I'm building this circuit from an old schematic for a stroboscope (spins a disk with patterns on it and strobes a light based on input frequency). I've already built the frequency divider (it is extremely cool ripple counter with clock chopper feedback that calculates a 1/(2^n) equation), and the input strobe circuit (converts an audio signal to a square wave). I'm stuck on the motor circuit.
I believe it is controlling the speed of a 50V AC motor via frequency because the input on the lower left NPN is a 50% duty cycle square wave that varies from 30-90 Hz.
There's 50V DC supply on top there, and the input signal is 5V (through the 4K7 on the lower left). The transistors stacked below the 50V are all listed as generic high Vce devices, and the 71963-1 devices are just generic NPNs. I have no info on the motor, other than I assume it is a two pole 50V AC motor.
Three things about the circuit confuse me.
1) Why the need for the NPN with 1K tied to 50V on top?
2) I understand the need for a Darlington array to drive the high voltage and current, but why the PNP on the bottom? (Is it to pull to ground faster b/c of the current in the coils?)
3) Why would the cap values into the windings be so different (100n & 5u6)? Is it something specific to the SKU of motors they had made for the device, or is there some other standard trick here that I don't get? (note: the text on the lower cap says 5.6 NP, not NF, I assume that means non-polarized?)
Thanks in advance, I know it's a lot of Q's but searching for "basic analog circuit analysis" on google brings up much crap.