TRIAC Dimmer troubleshooting

Thread Starter

farzad latifeh

Joined Oct 3, 2017
63
I made a TRIAC dimmer circuit using this reference,
it works very smoothly without snaps-on, but the C1 timing cap. has to be 100nF exactly, if it turns below some points like 85nF, then the lowest point of POT cannot turn off the TRIAC perfectly, for instance, in my work, I bought some 100nF cheap MKT cap. which were 82nF in real and at the lowest point there is still 50V on the load which is undesirable, if I use 150nF caps then the TRIAC triggers at 50% of POT which is again not good, I know I can buy better caps to overcome the problem but the question here is how to change the circuit in some way to be adaptable independent of the C1 timing cap? I mean is there any practical way to adjust the off point of TRIAC with some like POT or something else?
appreciated of your informative answers.
 

Dodgydave

Joined Jun 22, 2012
9,848
It's just a combination of resistance and capacitance making a timer, then when it reaches 28V( which is the diac strike voltage) , increasing either will make the time between firing longer, and thus dim the light.
You could also put a 1M preset in parallel with the potentiometer to narrow the span control.
 
Last edited:

Thread Starter

farzad latifeh

Joined Oct 3, 2017
63
It's just a combination of resistance and capacitance making a timer, then when it reaches 28V( which is the diac strike voltage) , increasing either will make the time between firing longer, and thus dim the light.
You could also put a 1M preset in parallel with the potentiometer to narrow the span control.
Actually it worked with 100K for R2, but the full power reduced to 180 Volts, I guess using 1M in parallel with POT may help to narrow the span control, anyway thanks for informative answer.
 

Dodgydave

Joined Jun 22, 2012
9,848
Can someone explain the role of the Bridge BR1? Never seen one like this in such circuits.
Yes the bridge rectifier is to rapidly discharge the capacitor on the next half cycle of the mains, this is to remove sluggish turn on at low levels.

If you have a mains dimmer and noticed that the light comes on rapidly at low levels , but dims perfect to off , that is the cause so to eliminate that the bridge rectifier is used, ..
 

Thread Starter

farzad latifeh

Joined Oct 3, 2017
63
the circuit is perfect in case of using it for resistive loads like incandescent lamps, but in the case of using it for induction loads (especially for Shaded-pole motors) the very smooth increase in Voltage don't work, actually, the lowest voltage to turn the motor on is something around 140V (for 220V, 1.2W motors), as @Dodgydave explained the control of RC timing values can control the minimum and maximum Voltage, accordingly, I changed 100nF cap with 33nF and it works, but the question here is, is this circuit actually OK for induction loads like what I mentioned to control the motor speed or there is something I missed?
 

Ramussons

Joined May 3, 2013
913
Yes the bridge rectifier is to rapidly discharge the capacitor on the next half cycle of the mains, this is to remove sluggish turn on at low levels.

If you have a mains dimmer and noticed that the light comes on rapidly at low levels , but dims perfect to off , that is the cause so to eliminate that the bridge rectifier is used, ..
How does it function?
 

Dodgydave

Joined Jun 22, 2012
9,848
As explained above, it rapidly discharges the capacitor on the next half cycle, because the capacitor is charged say positive so on the next half cycle the capacitor has to charge up negative, the bridge rectifier only conducts to discharge the capacitor .

If you click on the link in the first post it will explain it.
 

Ramussons

Joined May 3, 2013
913
As explained above, it rapidly discharges the capacitor on the next half cycle, because the capacitor is charged say positive so on the next half cycle the capacitor has to charge up negative, the bridge rectifier only conducts to discharge the capacitor .

If you click on the link in the first post it will explain it.
Thanks.
 

DickCappels

Joined Aug 21, 2008
6,788
1609851539238.png
I see a small shock hazard. C2 needs a bleeded resistor so a charge doesn't stay on C2 after, for example, pulling the plug out of the wall 100k or so should do it. It think it also wise to use this resistor when the circuit doesn't have a plug that way you don't have to remember to discharge C2 before handling or testing the circuit.
 

Kjeldgaard

Joined Apr 7, 2016
435
I see a small shock hazard. C2 needs a bleeded resistor so a charge doesn't stay on C2 after, for example, pulling the plug out of the wall 100k or so should do it. It think it also wise to use this resistor when the circuit doesn't have a plug that way you don't have to remember to discharge C2 before handling or testing the circuit.
There is a discharge path through R4 or R5, and through the bridged rectifier.
 
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