I have two Danfoss RDS 20 dc motor control boards. Don't have circuit diagrams for them. The boards are double sided, difficult to trace. What i found so far:
They use 555 timer as oscillator to drive SCR rectifier. Duty cycle is controlled with a 5K pot through 324 opamps. On one board there's 12V supply on the timer and nothing else. The other is stuck on 12% duty cycle and the pot doesn't affect it. What i can't figure out is the circuit for 555 they use. It doesn't look like anything from datasheet's possible applications. Maybe someone can explain to me how they regulate duty cycle of 555 based on my measurements (see attached). Also Vpp might be a little off, was measuring with a cheap pocket scope.

 

I have a dc motor control board RDS 20 by Danfoss electronics. Its a discontinued model. No circuit diagram available. I already replaced it with KBIC-225, but i'm trying to understand how it works for educational purpose. Its a 240V input 180V output half-controlled bridge rectifier. I've traced part of the diagram (see attached). But i have trouble understanding how it works. Gates are connected to the output of a 555 timer working in astable mode without any external regulation at 50% duty cycle. Speed is controlled by a potentiometer connected to one of four (or maybe several, didn't trace that part of board) 324 opamps. The only connection between opamps and 555 is through point "1" on the diagram i drawn. The way i understand it, the outputs 1 and 8 of 324 draw the gates of the SCRs to the negative voltage, thus reducing the duty cycle even more. Now the question: if i understood it correctly, what's the point of this design. it seams like the max possible duty cycle is 46%, or am i missing something?

 

We need to see how the lm324 is wired to the Scrs, I presume there are 2 diodes and 2 scrs fed from an AC supply, and fired each half cycle.

The 555 circuit can only be altered by the Control pin 5, or by altering the timing resistors and capacitor on pins 6,7.

The 10k on pin4 and the 3k on pin 3 can be omitted.

 

We need to see how the lm324 is wired to the Scrs, I presume there are 2 diodes and 2 scrs fed from an AC supply, and fired each half cycle.

You can see how they are wired on a photo attached to the second message.

324 drives the gates to a negative voltage at 120hz. My question is whats the point of this uncontrolled 555 timer?

 

Took some measurements at test-points 1,2,3,4 on the diagram