LED lamps (and some fluorescent ones) may glow or flash when they’re supposed to be off when used with electronic switching devices like solid state relays and dimmers. Here’s why.
The main reason is usually the snubber network across a triac. This is usually a resistor and capacitor connected in series and appllied across the main terminals of the triac to attenuate sudden high speed glitches and transients that could result in the triac turning on when it wasn’t supposed to. Triacs have a characteristic called DV/DT Where the D stands for delta, meaning “change in” so the characteristic relates to change in voltage / change in time. This relates to the triacs ability to turn off or remain off when a rapidly rising waveform is applied across it, like a pulse or transient. Typical values for a snubber network are 100 ohms and 100nF. Normally with a heavy load the capacitive coupling through the snubber network is not an issue as it gets shunted by the load. But with very low loads like LED lamps or small motors it may cause a situation where a lamp keeps glowing dimly or a small effects motor keeps running or shuddering slightly.
In some cases you can solve the problem by using another snubber network across the load as it then forms a divider with the one across the triac, and that can lower the leakage voltage to a level that the affected loads will not light or move.
A common approach to solving this issue in the theatre industry is to use a ghost load where a traditional lamp is used to load down a dimmer circuit controlling a small effect, and also help ensure the channel turns on properly, as triacs need a minimum holding current to remain latched.
If you enjoy this channel you can help support it with a dollar for coffee, cookies and random technical stuff at
