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TV Flyback: finding HV return

Discussion in 'Electronic Basics' started by Dominic-Luc Webb, Jan 15, 2005.

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  1. I am after high voltage (for photomultiplier tubes) and considering
    use of TV flyback transformers. I have a couple, including some
    "known good" specimens that were extracted from working TVs.

    I am able to find windings (2 sets, I suspect) on the base pins that
    certainly must be 2 primary coils. Using an Ohm meter and checking both
    polarities, I do not find a return for the high voltage output (red with
    suction cup), or the two ouput wires coming from the side that I suspect
    are somewhat lower HV (horizontal and vertical control, 10 kV ???).

    Is there some nice trick to identify the HV return, or should I try to see
    where I can generate a sparc from the remaining pins that are not primary
    leads? Should this return be common to all three HV outputs? As I mention,
    I want to power PMTs, so even 10 kV is escessive for my needs.



    Dominic
     
  2. If the suction cup lead is included in your resistance measurements,
    then a high voltage rectifier (several silicon junctions in series)
    are also included. Your ohm meter probably does not contain a high
    enough voltage source to forward bias this string of diodes to
    indicate approximate coil resistance. Add a couple 9 volt batteries
    connected together in series and use the milliamp meter to find out
    which low voltage winding is also connected to the high voltage output
    (positive) lead. You will need to connect the negative side of the
    battery to the the suction cup to forward bias the rectifier, since it
    is designed to produce a positive output voltage (and block it from
    going back into the coil).

    Unfortunately for you, PMTs need a high negative voltage with respect
    to the near ground output. So you will either have to find a flyback
    that has a diode that can be reversed, or has the diode external to
    the flyback transformer (rare these days).
     
  3. Hi John,

    Thanks for the info and I will look into this. You are of course correct
    about the negative voltage, especially when it comes to classic PMTs like
    RCA 1P21, 931A and 931B used in astrophotometry. However, there are some
    that use positive voltage. I never investigated these because they were
    not quite right for the UVBRI measurement system commonly used in
    astronomy. In the present case, I will indeed have to end up with negative
    voltage.

    For the moment, I am exploring the use of flybacks to generate the needed
    voltage via my 555 timer circuit. For the moment, it is a learning
    exercise to see how well if I can get good coupling, what frequency, etc.
    I have access to quite old flybacks, so I suspect I can escape the diode
    problem.

    Dominic
     
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