Triac/light dimmer question

Thread Starter

hakaplan

Joined Jun 10, 2020
4
I am a hobbyist and a general fixer around the house. I know just enough electronics to get into trouble. I have an old inline cord lamp dimmer with switch that stopped dimming. It only turned full on or off. In reading online I gathered that the triac was likely ruined by a power surge. The original triac was X25783. I thought that as long as the ratings exceeded the requirements, I could use anything. I purchased a BTA16-600B, soldered it as the previous had been and tried it. Now the pot travels nearly the entire way until the very end where it lights completely and only limited dimming is possible by slightly backing off. How did I screw up? Thanks very much.
 

Thread Starter

hakaplan

Joined Jun 10, 2020
4
possible the pot burnt in the surge, only the end of the track is now connected,
Well before I replaced the triac, as soon as I would turn it on, it lit and stayed lit all the way turning the pot--so doesn't that mean the pot was conducting from the very start point?
 

Hymie

Joined Mar 30, 2018
846
I would suspect that the triac has failed short circuit; once the dimmer switch (which is integral to the pot, either through rotation or push on/off) switches power to the triac – full power is applied to the lamp.
 

Thread Starter

hakaplan

Joined Jun 10, 2020
4
I would suspect that the triac has failed short circuit; once the dimmer switch (which is integral to the pot, either through rotation or push on/off) switches power to the triac – full power is applied to the lamp.
So Hymie do you agree with andrewmm that with the new triac in place that the pot is likely bad?
 

KeithWalker

Joined Jul 10, 2017
1,144
The problem may be caused by the new triac having a different construction to the older one. I can't say for sure because I can't find a data sheet for the X25783 but it's equivalent, Q2010LT has a diac built into the gate circuit. The BTA16-600B does not. You will have to add a diac in series with it's gate to get it to work in your dimmer.
Regards,
Keith
 

Thread Starter

hakaplan

Joined Jun 10, 2020
4
The problem may be caused by the new triac having a different construction to the older one. I can't say for sure because I can't find a data sheet for the X25783 but it's equivalent, Q2010LT has a diac built into the gate circuit. The BTA16-600B does not. You will have to add a diac in series with it's gate to get it to work in your dimmer.
Regards,
Keith
Keith,
I obtained a DB3, inserted it as you instructed, and it now works perfectly. And I learned something I didn't know. So thanks very much for your help.
Howard
 

BobaMosfet

Joined Jul 1, 2009
1,177
I am a hobbyist and a general fixer around the house. I know just enough electronics to get into trouble. I have an old inline cord lamp dimmer with switch that stopped dimming. It only turned full on or off. In reading online I gathered that the triac was likely ruined by a power surge. The original triac was X25783. I thought that as long as the ratings exceeded the requirements, I could use anything. I purchased a BTA16-600B, soldered it as the previous had been and tried it. Now the pot travels nearly the entire way until the very end where it lights completely and only limited dimming is possible by slightly backing off. How did I screw up? Thanks very much.
You can't use just 'any' component. Engineer uses specific components because that is what engineering is about.

Likely the correct trying is the ubiquitous MCR100.
 

PAFMC01

Joined Mar 20, 2013
11
I am a hobbyist and a general fixer around the house. I know just enough electronics to get into trouble. I have an old inline cord lamp dimmer with switch that stopped dimming. It only turned full on or off. In reading online I gathered that the triac was likely ruined by a power surge. The original triac was X25783. I thought that as long as the ratings exceeded the requirements, I could use anything. I purchased a BTA16-600B, soldered it as the previous had been and tried it. Now the pot travels nearly the entire way until the very end where it lights completely and only limited dimming is possible by slightly backing off. How did I screw up? Thanks very much.
I had a bit of trouble finding a datasheet on the X25873, but then I did not look hard. What I did find is a post on another site
https://electronics.stackexchange.com/questions/205968/part-identification-in-this-dimmer-switch
which identifies the X-thingy as a quadrac - NOT a triac. A quadrac is a triac with an inbuilt diac. I agree with you that any of a number of triacs should -no - WILL work in your repair job. Just add an external diac. I would also add a resistor from Gate to T1 to shunt thermally generated leakage current which will likely cause false trigering if the triac gets hot. Values around 560 to 3300 ohms depending on the other circuit values. A lot of "simple dimmer circuits" do not show this resistor, but my 4layer bible in the late 60's early 70's was the GE SCR Manual and they often included it in practical circuits.

My own version is the 4th edition, acquired in 1967, and I still do not know of a more instructive source than it, unless it might be
the 5th edition, found here:
https://www.scribd.com/document/91570204/GE-SCR-Manual-1972
 
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