I am at a complete loss (Dimming LED anomaly)

[R Weiland]

Actually, not so much for the case I was interested in. If you do run too few lights that can cause the flicker, but my OP was about more lights causing the issue.  It has be now proven to be the capacitance of the wire. 

As I replied to klb:

 

Remember the LED is a non-resistive load, In fact it is a non-linear load that distorts the current waveform, this combined with the low current causes the issues. The more LEDs you hang on a TRIAC the more distorted the waveform and hence the triggering. Adding a snubber improves the situation by damping some of the waveform distortion.

[plasmadrive]

My strings are 70 lights long and that is about 40mA.  That is just about the hold on current for most triacs of the type LOR uses.. (though I didn't look at their specs).  With many strings end to end the lack of hold on current in the cycle won't be the issue.  

[plasmadrive]

As I replied to klb:

 

Remember the LED is a non-resistive load, In fact it is a non-linear load that distorts the current waveform, this combined with the low current causes the issues. The more LEDs you hang on a TRIAC the more distorted the waveform and hence the triggering. Adding a snubber improves the situation by damping some of the waveform distortion.

Yes, non linear meaning that it has an on/off threshold, but once on they are very linear with increase in voltage, so goes the current.. However, a sign wave is a non linear source.  The distortion you see that goes down with the dummy load addition has to do with the capacitance of the wire and how it interacts with the voltage and currents that are coming thru the triac. 

 

[R Weiland]

You listed hold on current as part of the issue. The current drawn never goes down as the number of strings goes up. Plus, I don't think I have ever seen a string flicker lower than what the output would be with some linear load. Only brighter.

With a string or two, the output voltage for 100% on tracks line waveform beautifully. Current looks much like you sliced the middle third out of the waveform. A nice curved positive peak, a flat line, then a nice curved negative peak. And phase angle dimming looks just like you would expect. The leading edges of these waveforms get trimmed off.

When you add more strings, at about 2/3 of the way through the half cycle, you start seeing a deviance. The voltage starts to have an inflection point and a straight line (slow discharge) that does not reach zero by the next half cycle. Current does not change behavior, just the voltage, because you now have more capacitance than the LED strings can drain once below their voltage knee.

It appears to be this stray voltage on the load side of the junction that is causing it to retrigger right after zero crossing, even if the phase angle dimming is not calling for it to be triggered.

The non linear dimming? Remember the current curve being flat line near the ends of the half cycle? That is throwing off the trigger delay to brightness curve.

Very interesting. I expected the capacitance of the wiring to be insignificant. Since I haven't dissected an LED string is there a capacitor integral to the LED string as part of the power circuit? I know LED lighting bulbs have a capacitor as part of their power supply, just wondering. Great work !! 

[R Weiland]

Yes, non linear meaning that it has an on/off threshold, but once on they are very linear with increase in voltage, so goes the current.. However, a sign wave is a non linear source.  The distortion you see that goes down with the dummy load addition has to do with the capacitance of the wire and how it interacts with the voltage and currents that are coming thru the triac. 

 

This is what I meant be Non-linear.

 

In an AC circuit, Linear means that the application of a sinusoidal voltage results in a sinusoidal current. As the instantaneous voltage changes over the period of the sine wave, the instantaneous current rises and falls in proportion to the voltage so that the waveform of the current is also a sine wave.

A rectifier circuit with a capacitor filter is very non-linear. The capacitor is selected so that it remains charged to nearly the peak value of the AC input voltage. When the load is applied, the capacitor discharges. The current is not proportional to the voltage but behaves non-linearly.

 

The problem does appear to be a capacitance issue and a snubber will "rectify" the problem. Pun intended.

[TonyD]

Maybe some experts will post so we know what the real answer is.      

[plasmadrive]

Very interesting. I expected the capacitance of the wiring to be insignificant. Since I haven't dissected an LED string is there a capacitor integral to the LED string as part of the power circuit? I know LED lighting bulbs have a capacitor as part of their power supply, just wondering. Great work !! 

I thought the same thing about the wire... Turns out in my case using what someone else gave me for pf value per ft of wire I came up with .0067uf or some such. 

 

There is no cap integral to the DC circuit that I am able to see. 

 

The new LED bulbs need that cap.. the Christmas lights.... not so much me thinks. 

[plasmadrive]

This is what I meant be Non-linear.

 

In an AC circuit, Linear means that the application of a sinusoidal voltage results in a sinusoidal current. As the instantaneous voltage changes over the period of the sine wave, the instantaneous current rises and falls in proportion to the voltage so that the waveform of the current is also a sine wave.

A rectifier circuit with a capacitor filter is very non-linear. The capacitor is selected so that it remains charged to nearly the peak value of the AC input voltage. When the load is applied, the capacitor discharges. The current is not proportional to the voltage but behaves non-linearly.

 

The problem does appear to be a capacitance issue and a snubber will "rectify" the problem. Pun intended.

Yes, linear with respect to the wave form it follows.. Very non linear for the DC/LED current with respect to Lumen output for sure..

And yes, it certainly does "rectify" the problem.  LOL