm resistors in series (500 ohms) to limit the current. The LEDs flicker anyway.

I am out of ideas to make this work.

Guys,

This is the LED datasheet, just in case.

Thanks for the help!

 

DickCappels,

Thank you for the help. I tested the variac as you suggested, and you are right! No isolation, so I included the transformer on the left of my drawing above.

I also tried taking the variac out of the circuit, connecting the bridge directly to the transformer, but in this case using 5x 100 ohm resistors in series (500 ohms) to limit the current. The LEDs flicker anyway.

I am out of ideas to make this work.


Guys,

This is the LED datasheet, just in case.

Thanks for the help!

Some ideas:
1. Try another string of LEDs (if available).
2. Assuming the LEDs flicker off long enough for a meter (V or A) to respond (even slightly): With power disconnected, insert a small resistance (e.g. 1 ohm) in series with the LEDs string. Reapply power. Use voltmeter to measure drop across resistor. Is the voltage steady or erratic as the LEDs flicker? An erratic voltage might indicate an intermittent connection in the LEDs string. (Alternatively, if your meter can safely measure 0.5A, insert the ammeter in series with the LEDs string, again looking for instability in the reading.)
3. If by "flicker" you mean there is a brightness variation, but not an on/off variation, then there may be intermittent shorting of one or more LEDs that causes the other LEDs output to vary.

 

It sounds like a faulty LED to me. As it heats up, a problem surfaces. It could be a suspect solder joint.
You could limit the current and run each string one at a time to see if there is a common problem or just one string suspect.