Author Topic: Phase shift mains AC to de-flicker LEDs  (Read 4687 times)

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Offline Lupin III.Topic starter

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Phase shift mains AC to de-flicker LEDs
« on: March 20, 2020, 12:18:06 pm »
I have a few COB LEDs with integrated mains "power supply" (like these: https://www.ebay.com/itm/20W-LED-COB-Chip-110-220V-Smart-IC/232273725866 ). They are really cheap. Maybe not the best for living room ligthing, but definitely good enough for shop or workbench lights. The only problem I got with them is that because they ride the mains voltage sine they have visible flicker when you move your eyes quickly.

So I was thinking, if you get two of those, but shift the mains AC phase to one of them by a series capacitor, the visible flicker could be substantially reduced. Similar to how you create another "phase" with a series capacitor if you want to run a 3-phase motor on a single phase.

This should work, but what value capacitor would I need approximately to shift enough for a 20, 30 or 50W COB? Would something similar work with an inductor in series to shift in the other direction?
 

Offline Zero999

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Re: Phase shift mains AC to de-flicker LEDs
« Reply #1 on: March 20, 2020, 12:28:15 pm »
Stupid question: why not just use LEDs with a proper filter capacitor which doesn't flicker?

Trying to bodge crappy LED drivers isn't worth the expense.
 

Online Circlotron

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Re: Phase shift mains AC to de-flicker LEDs
« Reply #2 on: March 20, 2020, 12:37:08 pm »
Maybe put the mains into a bridge rectifier and filter capacitor then feed this dc to the LED lamp.
 

Offline richard.cs

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Re: Phase shift mains AC to de-flicker LEDs
« Reply #3 on: March 20, 2020, 01:45:47 pm »
This should work, but what value capacitor would I need approximately to shift enough for a 20, 30 or 50W COB? Would something similar work with an inductor in series to shift in the other direction?

This was historically done on some fluorescent fittings. One tube with an inductive ballast and one with a capacitive one so the two flicker out of phase. I suspect for what you want a better LED driver would be the preferred solution (by far).

If you do want to try it I suspect your first step should be to de-goop one and reverse engineer the supply circuit used. This might work without modification from DC, though this is far from guaranteed. Almost certainly they just conduct over the top portion of the AC waveform and if they regulate current at all they may do so at a value that blows them up if you go to 100% duty cycle by feeding them DC. Note that you will need to drop significant voltage across your reactance to get a phase shift approach to work, this might mean running 110 V devices from 220 V, which means if you're in a 110 V country the idea may be unworkable.

 

Offline Lupin III.Topic starter

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Re: Phase shift mains AC to de-flicker LEDs
« Reply #4 on: March 20, 2020, 01:52:01 pm »
Please stay on topic. If you know it won't work, tell me why. If you know it will work and how to calculate the capacitor value, I would appreciate it as well, if you'd tell me. But "Why..." or "Why not..." is not helpful.

Stupid question: why not just use LEDs with a proper filter capacitor which doesn't flicker?

Trying to bodge crappy LED drivers isn't worth the expense.

Putting a capacitor that costs less than 1€ in series with a <2€ LED is hardly an expense, neither in time nor money.


Maybe put the mains into a bridge rectifier and filter capacitor then feed this dc to the LED lamp.
The drivers need the off-time, because that's what they are built for. On DC they'd overheat. Also a bridge rectifier and a smoothing cap for an up to 50W device would be quite big.
 

Offline Lupin III.Topic starter

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Re: Phase shift mains AC to de-flicker LEDs
« Reply #5 on: March 20, 2020, 01:57:26 pm »
If you do want to try it I suspect your first step should be to de-goop one and reverse engineer the supply circuit used. This might work without modification from DC, though this is far from guaranteed. Almost certainly they just conduct over the top portion of the AC waveform and if they regulate current at all they may do so at a value that blows them up if you go to 100% duty cycle by feeding them DC. Note that you will need to drop significant voltage across your reactance to get a phase shift approach to work, this might mean running 110 V devices from 220 V, which means if you're in a 110 V country the idea may be unworkable.

The circuit is simple it's pretty much a one-chip solution per 10W of LED string. The flicker comes from only conducting when the mains sine is higher than the foward voltage of the LEDs, then the chip starts regulating the current. I'm in Europe, so 230V. If I go by AC motor running caps they are in the order of a few µF for motors <100W to create the shifted phase. Would the a cap for the LED load be in the same order?
 

Offline Zero999

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Re: Phase shift mains AC to de-flicker LEDs
« Reply #6 on: March 20, 2020, 02:28:17 pm »
No one can answer the question because there isn't enough information.

Do you know the internal schematic of the LED driver?

If it's a capacitive dropper, all a series capacitor will do is reduce the brightness slightly.

If the driver has a bridge rectifier on its input then adding an extra capacitor to the output or running it on DC should work.

My advice is to avoid cheap LEDs like that because they often fail prematurely. Cheap often doesn't save money in the long run.
 

Offline greenpossum

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Re: Phase shift mains AC to de-flicker LEDs
« Reply #7 on: March 20, 2020, 02:39:09 pm »
My advice is to avoid cheap LEDs like that because they often fail prematurely. Cheap often doesn't save money in the long run.

As far as I can tell they are like LED light bulbs, if they fail you throw them out as they are only about $1-2 each.

OP, I do think it's a bit like the Irish road directions: If you want to go to X, I wouldn't start here if I were you.  ;)
« Last Edit: March 20, 2020, 02:41:49 pm by greenpossum »
 

Offline StillTrying

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.  That took much longer than I thought it would.
 

Offline richard.cs

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Re: Phase shift mains AC to de-flicker LEDs
« Reply #9 on: March 20, 2020, 02:52:36 pm »
I've attached a very simplified LTSPICE model.

The LED is modelled as having a forward voltage of 250 V, an on-resistance of 100 Ohms and an active circuit limiting the forward current to 200 mA. You would need to tweak this to match your LED and drive. Run the model and plot V(line,neutral) and I(I1).

I find that if I put a capacitor in series then about 4u7 will shift the conduction earlier by around 1.2 ms, which is only 22 degrees. A smaller capacitor shifts it further but also significantly limits the LED current. A series inductor of around 500 mH (huge!) will shift it about 1.5 ms, around 27 degrees, the other way before the LED current starts to drop.

If I change the model to be closer to a 110 V device (say 130 V forward voltage, 50 Ohms on, 400 mA limiting circuit) then I can put in greater X:R ratio before the LED current starts to drop and I can get phase shifts of around 90 degrees each way but there is a strong danger of just blowing up the active current source feeding a 110 V from 230 V like this. Essentially you must have enough series reactance to drop it out of regulation or as soon as the current source starts to cut in it will increase the voltage drop and be destroyed (unless they use the high voltage components in the lower voltage models).

TLDR: You can simulate it. You can't phase shift 230 V ones enough to matter without seriously dimming them, and whilst you can phase shift 110 V ones enough you can't allow the driver to operate without risking destruction so the only thing you're gaining over a driverless device is the rectifier.
 
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Offline Zero999

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Re: Phase shift mains AC to de-flicker LEDs
« Reply #10 on: March 20, 2020, 02:55:08 pm »
My advice is to avoid cheap LEDs like that because they often fail prematurely. Cheap often doesn't save money in the long run.

As far as I can tell they are like LED light bulbs, if they fail you throw them out as they are only about $1-2 each.

OP, I do think it's a bit like the Irish road directions: If you want to go to X, I wouldn't start here if I were you.  ;)
The trouble is the damage they might do, when they fail: think fire and smoke.

My advice is for the original poster to return them to the seller, giving flicker as a reason why they're unacceptable and demand a refund. Sellers shouldn't get away with selling crap like this. It's not the buyer's responsibility to fix defective goods.
 

Offline greenpossum

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Re: Phase shift mains AC to de-flicker LEDs
« Reply #11 on: March 20, 2020, 02:55:36 pm »
 

Offline StillTrying

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Re: Phase shift mains AC to de-flicker LEDs
« Reply #12 on: March 20, 2020, 03:01:20 pm »
That website was blocked by my browser because of an expired HTTPS certificate.

Me to, but I just clicked past the warnings.
.  That took much longer than I thought it would.
 

Offline greenpossum

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Re: Phase shift mains AC to de-flicker LEDs
« Reply #13 on: March 20, 2020, 03:12:00 pm »
Here's a safe website with a link to a long YouTube video teardown: https://hackaday.com/2019/02/15/cob-led-teardown/
 

Offline StillTrying

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Re: Phase shift mains AC to de-flicker LEDs
« Reply #14 on: March 20, 2020, 03:24:10 pm »
Here's a safe website

LOL I'm checking suspicious websites all the the time, that one isn't.

LED drivers are BP5132H.
.  That took much longer than I thought it would.
 
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Offline Zero999

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Re: Phase shift mains AC to de-flicker LEDs
« Reply #15 on: March 20, 2020, 03:32:43 pm »
Yes, I do remember Big Clive doing a video now, but he didn't reverse engineer the schematic.

Looking at the photograph, there appears to be a full wave bridge rectifier on the top right hand side of the board. Adding a suitable 400V rated capacitor across the DC side should reduce the ripple greatly, as well as increase the duty cycle, hence the brightness. The downside is it will get hotter more quickly. Hopefully the driver will have some over temperature protection and reduce the current, otherwise it will fail more quickly.


A better approach would be to power it off a DC power supply with a lower voltage, than the mains peak voltage but, it's still more sensible to use a higher quality LED lamp, rather than cheap crap.
 

Online tunk

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Re: Phase shift mains AC to de-flicker LEDs
« Reply #16 on: March 20, 2020, 04:06:04 pm »
Diodegonewild have also looked at them (including schematic):
 

Offline Zero999

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Re: Phase shift mains AC to de-flicker LEDs
« Reply #17 on: March 20, 2020, 04:18:17 pm »
Oh, I also forgot to say:

The metal circuit board must be earthed for safety's sake. It's also quite likely they don't meet the bare minimum safety standards as they will lack sufficient insulation between the live parts and earth. If you mush use them, then I repeat, ensure the aluminium board is properly grounded!
 

Offline Lupin III.Topic starter

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Re: Phase shift mains AC to de-flicker LEDs
« Reply #18 on: March 20, 2020, 06:16:29 pm »
I've attached a very simplified LTSPICE model.

The LED is modelled as having a forward voltage of 250 V, an on-resistance of 100 Ohms and an active circuit limiting the forward current to 200 mA. You would need to tweak this to match your LED and drive. Run the model and plot V(line,neutral) and I(I1).

I find that if I put a capacitor in series then about 4u7 will shift the conduction earlier by around 1.2 ms, which is only 22 degrees. A smaller capacitor shifts it further but also significantly limits the LED current. A series inductor of around 500 mH (huge!) will shift it about 1.5 ms, around 27 degrees, the other way before the LED current starts to drop.

Thanks Richard. I'll try that out. Even a 22° shift may already make a difference, since the "dark time" will be shorter. The flicker is at 100Hz, assuming the LEDs conduct about half of those 10ms, so 5ms, a 1.2ms shift is considerable.


Oh, I also forgot to say:

The metal circuit board must be earthed for safety's sake. It's also quite likely they don't meet the bare minimum safety standards as they will lack sufficient insulation between the live parts and earth. If you mush use them, then I repeat, ensure the aluminium board is properly grounded!


I know this, which is why the heatsink IS already earthed. And not only that, the COB is in a plastic/glass lamp housing (with vent holes for airflow), so the heatsink can't even be touched unless you poke throuh the holes with a screwdriver.
 

Offline Prehistoricman

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Re: Phase shift mains AC to de-flicker LEDs
« Reply #19 on: March 20, 2020, 07:26:06 pm »
May not be possible, depending on the driver. Some turn off the entire array when the voltage gets too low. More efficient ones will switch in various banks to drop more voltage as the input voltage rises. If you have one of these, then supplying a lower DC voltage will be fine and won't overheat the driver. If you have the former, then you may risk overheating it by running on DC.

Getting 230V or 120V DC isn't particularly easy. Perhaps a dimmer circuit into a bridge rectifier + cap would work, but the peak current might kill the dimmer.

Your phase shift idea is an interesting one but I don't think there's an easy way to do it. Phase shift by inductor or capacitor implies an attenuation in the signal (see any RC or LR bode plot). Then again consider overpowering the module; if you use a diode to switch over to your shifted AC, then you're power the module for more % of the time.


As for questions about why this project should exist, I agree with Lupin that they're unhelpful. I see this all the time on the forum. Some people just want to hack things, and they don't need to justify that to the world.

Offline strawberry

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Re: Phase shift mains AC to de-flicker LEDs
« Reply #20 on: March 20, 2020, 07:50:59 pm »
I recommend reduce LED bulb/TV current by ~20% to double useful life. As manufacturers push them to the limits to earn more money.
PFC choke could boost voltage up to LED threshold level and therefore limit fliker
 
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Online Someone

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Re: Phase shift mains AC to de-flicker LEDs
« Reply #21 on: March 20, 2020, 09:51:15 pm »
Stupid question: why not just use LEDs with a proper filter capacitor which doesn't flicker?

Trying to bodge crappy LED drivers isn't worth the expense.
Putting a capacitor that costs less than 1€ in series with a <2€ LED is hardly an expense, neither in time nor money.
But asking for everyone else to donate their time is no expense....

Maybe put the mains into a bridge rectifier and filter capacitor then feed this dc to the LED lamp.
The drivers need the off-time, because that's what they are built for. On DC they'd overheat. Also a bridge rectifier and a smoothing cap for an up to 50W device would be quite big.
How do you know it will overheat on DC?....

Please stay on topic. If you know it won't work, tell me why. If you know it will work and how to calculate the capacitor value, I would appreciate it as well, if you'd tell me. But "Why..." or "Why not..." is not helpful.
If it does overheat on DC then they have set the current limit to work with the rectified AC duty cycle, run it on DC and adjust the current limit so the heating (RMS) losses remain steady (or lower).

As mentioned above modifying a badly designed bargain priced device is usually a loss making exercise, throwing good money after bad (https://en.wikipedia.org/wiki/Sunk_cost#The_fallacy_effect). Any solution is likely to cost more in time and resources than simply buying a more appropriate device to begin with.
 
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Offline greenpossum

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Re: Phase shift mains AC to de-flicker LEDs
« Reply #22 on: March 20, 2020, 09:59:09 pm »
LOL I'm checking suspicious websites all the the time, that one isn't.

I'm not worried about my browser, but I don't want to support sloppy websites and their sloppy maintainers.
 

Online NiHaoMike

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Re: Phase shift mains AC to de-flicker LEDs
« Reply #23 on: March 21, 2020, 01:11:16 pm »
Connect a large capacitor in parallel with the bridge rectifier, then connect another capacitor in series with the AC input to limit the current. (Use an AC power meter to check if you got the right value, making sure it's the same or lower compared to the unmodified power usage.) It might be more economical to use a series inductor if you're supplying a lot of modules.
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Offline Siwastaja

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Re: Phase shift mains AC to de-flicker LEDs
« Reply #24 on: March 21, 2020, 02:30:19 pm »
Think about the fundamental physics:

The flicker happens because the mains cannot provide power to the LED during low line voltage part of the period.

How do you "shift phase"? By adding a delay. How do you add the delay? By storing energy and releasing it.

Now, how do all the proper, non-flickering LED lights work? By storing energy! No special gimmicks required: just add the filter cap to the rectified mains. Buying non-flickering lamps off-the-shelf obviously solves the problem. If you want to bodge the existing drivers, I'd just try to find where the DC link is and add capacitance there.
« Last Edit: March 21, 2020, 02:32:25 pm by Siwastaja »
 
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