Electronic – good reason to use an SCR based speed controller instead of a TRIAC


I tried to purchase a switch for a reciprocating saw with variable speed for my Dewalt DW304. I discovered the part is no longer available.

Instead, I bought a similar switch used on the later model DW304PK. Its electronic, however, baffles me. The original one has a switch housing a simple DIAC circuit for triggering an external TRIAC, but the new one a more complex SCR circuit.

Below is my shoddy attempt at a schematic. I was able to read values from what I believe to be diodes, capacitors and resistors. The black strips are resistors. The two longer ones are slide resistors (?). I was unable to obtain a datasheet for it, but the make is Marquardt and the part number is 2069.0504. The SCR is a TN1215.

I was under the impression that an SCR throws away half of the voltage, whereas a TRIAC uses the full voltage. Is there a good reason why the manufacturer would use a SCR circuit when you are only getting half the power? For machines designed to cut metal, a lower speed is required, but what about torque?

Are the two glass diodes serving to rectify the other AC half, making the SCR behave as a DC speed controller on a universal motor? Perhaps, the circuit is serving the same function as the DIAC circuit by triggering phase angles?

Original Switch Front Circuit Board
Original Board

New Switch revealing board
New Switch Revealing Circuit Board

Front of Board

Front of New Circuit Board


simulate this circuit – Schematic created using CircuitLab

Poor attempt at creating a schematic

Best Answer

Maybe this is what the circuit is:

enter image description here

This circuit uses back EMF as feedback to better regulate the speed with changes in torque. The C106B is a sensitive gate SCR good for a couple of A.

I lifted the image from this site, but it was most likely originally plagiarized from the General Electric SCR manual and the obnoxious watermark added. GE has long ceased making this kind of device. The part numbers probably indicate an origin ca. 1970-75, give or take 5 years.