LED Dimmer fires when connecting load - Page 1
Forum section: electronics / projects

author: kkawula
author_rank: Newbie
timestamp: June 28, 2016, 11:17:23 pm
content: Hello,I'm trying to design light dimmer for driving the LED bulbs.Initially design was based on standard triac circuit with optocoupler. Unfortunately that solution wasn't especially good for driving low power LED bulbs at small PWM values.I have searched internet and I found how to make alternative circuit with galvanic isolation provided by transformer. Circuit from the attachment has been built and tested. Every thing works ok, starting from very low PWM values to 100%.Unfortunately now I have to face up different problem. From time to time, during connecting the load, I can observe that triac occasionally fires and blink could be observed.I have tried triac which are not EMI sensitive like BTA312-800C, but it does not change anything.Any ideas what can I improve and why triac fires?Best regardsKrzysztof

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 29, 2016, 01:19:11 am
content: Try moving the diode to the other side of the transformer. My guess would be stray capacitance coupling in a pulse.

author: bktemp
author_rank: Super Contributor
timestamp: June 29, 2016, 06:19:19 am
content: Most LED lamps don't like being dimmed and will fail soon unless they are specifically designed for dimming.You could try adding a resistor (100-1kOhm) across G and T1. This decreases the gate sensitivity to prevent any voltage spikes from mains triggering the triac.

author: Fungus
author_rank: Super Contributor
timestamp: June 29, 2016, 06:21:46 am
content: Quote from: kkawula on June 28, 2016, 11:17:23 pmWhat 'bulbs'?Are they dimmable? You'll have big problems if they aren't.

author: kkawula
author_rank: Newbie
timestamp: July 01, 2016, 10:25:33 pm
content: After in depth investigation, the RC snubber circuit has been found as one of cause that triac is fires sporadically when load is connected.On load connection the contacts in the socket are “Bouncing” what in some simplification we can consider as switching load “on” and “off”. When it’s “on” the snubber circuit is charging and remains charged when load is disconnected. When load is reconnected triac is polarised with snubber voltage which need to be reverted by mains voltage. When disconnection time is long and mains voltage swings to opposite voltage, triac gets voltage shock and dV/dt can reach 800V/us (according to my observation.)After removing snubber, triac fires less often but still it happens…:(@bktempMost LED lamps don't like being dimmed and will fail soon unless they are specifically designed for dimming.You could try adding a resistor (100-1kOhm) across G and T1. This decreases the gate sensitivity to prevent any voltage spikes from mains triggering the triac.I have already try it with out bigger effectBtw triac BTA312-800C should be quite insensitive to any EMI problems )

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 02, 2016, 01:13:40 am
content: Try moving the snubber to be across the output and add a X rated cap directly across the mains input?

author: bktemp
author_rank: Super Contributor
timestamp: July 02, 2016, 11:30:51 am
content: Quote from: kkawula on July 01, 2016, 10:25:33 pmThat does not make sense to me.A triac triggers whenever a large enough current is flowing at the gate pin. This can be either an external signal applied to the gate, or an internal generated current by either capacitive coupling from T2 or overvoltage breakdown. For both cases a snubber should help because it reduces dV/dt a bit so reducing the capacitive coupling effect and reducing the amplitude of an inductive voltage spike. But in your case the resistance is quite high, so it may not do much. Try adding a TVS diode or a varistor accross the triac.