Motor Starting Switch - Page 1
Forum section: electronics / beginners

author: RyanG
author_rank: Regular Contributor
timestamp: March 27, 2017, 09:10:58 pm
content: I know what this circuit does but I don't really understand how the bottom half of the board works. I'm not very good at understanding the AC input stuff on this type of board. It replaces the old centrifugal switches used in the motors at work. Can anyone explain how it works? I have a general idea about the rectifying and the optotriac, but how does it turn the opto on and off? And, is there an easier way to do the same thing?

author: danadak
author_rank: Super Contributor
timestamp: March 27, 2017, 10:24:21 pm
content: The emitter is constrained to 12V. So when the ac line rises above 12 + Vq1vbe + Vd5 ~=12 + .7 + .7 =~ 13.4V then current starts to flow thru OpTo LED side.Regards, Dana.

author: Benta
author_rank: Super Contributor
timestamp: March 27, 2017, 10:30:08 pm
content: Is the circuit reverse engineered and the schematic built from there?Not so important, but it is very messy to look at.To help yourself, I'd suggest straightening out a lot of the connections, and perhaps suddenly the circuit will make sense.Not directly helpful, I know, but tidy schematics save a lot of headaches.It seems to be running off 240 VAC, if I understand the labelling correctly.

author: apelly
author_rank: Unknown
timestamp: March 27, 2017, 10:49:19 pm
content: I'd guess is it's a zero crossing detector

author: Paul Price
author_rank: Super Contributor
timestamp: March 27, 2017, 11:11:53 pm
content: Not so fast, danadsk and apelly!The 220uf cap across the 12V zener is being charged through 6.6k. This has a T=RC or .76 Sec and will take many cycles to reach 12V. At the same time there is an asymmetrical bias to the 2N4403, the base voltage is rising with a T=RC of .033-sec and during the negative half-cycle the cap on the base is discharging and the current through the opto-triac increases changing the duty cycle of the start winding current..apelly, The opto-triac is a random type, so the whole circuit design is not to create a zero-cross circuit.Adding to the complexity is the 100uF cap across the C-E of the 2N4403 and when the transistor conducts.During the charging of the 220uF capacitor over many cycles, the current through the opto-triac rises slowly having the effect of a slowly increasing duty cycle of the motor start winding by phase-control of the triac in series with the start capacitor, giving the motor a soft start.

author: Benta
author_rank: Super Contributor
timestamp: March 27, 2017, 11:30:55 pm
content: You're over-analyzing. At the cathode of D6 you have a somewhat steady +12 V supply for the optocoupler circuitry. R7 and R8 are just for limiting inrush current and ripple current in C5 as well as peak current in D6.The rest is so convoluted due to bad schematic artwork that it is unreadable. Just look at the connection to D6. Makes your head spin...

author: Paul Price
author_rank: Super Contributor
timestamp: March 27, 2017, 11:49:46 pm
content: I see no problem at all reading the schematic nor with the top connection to D6, it is simply an easy way to add the zener to the schematic without redrawing the schematic because of the tall size of the vertically placed zener schematic symbol.I don't think you can ignore the time-constants here in charing the 220uF to 12V, they serve a phase delay purpose in the starting the motor.The two 3.3k resistors have to be so large in resistance to limit the power delivered to the zener diode, but also create a RC delay. They are not limiting inrush current, small-valued resistors always do that and this is not their purpose, their use is in setting the maximum current and power to be dissipated by the zener. If R7 R8 were small enough in value to limit inrush current this circuit as drawn would attempt to use several tens of watts and magic smoke would come forth!The purpose of R6 is to discharge the cap across it and cause the 2N4403 to turn on during the negative half-cycle of the sine waye AC and change the phase-angle of the opto-triac as the voltage of the zener attempt to rise to 12V. The peak current in R6 is already limited by the zener diode voltage and not at all a consideration here, this is certainly not the purpose of R8 R7.Consider the original post's remarks about what this circuit replaces, the centrifugal start switches on a motor whose purpose was to switch off the start capacitor circuit when the motor reaches running speed, and this takes some time.While it is a waste of time to over-analyze a circuit, it is also a fail to rush to conclusions. even a motor-starting circuit knows this..

author: RyanG
author_rank: Regular Contributor
timestamp: March 28, 2017, 01:13:06 am
content: I did reverse engineer it from a board, and that is why the schematic is a mess. I understand that it doesn't look neat, but I do know that it is very readable in its current form. I just wanted to better understand how it works, and I might try to make these instead of buying them for $60+ a piece.