Hi,

Apologies if I have posted this in the wrong section,
Looking though my collection of PWM PCBs ,I would like to try and learn more about the various designs ,
I have noticed that some have 1 Triac and others have 2 triacs , and power the same type size of brush motor.

I would welcome info on why some have 1 and some have 2

 

Are you sure that the ones with 2 triacs, they are actually triacs or a pair of back-to-back SCRs? Could easily look almost the same.
Check out the semiconductor part numbers.

 

Could you post a schematic to use as reference?

 

Let me answer your question with the same amount of detail that you have provided in your post: "Because..."

 

So sorry I did not include more info on this , I have now taken 4 pics of the various PCBs , they are all UK 230v and all fit scroll saws.
At first glance I assumed they are all PWM , but now need to research and learn more about them.

I am now confused as a mixture of the following BTA 12 / TYN612M / BT152 (triacs / SCRs / Thyristors )

I am hoping the pictures maybe helpful,

 

When two SCR's are used as it seems they do here, they are two components of a bridge rectifier in order to produce phase angle controlled DC.
One of the boards looks as though it is possibly Mosfet controlled PWM. Which BTW, is different from SCR controlled AC.
Max.

 

Here is the circuit from the datasheet for the U2010B chip, it's used for mains control, ie light dimmers, motor speed, heating controllers etc....