DC Motor PWM controle with BJT and SCR - Page 1
Forum section: electronics / projects

author: maher471
author_rank: Contributor
timestamp: March 24, 2025, 10:23:31 pm
content: Hi, after i redraw a schematic by reverse engineering for the bernina sewing machine ,I would like to know:1- The role of the SCR (T80) in regulating the motor speed, which is already controlled by the PWM signal from the T77.2- Is the R23, R11, and R10 network used to measure back EMF or not?

author: moffy
author_rank: Super Contributor
timestamp: March 24, 2025, 11:58:51 pm
content: I think the SCR is the basic ON/OFF controller, but when switching OFF the SCR needs 0 current flowing through it, which greatly would reduce any voltage spikes, but not really sure as the hybrid circuit hides the controller logic.

author: jwet
author_rank: Frequent Contributor
timestamp: March 27, 2025, 01:36:34 pm
content: That's really interesting. The hybrid certainly muddies up the function.My best guess based on the age and the partial schematic-I believe that the SCR is a coarse speed control and the 10 Khz hi side PWM is the fine control. The hint to me is the snubbing circuits to the right of the SCR. If it was simply on and off, I don't think this would quite be so elaborate. The problem with 60 Hz "dimmer" type circuits running motors is poor torque and smoothness at lower speeds. Bernina's are high end machines. In a modern design, this all might be done with a HV MOSFET for IGBT but when this was designed, this topology was what they had. The SCR is an effective way to start the motor (using a high duty cycle on the PWM transistor). After start, the PWM can be adjusted dynamically for fine control. The SCR is triggered on zero crossing from the hybrid, the PWM likely synchronised at the start with a zero crossing but is then free running. I would think that the PWM duty cycles are kept fairly high (>50%) and the SCR is run closed loop to get smooth start and stop. The components that you mention across the motor are back emf sensing and close the loop on all this. The pot is an overall velocity feedback control. This is very sophisticated for the time especially. In a modern machine, this would all be done in a small uP with some modern IGBT or MOSFET drive.Are you repairing or modifying? Ed Nisley of "The Smell of Molten Projects in the Morning" and Circuit Cellar fame has done a bit of blogging about experimentation with his wife's sewing machine(s). Very complete analysis and entertaining like most of Ed's stuff. Follow this link and search for sewing machine- he wrote a lot about "deglaring" too- getting the lighting just right.EDIT before any comments were made-After a further look- it appears that the backemf speed feedback also has a component of current thrown in with the low valued sense resistor at the bottom. I think what this does is make the feedback a combination of torque (current) and volts. This combination is sort of "power feedback" which might be what someone sewing wants. I think if this is the case, then the pot is soft of a power profile vs. speed- how much of the feedback signal is torque vs. speed. I imagine its mostly speed but at some minimum torque. Consumer electronics is so clever when you analyze it carefullyLink-https://softsolder.com/Good luck.