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?
Front of New Circuit Board
Poor attempt at creating a schematic