Led phasing?

[Munty Scruntfundle]

Hi there.

Some years ago when I got quite obsessed with photography I invested in a bunch of led lights to shine through water droplets. I ended up with some good results, but until that point hadn't realised leds on ac power flicker.

I'm looking for a light/lamp for the garage, but I don't want a flickering lamp anywhere near rotating machinery. I notice there are all sorts of led lights being sold for workshop space, so have they got around the flickering problem, or just not explaining it?

I know this might technically be an electrical question, but I would hope others here have considered it too.

 

[Mark Hancock]

Hi

I changed over to led lighting last year and haven't experienced any flickering problems. I change all the tubes at ceiling height and all the lamps on the lathes.

 

[CHJ]

I use an LED light for close illumination of chuck mounted pieces to avoid shadow spots and have not experienced any strobing.

I do use fluorescent and incandescent lighting for deliberate 50HZ strobing for speed checks, I will get my speed discs out later and do a check with the led.

 

[HappyHacker]

Early Leds often had poor power supplies (drivers) that produced the low voltage for the Led at about 100Hz. Modern Leds usually have better drivers but the cheap ones may still be flickering at 100Hz. Most of us cannot see a flicker at 100Hz but it may give a stroboscopic effect with machinery as you have said. You would need to get your lights from a supplier/manufacturer who understands such things and can assure you that their drivers are producing a decent DC output which will stop flicker.

While I have installed lots of LED lights I have never installed any in a workshop so I have not done the research to make a recommendation or noticed if 100Hz does cause a stroboscopic effect.

 

[Woody2Shoes]

I think that there are two causes of flicker in LED lamps/luminaires:

1) The way that the AC mains voltage is modified to feed the LED chips themselves (which, being diodes, only conduct electricity and emit light when the voltage applied to them is "the right way round"). The cheapest way to do this is not to create a smooth DC voltage (which would be flicker-free of course), but to create a 'spiky' DC voltage, with the timing of the spikes linked to the frequency of the AC supply.

2) The persistence of the phosphors embedded in the chip. Most 'white' LEDs actually emit UV light, which is then converted to visible wavelengths by phosphors coating the chip. This arrangement is not so different from the old mercury-vapour fluorescent tubes. Some phosphors continue to emit light for quite a while after they've been 'excited' by a burst of UV - you can often see this if you turn off an LED or fluorescent tube in an otherwise dark room. I'm sure that many manufacturers specify/rely on persistent phosphors to allow them to save cost on the AC>DC conversion electronics because they 'smooth out' the light output even if the electical input to the LED is a bit spiky. On large production runs, fractions of a penny matter!

So...I think the answer to your question is: it depends on the details of what phosphors are used and what is the detail of the specific power supply arrangements. In short, if it's important, I'd check for a specific luminaire first.

Cheers, W2S

 

[CHJ]

I have checked the LED lights I have and cannot see any strobing between 100rpm and 3000 + rpm, So either they are using DC or a very high frequency (most likely) to drive them.

I used these strobe discs to check.

A link to Tony Jeffree's printable masters is in the Metal Working section of my web site.

 

[RogerS]

I use these LED panels. No flickering or strobing of any description.

 

[Racers]

I have led tubes in my fluorescent lights in the workshop no flickering at all, I think they fit smoothing capacitors these days.

Pete

 

[Racers]

I think that there are two causes of flicker in LED lamps/luminaires:

1) The way that the AC mains voltage is modified to feed the LED chips themselves (which, being diodes, only conduct electricity and emit light when the voltage applied to them is "the right way round"). The cheapest way to do this is not to create a smooth DC voltage (which would be flicker-free of course), but to create a 'spiky' DC voltage, with the timing of the spikes linked to the frequency of the AC supply.

2) The persistence of the phosphors embedded in the chip. Most 'white' LEDs actually emit UV light, which is then converted to visible wavelengths by phosphors coating the chip. This arrangement is not so different from the old mercury-vapour fluorescent tubes. Some phosphors continue to emit light for quite a while after they've been 'excited' by a burst of UV - you can often see this if you turn off an LED or fluorescent tube in an otherwise dark room. I'm sure that many manufacturers specify/rely on persistent phosphors to allow them to save cost on the AC>DC conversion electronics because they 'smooth out' the light output even if the electical input to the LED is a bit spiky. On large production runs, fractions of a penny matter!

So...I think the answer to your question is: it depends on the details of what phosphors are used and what is the detail of the specific power supply arrangements. In short, if it's important, I'd check for a specific luminaire first.

Cheers, W2S

Mmmm Both points are not quite right.

Early leds didn't have a smoothing cap to fill the gaps between the positive half cycles of the rectified ac so they flickered.

No phosphor in a led the colour is determined by the chemical make up of the led.
Leds turn off very quickly so they get used in data transmission switching on and off at several megahertz.

Its the lack of smoothing cap that caused the flicker, or if the cap dries up the flicker will appear.

Pete

 

[chaoticbob]

...
No phosphor in a led the colour is determined by the chemical make up of the led.
...
Pete

That's right for 'single colour' LEDs (the colour is determined by the semiconductor bandgap) , but W2S's comment was about 'white' LEDs which do use indeed use fluorescent materials - see, for example, this wikipedia article.

Bob.

 

[Inspector]

I put 16 LED fixtures in my shop when it was built 3 years ago. I’ve never detected any flickering.

Pete

 

[Woody2Shoes]

I think that there are two causes of flicker in LED lamps/luminaires:

1) The way that the AC mains voltage is modified to feed the LED chips themselves (which, being diodes, only conduct electricity and emit light when the voltage applied to them is "the right way round"). The cheapest way to do this is not to create a smooth DC voltage (which would be flicker-free of course), but to create a 'spiky' DC voltage, with the timing of the spikes linked to the frequency of the AC supply.

2) The persistence of the phosphors embedded in the chip. Most 'white' LEDs actually emit UV light, which is then converted to visible wavelengths by phosphors coating the chip. This arrangement is not so different from the old mercury-vapour fluorescent tubes. Some phosphors continue to emit light for quite a while after they've been 'excited' by a burst of UV - you can often see this if you turn off an LED or fluorescent tube in an otherwise dark room. I'm sure that many manufacturers specify/rely on persistent phosphors to allow them to save cost on the AC>DC conversion electronics because they 'smooth out' the light output even if the electical input to the LED is a bit spiky. On large production runs, fractions of a penny matter!

So...I think the answer to your question is: it depends on the details of what phosphors are used and what is the detail of the specific power supply arrangements. In short, if it's important, I'd check for a specific luminaire first.

Cheers, W2S

Mmmm Both points are not quite right.

Early leds didn't have a smoothing cap to fill the gaps between the positive half cycles of the rectified ac so they flickered.

No phosphor in a led the colour is determined by the chemical make up of the led.
Leds turn off very quickly so they get used in data transmission switching on and off at several megahertz.

Its the lack of smoothing cap that caused the flicker, or if the cap dries up the flicker will appear.

Pete

I think LED power supplies have always had some kind of smoothing capacitor in them but most of the 'smoothing' is not done by capacitors - they use similar techniques to those used in 'switched mode' PSUs https://en.wikipedia.org/wiki/Switched- ... wer_supply - which is basically using semiconductors to chop up the AC waveform at a high frequency and use (relatively small) inductors rather than relatively large (and possibly more expensive/less reliable) capacitors - here's a basic example (not necessarily what might actually be found in an LED luminaire): https://uk.rs-online.com/web/p/ac-dc-co ... s/1465675/ .

The colour point has already been covered but for more info - https://en.wikipedia.org/wiki/Light-emi ... eakthrough

Cheers, W2S

PS an interesting breakdown of a fairly 'state of the art' cheap LED luminaire (with schematic) : https://www.youtube.com/watch?v=Bv0kdFyYEAU

 

[Racers]

Caps are used in switch mode PSUs the high frequency allows smaller value caps to be used. They do dry out and need replacing , I have replaced lots of them in TVs monitors etc you can usually fix them for a few quid.

Pete

 

[Munty Scruntfundle]

Everything is pointing towards good news!

Thanks folks, order being placed!