Convert high voltage keying to positive keying

Discussion in 'Straight Keys - CW Enthusiasts' started by VK3MTW, Aug 27, 2021.

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  1. VK3MTW

    VK3MTW Ham Member QRZ Page


    Thank you have just tested that with my multi and then a test light to see better.

    Will work very nice

    Will post up my built soon.
    Thank you
     
  2. KG7WGX

    KG7WGX XML Subscriber QRZ Page

    So, here is a sketch showing how that might work. I didn't show the internal MFJ keying circuit, but that is what completes the path to the negative side (gnd) to energize the relay.

    Cutting the factory wiring to the jack is necessary to isolate the internal relay source and the external factory jack source.

    Relay schematic 2 sm.jpg
     
  3. VK3MTW

    VK3MTW Ham Member QRZ Page

    Oh my excellent.

    Thank you again.
    I was in the process of drawing and sending my idea to make sure I knew what to do.

    I'll still upload my drawing so you can see what I thought from your 2nd last msg... IMG_20210829_215659129.jpg
     
  4. KG7WGX

    KG7WGX XML Subscriber QRZ Page

    It should work.

    Whatever connector you use for the boat anchor key output should be rated for that voltage, of course.
     
  5. K7JEM

    K7JEM Ham Member QRZ Page

    Make sure that you place a reversed biased diode across the coil of the relay. This prevents the high voltage spike that is generated when the relay is deenergized from destroying the internal transistor.
     
  6. VK3MTW

    VK3MTW Ham Member QRZ Page

    Cheers. Do I put that between both contacts on the coil contacts?
     
  7. VK3MTW

    VK3MTW Ham Member QRZ Page


    Like this? IMG_20210829_222615388.jpg
     
  8. K7JEM

    K7JEM Ham Member QRZ Page

    The diode goes the opposite direction. The cathode goes to the supply voltage.
     
  9. VK3MTW

    VK3MTW Ham Member QRZ Page

    Will be a delay in the build as covid has shops closed.
    Thanks I will install the cathode side to the mains power.
     
  10. K7JEM

    K7JEM Ham Member QRZ Page

    Have you verified that the radio that you are attaching to has voltage on the key in excess of 70v? The transistor in the keyer should be good for that much, assuming it is a positive voltage on the key with respect to ground.
     

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