Solar Simulator White LEDs flickering - Page 1
Forum section: electronics / general

author: alatza
author_rank: Newbie
timestamp: August 16, 2019, 10:05:34 pm
content: Hello everyone,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.

author: f4eru
author_rank: Super Contributor
timestamp: August 17, 2019, 08:09:08 am
content: Quote1) you did not add current limiting, or better, current regulation. That is a must.2) your bridge rectifier outputs 320V, not 220V. your LEDs are at risk3) PFC is better for a high power, continuous draw4) don't forget to account for surges in your system, or they will destroy your LEDsYou may have a defective LED in the chain.

author: mariush
author_rank: Super Contributor
timestamp: August 17, 2019, 09:07:47 am
content: 13 x 20v = 260vAC voltage rectified to DC results in a peak DC voltage of = Vac x 1.414 (square root of 2) minus 2 x voltage drop on diodes of bridge rectifier = 220v x 1.414 - 2 x 0.7v = ~310vAccount for AC over-voltage... it's not uncommon to get up to 250v AC, which will result in peak DC voltage of 250 x 1.414 - 2 x 0.7 = ~ 350vYou can estimate the capacitance required to keep the minimum voltage above some threshold at a particular current with the formula :C = current / [ 2 x Mains Frequency x ( Vdc peak - Vdc min desired) ]so for example, for 100ma (0.1A) and in a 50 Hz country , and let's say you have peak DC of 320v and you want minimum 270v (13x20 + some margin)C = 0.1 / [ 2 x 50 hz x (320-270) ] = 0.1 / 2 x 50 x 50 = 1/5000 = 0.0002 Farads or 200uFFor your peak 450 mA of current, in order to keep the minimum voltage above 270v, you're need at leastc = 0.45 / 5000 = 0.00009 Farads or 900 uFYOU can go the other way and put the capacitance in the formula ... let's see what's the estimated minimum voltage with 680uF0.000068 (680uf) = 0.07 (70mA) / 2 x 50 x [ 320 - Vdc min ]0.000068 x 100 x (320 - Vdc min) = 0.07320 - Vdc min = 0.07 / 100 x 0.000068 = 10.29=> Vdc min = 320 - 10.29 = ~310vSo with 680 uF, there's enough capacitance to allow the minimum dc voltage to rise fairly close to 310v, just 10-20v below the peak dc voltage that comes out your bridge rectifier.You need to limit the current going in the leds ... a simple resistor will do the jobThe formula is [ Vdc - ( number of leds x voltage led ) ] / currentWith 320v DC input voltage and 13 leds x 20v and 70mA current (0.07a) , you need a R = (320 - 260 ) / 0.07 = 60/0.07 = 4.2 ohm ... which isn't a standard value ... so either use 4 x 1 ohm in series or you go with 4.7 ohm and you get less current.The resistor must be rated for the amount of power it's gonna dissipate, to handle the heat. The formula is P = I x I x R ...So if we go with 4.2 ohm and 0.07A , then power wasted in resistor is P = 0.07 x 0.07 x 4.2 = 0.02058 watts, which means a 0.125w resistor would be good enough but you could certainly use one rated for 0.25w

author: alatza
author_rank: Newbie
timestamp: August 18, 2019, 05:54:15 pm
content: Quote from: f4eru on August 17, 2019, 08:09:08 amf4eru,Thanks for your reply.1) I added a 100R resistor now. Thanks.2) Yes, I know. The intention is to reach 260VDC adjusted by the variac. Each LED is 20V/450mA.3) I will try it. Any specific configuration? Thanks.4) Will do it too.5) All LEDs look fine, brightness looks good until they start flickering.I really need to solve this problem.

author: alatza
author_rank: Newbie
timestamp: August 18, 2019, 06:08:28 pm
content: mariush,Thank you for the calculations. Excellent for me to check my math.Quote from: mariush on August 17, 2019, 09:07:47 amOk. Since we have 60Hz here. I got:C = 0.45 / [ 2 x 60 hz x (320-270) ] = 0.000075 F = 75uFI am using 2x 680uF = 1360uF !!!Quote from: mariush on August 17, 2019, 09:07:47 amRight. I included the resistor. But since the desired current is 450mA, I used 100 ohm instead.I thank you very much for your help. But the LEDs are still flickering.Any other suggestion?

author: alatza
author_rank: Newbie
timestamp: August 18, 2019, 06:13:49 pm
content: Guys,Here is my circuit and the LEDs datasheet:LEDs DATASHEET:https://cdn.shopify.com/s/files/1/0344/6401/files/YJWJ040-1.1_YJ-VTC-135L-G01.pdf?8015580825686102054CIRCUIT DIAGRAM:Any help is welcome! Thank you!

author: Kleinstein
author_rank: Super Contributor
timestamp: August 18, 2019, 06:28:36 pm
content: Solar cells don't work very well with white LEDs, as the spectrum is quite different.Chances are one would still use halogen incandescent lamps as the spectrum is reasonable close to the solar spectrum.For visible light white LEDs are much better, but for the solar cells the halogen lamps may be nearly as good, as the solar cells have the best efficiency in the near IR range, just like the lamps.

author: alatza
author_rank: Newbie
timestamp: August 18, 2019, 08:12:06 pm
content: Kleinstein,I am using natural DSSCs, they are made from chlorophyll and other pigments. Besides, this LED has a very good spectrum in the visible range, I tested them using spectrometer.My problem now is the flickering.Thank you anyway!

author: Kleinstein
author_rank: Super Contributor
timestamp: August 18, 2019, 08:27:49 pm
content: The LEDs instantly react to the mains frequency. To suppress the ripply it needs buffering. At that power level active power factor correction would be a good idea too. So it would be more like starting with a 24 V or 48 V SMPS, and not directly from mains. This is also a lot safer.Since the LED voltage is rather constant, the buffer caps in the shown circuit are not very effective and already a low ripple voltage can cause quite some voltage variations. It may get better with an active current sink circuit instead of the 100 Ohms resistor.Even the 3200 K LEDs don't have a good spectrum in the red and NIR. So maybe one could use these LEDs for maybe 1/3 the power and add NIR (e.g. 850 nm) and deep red LEDs for the rest.