Dimmer repair

Hi Walid,

If you only have off and on and when the dimmer is in the on position it is always at full brightness then the most likely problem is the Triac. You can test this with a continuity tester when the dimmer is removed from power by checking the continuity between A1 and A2 terminals. It should be open under normal circumstances. If it is shorted then the triac is bad. The other components are probably OK. The BTB 04-600A is a 4 Amp 600 Volt triac and can be replaced with pretty much any triac that has ratings of more than 4 amps and 600 volts. I have some BTB 06-600 which would be 6 amps and 600 volts. If you are inside the USA I will send one to you. If you are looking at the triac with the pins down and the label facing you the A1 and A2 terminal are the left pin and the center pin. The right pin is the gate and should be attached to a DB3 trigger diode if you have a standard design.

John

Hi John,

Thank you for your help I really appreciate your kind answer but I am too far from you that would make this triac very expensive So, I am experiencing exactly what you described and already replaced the old triac with a new one labeled BTB08-600BW (the only "equivalent" triac I found in the neiborhood) which didn'ty solve the problem. So I just tested both of them and both of their A1 and A2 show no continuity....does that mean they're both open and ok ? When I tested the capacitor I got a 0.0µF instead of 0.033µF...is it possible that my tester is not sensitive enough or is my capacitor is dead ? In that case could he be the origin of the problem ?

Best regards,

W

Good question indeed, I am using a regular incandescent bulb.

Hi Walid,

In order to do some experimenting I built a dimmer circuit on bread board.

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The first thing I learned from doing this is that my explanation of the circuit a few posts ago was at best simplistic and probably incorrect. My Experiments indicate that your Diac is likely OK. The capacitor value is fairly critical. Too much capacitance and the light stays on high. Too little capacitance and the light stays on high. No capacitance and the light stays off. Shorted Diac or one with parallel resistance and the light stays off. No Diac in circuit and the light stays off. I am sorry as this may not simplify or help much. If you have experiments that you would like me to run let me know. I will leave the bread board assembled for a few days.

John

Tonight I hooked up the oscilloscope as I wanted to see the change in the signal on the gate of the triac as the potentiometer was turned from a dim setting to a bright one. Channel one (Yellow) shows the source AC voltage and syncs the horizontal trace. Channel two (blue) is the gate of the triac and the trace on the Hantek Scope is across the bulb. Note the RMS voltage as we go from very dim to full brightness.

Dim Setting

Bright Setting

Note of Caution. Taking readings of AC line voltage can be dangerous. I used an isolated bench AC source for this experiment and a portable secondary scope so than I would not have an Earth or Common ground conflict.

John

Hi John,

I am really glad to see that here are persons like you whom put all that effort to help others. So, I changed the Diac and nothing changed still from Off to On full brightness. This, with your last message about the importance of capacitance lead me to think that, in fine, it's the capacitor that's dead or faulty and unfortunatly I couldn't find the exact same one to replace it. I took some photos of the controller and the DMM I borrowed from a friend so if you have any suggestions or ideas please let me know, I would be happy to explore further more.

Best regards,

Walid

If you put the DMM in the 200 nF range,it should give a good measurement of that cacitor. : The value 33 - or something within 10% of that.

You need to fit the capacitor legs into the 2 slots with capacitor symbol.

Discharge the capacitor first.

It's indeed a good DMM.

Hi Jan,

I tested both: brown one on the PCB and the yellow - original one, they both read 00.0 µF :/ I am even starting to doubt the DMM....

I found this page which I think is quite interesting and clear :

https://www.astuces-pratiques.fr/electronique/variateur-a-triac-fonctionnement-et-schema

Is this how you measured them? If yes, I think they are defect.

Good call on your note of caution JW. Nastie sparks can happen when you fail to recognize the isolation.

I would like to know if the lamp lights with the DIAC out of the circuit.

John

I would like to know if the lamp lights with the DIAC out of the circuit.

John

No John, the light stays off whithout the DIAC...

Thanks for running the experiment which confirms that the Triac is OK and that the Diac is also OK.  Verify that the resistors have the proper values and then the only thing left is the capacitor.

John

So here are the mesured values:

C1 (yellow original one) labeled 0.033 µF reads 3.1 when DMM on 200 nF

C2 (small capacitor behind the big one) labeled 683K reads 70.1 when DMM on 200 nF

R1 (red, red, orange, silver or grey) supposed to be 22 Kohm reads 0.02 when DMM on 2 Mohm

R2 (blue, grey, brown, gold) supposed to be 0.68 Kohm reads 0.665 when DMM on 2 Kohm

C1 is not OK

C2, R1 and R2 measurements are OK

Thank you for the feedback Jan, problem is from what I understood C2, R1 and R2 are part of the main syteme that works to dim the light...C1 and the coil are here to reduce the amount of parasites emitted by the Triac and thus are "optional"....still the system doesn't work...

What is minimum and maximum resistance of the control?

Can you please be more specific ?

The knob that you turn to select dim or bright is a variable resistor. It has two terminals that attach at the edge of the board.  When it is at dim I would expect a reading of several hundred K Ohms . When it is at the bright position I would expect a low Ohm reading.

John

That knob is also used to go from OFF to min light, at that point (even when in off position) it reads 14.6 when DMM is in 20 Kohm and when max bright light it reads 1.8 when DMM is in 200 ohm.

See if you can remove the solder from one leg of the control so that it is separated from the circuit trace. Try the dimmer with power on. The light should stay off. Unplug the unit and check the resistance of the control again with the one leg still detached. When you read the meter watch to see if a "K" is displayed with the number on the display. What are the minimum and maximum position resistances?

John