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

Hi Walid,

I looked a little closer at the standard dimmer circuit and the capacitor is also quite important. My schematic shows a 0.33 uF but that is probably for 115 VAC and you are probably working with 230 VAC. If your meter can check capacitors the 0.033 should not be too small for it to read. It should indicate 33 nF. I was wrong before about testing the Diac. It should not show any resistance in either polarity on your ohmmeter. Diacs do not break down and trigger until they are in the area of 30 V plus.

John

Hi John,

My meter is digital and unfortunatly not a fancy one...I tested both triacs, BTB04-600A shows 1.9 mega ohm between A1 and A2 and goes to 0.6 kilo ohm when A2 and Gate shorted. BTB08-600BW shows 13.3 mega ohm and then 80 ohm when A2 and gate shorted.

I tested again the original box capacitor and it shows 0µF instead of 0.033 µF (or OL?). I have a spare polypropylene capacitor marked 333K (and 400VMY) which should be 0.033 µF but when tested it shows 677 µF :/

I found another spare polypropylene capacitor marked 103K (and 630 FN) and showing 0.1 µF when tested, maybe this one could replace the original one ?

What do you think ?

Thank you in advance.

W

wal_e  wrote:

 

Hi John,

My meter is digital and unfortunatly not a fancy one...I tested both triacs, BTB04-600A shows 1.9 mega ohm between A1 and A2 and goes to 0.6 kilo ohm when A2 and Gate shorted. BTB08-600BW shows 13.3 mega ohm and then 80 ohm when A2 and gate shorted.

 

The TRIAC measurements are OK

I tested again the original box capacitor and it shows 0µF instead of 0.033 µF (or OL?). I have a spare polypropylene capacitor marked 333K (and 400VMY) which should be 0.033 µF but when tested it shows 677 µF :/

I found another spare polypropylene capacitor marked 103K (and 630 FN) and showing 0.1 µF when tested, maybe this one could replace the original one ?

What do you think ?

Thank you in advance.

W

OL is usually range overload. That means that te capacity is higher than the DMM can show in the active range.

33 nF is typically supported by a DMM. Does your meter have an option to change range for capacity measurements?

333K is 33 nF, So that is a good one.

I found another spare polypropylene capacitor marked 103K (and 630 FN) and showing 0.1 µF when tested, maybe this one could replace the original one ?

That's also an off measurement. 103K should be 0.01 µF / 10 nF.

I would not put a 10 nF where the 33 nF is expected ...

Hi Jan,

So I can solder back the original triac on the pcb ?

My DMM has auto range selection I can't change anything, in the manual, for capacitance it says :

Measuring range 600 µF -> résolution 0.1 µF

Measuring range 6000 µF -> résolution 1 µF

In both cases: accuracy : +- 4% reading + +3digits

So I tried putting back the original triac and replacing the capacitor with the 333K and it didn't work...same problem: from OFF to ON max power but no dimming function...

Did you test the DIAC. Small cylindrical component with two leads that is between the gate of the Triac and the capacitor?

John

The Trigger diode or DIAC is a critical part of the dimmer circuit. It is designed to show a very high resistance to AC current until the voltage exceeds its thresh hold at which time it conducts.

https://en.wikipedia.org/wiki/DIAC

In the dimmer circuit the control (probably 500K Ohms) in series with a current limiting resistor( probably 10K) and the capacitor form an RC timing circuit. The setting of the control causes the voltage on the capacitor to rise at different rates. As soon as the voltage on the capacitor exceeds the threshold of the DIAC it fires and turns on the Triac which remains on until the AC current in the circuit passes through the zero point. The period for 60 Hz line voltage is 1/60th of a second and in this time the current passes through Zero twice. The AC line current will automatically turn off the Triac every 1/120 th of a second. If the Diac is shorted there will be no delay since the threshold of the diac will be zero volts and the triac will be triggered very early each 1/2 cycle which will give the appearance of full brightness regardless of control setting. If the DIAC is working properly the triac will not be triggered until much later in the half cycle and it will give the appearance of being dimmer.

John

Here is the Newark listing for the DIAC DB3 which is the most commonly used DIAC in the dimmer controls.

https://www.newark.com/stmicroelectronics/db3/diac-28v-to-36v-do-35/dp/89K1258

John

Here is the Newark listing for the DIAC DB3 which is the most commonly used DIAC in the dimmer controls.

https://www.newark.com/stmicroelectronics/db3/diac-28v-to-36v-do-35/dp/89K1258

John

It is too narrow for me to try disconnecting one of the leads it is a bedlight dimmer and everything is compacted. My DMM shows OL when testing de Diac in Ohm mode. I asked a friend for his professional DMM. I will test the db3 c531 and let you know but will most certainly replace it anyways...

Never thought to ask: What type of bulb are you using in the lamp?

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.