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| Making a triac phase control circuit - are my values correct? |
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| ExtraThiccBoi:
Hi, I need to build triac phase control for industrial application. I base my circuit off an old one which I need to recreate to replace in other machines. The original features: KT728/800 triac (800V, Imax = 15A, Igmin = 100mA, Igmax = 5A), 100nF/630V cap, 100kohm pot, some rando diac. My new design: BTA16-800CWRG triac (800V, Imax = 16A, Igmax = 35mA) 47Kohm 500V 2W pot 6.8kohm R1 (in series with the pot to limit gate current to 35mA when pot is all the way shorted) 100nF 630V cap - should i keep the original value or adjust it to the new resistor values and how? ER900 diac This is my first real electronics project, I'm normally a hobbyist. Thanks for any help |
| floobydust:
What is your mains voltage and frequency? I think 0.1uF is a bit big and with 47k pot too small, it does not give enough delay. Roughly, you would like the potentiometer at 1/2 way to give a 1/8 cycle mains delay so the control has a good range. This is how I select the potentiometer and timing cap. About a 2-2.5msec delay to charge the timing cap to diac 32V. For 120VAC 60Hz I use 0.047uF 250k potentiometer, and 240VAC 50Hz 0.047uF 500k potentiometer. Spice simulations show DB3 diac gives >500mA peak Igt so lots. I add the extra diodes which discharge (reset) the timing capacitor every cycle and give better low end control. This is the circuit schematic that works for me. More can be found in figure AN1003.12 or Teccor App note AN1003 |
| ExtraThiccBoi:
This circuit runs on 240ish 50Hz. It drives primary of a transformer. Thanks a lot for the schematics! I'll build it like you recommend. Can I use a low power bridge rectifier (like 100mA)? Before I put it in the machine, can I test it with like an incandescent bulb to see if it works properly? Also thanks for the delay info, I'll use that in the future! |
| floobydust:
That circuit was originally an eBay "4000W SCR" speed controller I bought, that did crappy and the pot smoked and the triac shorted so I fixed it. The 33k resistors need to be 1-2W, I did not try optimize those. I've never seen a 100mA bridge rectifier, but the diodes see maybe 10mA at peak mains voltage at least 350V. Yes use a light bulb to try it out. I was surprised a smaller timing capacitor 33-47nF did better. |
| ExtraThiccBoi:
So I got some more specific requirements. The circuit needs to be as simple and as compact as possible. The triac you recommend is massive overkill, the 16A one is enough (and costs third as much). I will also omit the FBR discharge circuit. I understand that it can significantly improve the performance, but we are talking +-1A control (it drives transformer that provides power to heat up hoses for polyurethane foam components) and the original schematic doesn't have it. I tried simulating the circuit but got this mess, any tips? I have been off electronics projects for 6 months, so most of my knowledge is a bit rusty, sorry about that :-[ Should I adjust the resistors for the BTA16-800CWRG? I wanna use this triac since it's in stock at my local store for few pennies. Probably a noobie question, but why is there that complicated resistor network for the gate current? Why does there have to be another 470K resistor in parallel with the 470K pot? And the 3.6k resistor in series with that? Most schematics I've seen had only the pot. Thanks and sorry for the nooby questions :D Edit: simulation errors solved, now it's working fine |
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