What is the purpose of R1/R2 in this rectifier

Discussion in 'General Electronics Chat' started by adr1an, Feb 11, 2010.

  1. adr1an

    Thread Starter New Member

    Feb 11, 2010
    Good evening all!

    I've often referred to the fantastic information in the other parts of this site.. but today... I have to unfortunately hassle everyone and ask a (probably inane) question. I really should know the answer to this, but, well.. I'm having one of those days! (weeks?) :)

    In the following schematic - what is the purpose of R1/R2 in the Rectifier Unit ? And to what degree can I (potentially) alter their values ? The BoM for this refers to 39k 2Watt MFR's.. but I can only get 1 Watters easily, so need to run some in parrallel to get the dissipation that the original author felt was neccesary.

    And silly question - but how do I determine the current through the circuit when the 'source' is off an Alternator that obviously changes its generated output based on speed etc... do I just go and measure the maximum output at peak-RPM's in the existing Rect/CDI setup, or - more likely - is there some formula I've missed here ? :)

    I really appreciate any hints, tips or feedback anyone can offer.. Thanks very much!

  2. JDT

    Well-Known Member

    Feb 12, 2009
    The purpose of R1 and R2 is to ensure that the reverse voltage experienced by D3 and D4 is equal.

    Presumably, the reverse voltage on this alternator winding is more than 1 doide can stand. So, 2 diodes in series. Without these resistors, if one diode had more reverse leakage current than the other then the reverse voltage will not be shared. These resistors swamp out the diode leakage current. You could try 1W resistors and see how hot they get. But DON'T TOUCH THEM WHEN IT IS WORKING!

    As far as determining the current: This circuit charges up the capacitor C3. C3 will charge uop in a series of steps. The output of the alternator coils will be a series of pulses of current as the rotating magnets (in the flywheel?) fly past the coils so a straightforward measurement is not easy to do.

    I notice that an extra winding on the alternator (sig) triggers the thyristor Q1 probably once per revolution, discharging C3 into the ignition coil.

    Capacitor C3 will be charged to a high voltage, 400V + so be careful!
  3. adr1an

    Thread Starter New Member

    Feb 11, 2010
    Thank you very much! :)

    It of course makes sense now.

    I'll give the 1 Watt's a go and test it after a brief run.

    And yes, I have noticed the extra winding for the trigger. Its actually for a friend of mine who is rebuilding one of his many bikes - this is a 1973(??) H2 750 Kawasaki (its a triple cylinder two-stroke), which apparently, had notoriously bad CDI's (the caps would fail and then blow R1 in the CDI). This is an 'improved' design he passed to me that is apparently highly recommended.

    Once again - I appreciate you taking the time to respond. :)
  4. studiot

    AAC Fanatic!

    Nov 9, 2007
    I'm sure the 39K value is not critical.