Current profile with triac dimmed magnetic transformer and resistive load? - Page 1
Forum section: electronics / projects

author: e100
author_rank: Frequent Contributor
timestamp: September 01, 2018, 08:12:43 am
content: (A Google image search didn't turn up much and I don't have the hardware to safely do the test myself.)What does the current profile of the secondary look like when you use a leading edge triac dimmer on the primary side of a mains magnetic transformer (not the newer switch mode type) with a resistive load on the secondary like a 12v halogen light?When the triac triggers does the sharp dv/dt increase across the transformer get blocked by the inductance of the transformer distorting the voltage and current waveforms in the secondary so that it looks substantially different from the voltage and current profile on the primary?

author: Zero999
author_rank: Super Contributor
timestamp: September 01, 2018, 03:29:03 pm
content: I don't know. I've never tried, since it isn't recommended to use an iron transformer with a lamp dimmer.I expect the waveform won't be much different, to with a resistive load, if the transformer is fully loaded, because it will appear to be a mostly resistive load.

author: floobydust
author_rank: Super Contributor
timestamp: September 01, 2018, 05:33:58 pm
content: Primary and secondary voltage waveforms from my 20W halogen desk lamp for a few phase angles.It's a resistive (lamp) load so the secondary voltage and current are the same waveshape (profile?).The triac mains dimmer has 0.1uF 100R snubber and extra "hysteresis" diodes.Nothing out of the ordinary here.

author: e100
author_rank: Frequent Contributor
timestamp: September 01, 2018, 06:20:36 pm
content: Quote from: floobydust on September 01, 2018, 05:33:58 pmThe first picture looks like the typical triac waveform you see in books or this videowith a sudden conduction at the trigger point, but what's happening in the second picture. It looks like it's conducting from zero until about 50 degrees, then there's a decay, then it triggers again.

author: floobydust
author_rank: Super Contributor
timestamp: September 01, 2018, 07:12:23 pm
content: I'll check if I was measuring line or load side. The scope trace changed at long conduction angles (bright lamp).The dimmer is a $3 fleabay special from china, it comes with a curse. 4000w 220VACAfter a while, the potentiometer arced and I added a resistor and put in a new pot.Then the 25A triac gate shorted on a 1A fan load. I put in another triac. I kind of hate the thing.The circuit dims OK and works on a window fan I use in summer.There are two versions of the circuit, this has an added bridge rectifier to give "hysteresis free" or "no snapback" effect, like you see with lamp dimmers. It's also helps with inductive loads. The extra diodes discharge (reset) the firing delay capacitor every cycle.Circuit is from figure AN1003.12Teccor App note AN1003

author: e100
author_rank: Frequent Contributor
timestamp: September 02, 2018, 05:34:44 am
content: I had a look at the application notes but I didn't see any pictures of waveforms with zero degrees to some angle conducting, then switched off, then a few degrees later conducting again. It's like the triac keeps conducting beyond the 180 degrees polarity reversal (and expected turn off point) then eventually turns off, then gets triggered again, but that doesn't make sense, unless I'm missing something.