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Radiojones

Dimming Issues

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I'm running into an issue that hopefully one of you will have a solution to. My mini led lights dim just fine until I have 5 or more strands on a single channel. At that point they just turn completely on or completely off. Does anyone know what is causing this and how I might overcome it? Thanks for your help!

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Snubbers are great but if you dont want to hassle with all that, just add a C9 to the string....you can paint it black so it disappears and that should solve your problems.

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Snubbers are great but if you dont want to hassle with all that, just add a C9 to the string....you can paint it black so it disappears and that should solve your problems.

Bill - what does the C9 string do that solves this problem?

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Bill - what does the C9 string do that solves this problem?

A snubber is nothing more than a way for the LED capacitor to dump their power. A night light, a Glade Plugin, a C7/C9 bulb, wall wart, just about anything that will provide a connection between the hot and neutral wires will work.

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Bill - what does the C9 string do that solves this problem?

Not quite sure other than adding a bit of a load or resistance to the triac to make it perform properly. I've had this same problem for years when using more than 10 or 12 strings together. I've always just added a C9 or a string of mini's which is hidden and the problem is no more.

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Not quite sure other than adding a bit of a load or resistance to the triac to make it perform properly. I've had this same problem for years when using more than 10 or 12 strings together. I've always just added a C9 or a string of mini's which is hidden and the problem is no more.

Brilliant. Never knew that about LEDs, this may solve problems I ran into last year. Thanks for the tip.

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Triacs (the power component that switches the AC load) latch on using a gate pulse voltage and stay on until the current flowing through them drops below a cutoff.

Dimmer circuits monitor the AC circuit and track the zero-crossing points (when the wave goes from positive to negative). They they wait a calculated time following the zero crossing to apply power to the gate and then remove power from the gate before the next zero crossing. By adjusting the time that this gate is powered and causing the triac to trigger, you can adjust the total power applied to the circuit and the intensity of the lights.

If the circuit is non-reactive (purely resistive like incandescent bulbs with a power factor of 1.0 -- measurable with a kill-a-watt), then the voltage and the current through the triac are proportional. So, the zero crossing and the cutoff current coincide well. As the circuit becomes reactive (PF < 1.0), then the voltage and current start to get out of sync. This is typical for inductive loads such as AC motors or transformers. Then it becomes trickier for dimmer control circuits to adjust the gate pulse timing since the triac is no longer turning off at the zero crossing, but at some time before or after. If the pulse is mistimed, it will clearly cause the dimming to malfunction.

Of course dimmers like those used in our displays are not going to be designed to handle all kinds of reactive and non-reactive loads. So, they work on the assumption that the zero crossing is the cut off point as well and provide a little guardband on either side when the gate pulse is not sent. To ask for more would require putting load monitors on each output circuit -- prohibitively expensive.

Different LEDs will do different things. Most half-wave sets work fine because they don't store any charge and the current through them goes to zero when the voltage goes to zero. The wiring itself, however, can act like an inductive load. And since LED's consume very little current even at low voltages, it is hard for the string to "dump" this residual current flow. When you see flickering with half-wave strings, it is probably due to the inductance created from the wiring or field coupling with nearby light strings. Adding a small resistive load to the string will suppress the current flow and the dimmer can function correctly again.

Full-wave sets with voltage doublers are even worse since they can effectively charge the capacitors in the string during the peaks voltages on the AC sine wave and run off the capacitors for awhile. So at low points on the the wave, there is even less current flow. So, any string inductance or coupling from other strings is worse. Again, adding a small resistive load to the string will bring those "free range" currents back down to zero at the zero crossing.

The snubber doesn't have to be terribly large and that's why a single C7 or C9 light bulb is often more than sufficient.

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