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My first question here so bare with me if I'm a bit "off" or so regarding the level of knowledge etc. My purpose is to try to learn things. So I started off by buying some LED lamp DIY kits of this kind:

http://www.aliexpress.com/item/60-LEDs-Energy-Saving-Lamps-DIY-Kits-Electronic-Kit-Electronin-Fun-without-LEDs/32604794532.html

Now, I have assembled one and tried to understand the components and what parts they play. As I understand (please do correct me if I am wrong on any part) the LED driver is a "capacitive dropper" containing a current limiting thin film capacitor with a "discharge resistor" in parallel so that there won't be any energy left if I unscrew the lamp and touch the Edison screw fitting of the lamp. As I understand I can make the lamp run dimmer and thereby save lifetime of LEDs by choosing a smaller value of the thin film capacitor (as long as it is still rated for 400V at least if I have 240V rated AC outlet)?

Further there are four diodes for making up a "bridge rectifier" - in practice removing the negative side of the AC sinus voltage. There is also an electrolytic "smoothing capacitor" of 4.7µF 400V rating and a resistor in parallel to that. The capacitor is meant to smooth out the hunchback voltage after the rectifier. Then there is also a resistor in series with the LED beads to limit "inrush current" maybe...

Now, I have assembled one such set using some white LED beads and it seems to be working (yay :) ), all LED beads light up and that's great! But, the lamp does flicker even with the smoothing capacitor in place so my guess is that it is under-dimensioned for the lamp setup, but I am not sure I am correct?

Using this lamp DIY kit as a concrete example, how would one go about dimensioning the smoothing capacitor (and any other parts that would have to be replaced with different values)? The goal is to get a flicker-free lamp (to the extent possible).

So, here the electric infrastructure is 240V AC 50Hz. The LEDs seem to arranged in parallel pairs in series (30 pairs making up a total of 60 LED beads).

I have Googled a bit but I am still a bit unsure of how to go about this correctly. Thank you for your help!

UPDATE: Picture of schematic added. It was not included in my kit, but I found this which comes from a kit just like mine (only it has 38 LEDs).

UPDATE 2: Now I also found a second schematic, with the difference in "current limiting capacitor" value between the 38 and 60 LED models. All other parameters are the same (except number of LEDs, but they are connected in the same way) as depicted there.

UPDATE 3: I found this on a forum (http://www.fieldlines.com/index.php?topic=143017.0) with regard to sizing a smoothing capacitor for minimizing ripple:

You have to decide max ripple. You can never get 100% clean DC. Then the reservoir capacitor can be found using:

C = (t * I) / V

Where:

I = average load current drawn on DC side

V = ripple voltage peak-to-peak

t = duration between charging cycles and is equal to: 1/twice mains frequency.

In Europe the mains frequency is 50Hz, so: t = 1/100 = 0.01 seconds.

So, am I doing this right? I = 0,04A (20mA through each LED, connected as parallel pairs) per schematic below

t = 0.01 (2 x 50Hz, Full-Wave rectifier)

As for choosing V - the maximum allowed ripple, let's first calculate what 4.7µF would give me for the 38 LED beads lamp.

C = (t * I) / V can be written as V = (t * I) / C = (0.01 * 0,04) / 0,0000047 = 85V ripple?

But how much ripple should I aim for?

** UPDATE [YYYY-MM-DD]: 2016-05-01 ** I have received the 10µF 400V capacitors and I have tried to replace the 4.7µF included one with the 10µF. The results are noticeable but the flicker is still there. I tried to connect 2 x 10µF capacitors in parallel for a total capacitance of around 20µF in another lamp and yes that was one notch better still. However to get rid of the flicker completely I will need more capacitance as it seems. Probably 33µF-47µF would do the trick. I will have to wait for the 22µF, 33µF and 47µF capacitors to arrive (hopefully within a week or two) and see if what size I can fit as well as what the results will be.

Schematic

enter image description here

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    \$\begingroup\$ A schematic would help - is there one in the kit you can post? \$\endgroup\$ – Andy aka Apr 11 '16 at 10:31
  • \$\begingroup\$ @Andyaka There was not any included with the kits, but I found one form a similar kit and as far as I can see they are identical. I have updated my original question above. \$\endgroup\$ – 10100111001 Apr 11 '16 at 11:59
  • \$\begingroup\$ @Andyaka Update 2: Now I found a better schematic, see above. It shows the difference between the 38 LED model and the 60 LED model is only in the "current limiting capacitor". All other values are identical - most critically (I suppose) the "Smoothing capacitor" value of 4.7µF. I hope this helps. \$\endgroup\$ – 10100111001 Apr 11 '16 at 12:59
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    \$\begingroup\$ Realistically, to answer this requires knowledge of how much current the LEDs are taking - maybe you can measure the volt drop across R2 (taking care of course because there are high voltages around). \$\endgroup\$ – Andy aka Apr 11 '16 at 13:11
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    \$\begingroup\$ BE AWARE THAT WHILE THIS IS FINE ENOUGH FOR USE IN A WHOLLY ENCLOSED AMP IT IS LETHALLY DANGEROUS TO TOUCH ANY PART OF THE CIRCUIT . All parts of the circuit should be considered tp be at full mains voltage. \$\endgroup\$ – Russell McMahon Apr 12 '16 at 21:49
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As shown when comparing 38 and 60 LEDs, the "dropper" cap size affects the current capability of the circuit, so I recommend using one larger than the recommended for the 60 LEDs. Also, I would recommend a "smoothing cap of at least 47uf 400v, or as large as you can get and fit in the space available. I believe these changes will remove the flicker.

The warning Russell posted cannot be overlooked.

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  • \$\begingroup\$ Thank you for your reply! I believe though that the first point about the "Dropper capacitor" is not entirely true? Because it is a capacitor in series with AC, it will basically emit a portion of its charge (current) based on the voltage it has to drop (AC-voltage RMS minus voltage over the LEDs), no? Because if I change the dropper capacitor to a smaller value I simply get a dimmer lamp which draws less power and should prolong the LED lifespan of the lamp a lot (Source of learning: bigclive.com/ledlmp.htm). I tried this with a lamp where the LEDs were pushed too hard and it worked. \$\endgroup\$ – 10100111001 Apr 15 '16 at 21:19
  • \$\begingroup\$ Regarding the smoothing capacitor - I have sort of simulated the circuit and if I have done it correctly (haven't found a way to alter the forward voltage of the LED diodes there) a value of 39µF "should" suffice. However, in the 38 LED lamp version I do not have high hopes of pushing a fat cap inside the small space available in the base of the lamp. The 60 LED version gives me some hope though since it is much larger in the base. But if I go all the way from 4.7µF cap I have now to say a 47µF one...will I get problems with inrush current, should I adjust any resistor value in the circuit? \$\endgroup\$ – 10100111001 Apr 15 '16 at 21:24
  • \$\begingroup\$ Tell you what...I am in the process of ordering some capacitors from AliExpress of 10µf, 22µF, 33µF and 47µF and it will take a while for them to arrive probably but I will post my findings here. The warning from Russell is very valid I think and I will do my best not having to poke around with in the lamp while connected to the mains and also make sure to discharge any capacitors before I thouch the driver circuit. \$\endgroup\$ – 10100111001 Apr 15 '16 at 21:28
  • \$\begingroup\$ I have posted an update on this. See headline 2016-05-01 if interested. \$\endgroup\$ – 10100111001 May 1 '16 at 22:47

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