The dimmer apparently will not reduce the AC input voltage, that is, connected to an incandescent light bulb, the bulb appears to have maximum brightness. Rotating the brightness control doesn't produce any change in the brightness. Opening it up, all of the passive components appear to be in good condition. Testing the pot of the dimmer, resistance between the wiper and another terminal can be varied from about 500 Ohm to 280 k Ohm. What appears to be a DIAC connects to the gate of the dimmer's Triac. Would a safe assumption be that the DIAC has failed?

If the TRIAC has failed, I would think that no voltage would make it to the light bulb.

 

Triac shorted?

 

With the dimmer not connected to AC voltage or a load, I tried measuring resistance between the MT1 and MT2 terminals. I got 0 Ohms resistance for both DC voltage polarities of the meter. This should indicate that the TRIAC has failed, shouldn't it?

I tried searching the part number of the TRIAC on the web and didn't find anything. The dimmer is rated 120 V and 600 Watts. On hand I have a NTE 5608 TRIAC that is rated 8 A. Would it be worth it to try replacing the original TRIAC with the 5608, or is there a good possibility that the triggering circuit won't be a match with my replacement TRIAC?

The MT1 and MT2 terminals aren't shorted.

 

A failed triac is the common fault in dimmers. The failure mode is usually a short between MT1 and MT2. I keep a stock of L4008 or Q4008 for this reason. 5608 will work.

 

A failed triac is the common fault in dimmers. The failure mode is usually a short between MT1 and MT2. I keep a stock of L4008 or Q4008 for this reason. 5608 will work.

Thank you for the feedback.
-Pete

 

Worn out potentiometer comes second after triac failure.
but if you measure 0Ω between MT1 and MT2 hen the triac is shorted: replace it.

 

A major difference between the manufacturer's TRIAC and the NTE5608 TRIAC is that the mounting tab/ heat sink (TO220 package) of the 5608 is at the MT2 voltage. Not so with the original TRIAC; the tab of the original is isolated from MT1 and MT2.

After installing the 5608 in the dimmer, there is no control of the light output of a bulb as the load for the dimmer; all of the voltage is going to the bulb regardless of the resistance setting of the dimmer.

 

Is the tab creating a short circuit between MT1 and MT2? If so, use an isolated tab triac such as BTA12-600B.
Is the tab creating a short circuit to earth? If so, make sure you don't kill someone.

 

It might possibly be that the bypass switch has become stuck. That also happens on occasions.

 

The two capacitors in the circuit of the dimmer are marked .062K / 200V (decimal point 062). So capacitance of these capacitors equals .062nF, is that correct? If they are, that is much lower capacitance than what I've seen in the circuit diagram of a similar dimmer. The capacitors appear to be film polyester and much larger than a typical pico-farad cap; this type of cap would almost never fail, isn't that true?

Testing the diac with my ohmmeter, it is open circuit.
'

 

A diac will test open with a multimeter, even if it is ok.

 

A diac will test open with a multimeter, even if it is ok.

Measuring resistance of the DIAC with a DMM (low voltage for the DIAC) and it measures very high resistance indicates that at least it hasn't been damaged to the point of passing current at any voltage.

 

68pF is outside the measuring range of my DMM. However I did measure resistance of the two capacitors in the circuit of the dimmer and one of them held at 0 Ohms. Looking at the Mouser website, I see that such a capacitor with a high voltage rating isn't unheard of.

Connecting a 47k Ohm resistor in series with the DIAC of the dimmer to my bench power supply, and measuring voltage drop across the DIAC as I increased DC voltage output of the supply, the voltage drop across the DIAC increased to 35 V and then decreased for a further increase of the supply voltage. This was the case connecting the end terminals of the resistor/ DIAC both ways to + / - output terminals of the power supply. So I'm assuming that I don't need to replace the DIAC.

 

That sounds like what a diac is supposed to do, I think.

 

However I did measure resistance of the two capacitors in the circuit of the dimmer and one of them held at 0 Ohms.

So you‘ve found the fault: a short-circuit capacitor.

 

You need to open the circuit on at least one side to check a capacitor. If it is zero when isolated then that is a failed part.

 

You need to open the circuit on at least one side to check a capacitor. If it is zero when isolated then that is a failed part.

Yes, I did open/ isolate one side of both capacitors in testing whether or not they would take a charge.