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Measuring PTs

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  • Measuring PTs

    So after reading this article :

    I'm pretty inspired I've always wanted to try winding pickups but winding a PT would be so much more useful for what I'm doing right now and I'm all over the idea of recycling PTs from old microwaves and stuff. My question is to do with safety though. I've always been told not to connect a multimeter to a mains, hooking it up to the other end of a step up transformer to measure it seems even worse. I just don't want to electrocute myself, so I want to confirm a few safety things before I go ahead and try measuring a PT out of a microwave or something like that. if I want to measure the voltage of the secondary of a PT I would measure from either side of the secondary winding (but not the center tap, or does it matter) to ground? what I'm not sure on here and I just feel like I should know this stuff before I even think about trying anything is if I disconnect the ground am I going to get electrocuted, and why? because I'm now the ground? and why don't I get electrocuted when I'm holding the probes to the secondary and ground? because the multimeter draws so little current? and is the path of least resistance for the current?

    Can someone just shed some light on these basic things for me, I'm learning more and more about all this stuff on paper and its time for me to take a step back and just get some hands on stuff under my belt, and I'd like to live through it if possible.

  • #2
    I am hesitant to say do this and do that, because these voltages and currents can KILL YOU. But the only way to get electrocuted is for your body to become a CONDUCTOR. Current comes in one point and exits another. If it is from one side of your finder to the other, it hurts, startles you, maybe scares you, but doesn't really threaten your life.

    However, if that current comes in a fingertip and exits through your belly leaning against the chassis, the current can pass through your heart and stop it from beating. Hard to get happy when that happens.

    You cannot measure high voltage without connecting a measuring device to it. COmmonly we use a meter or a scope. Many voltages are measured with respect to ground, but not all. If you want to measure the high voltage secondary of the PT, if it has a grounded center tap, then you can indeed measure voltage from either end of the winding to ground. Hopefully each half will measure about the same and would then add up to the total voltage of the winding.

    But not all windings use the center tap or even have one. Then you measure from end to end. Only way to do it.

    You have to respect the mains. Not only is the voltage there lethal, but it has enough current behind it to cook you. But I have to respect the highway I drive on. Certainly I can die there as well. It all boils down to knowing what you are doing and being CAREFUL when you do it. How else would I find out what the mains voltage is without measuring it? I stick my two probes into the two slots on an outlet while the meter is set for AC volts and there on the screen is the reading.

    WHy don't I get electrocuted? I didn't touch anythig that was a conductor. My metal probe tips did, but I didn't. It is like picking up poop behind your dog. If you use your fingers you get poop on them. If you use rubber floves or a plastic baggie, you don't. The baggie INSULATES your fingers from the poop. Your meter probes are plastic or rubber cylinders with the metal probe running through the center. The rubber or plastic is an insulator, so the current can't get to your fingers. No shit!

    The key is to NOT TOUCH any conductor. And that includes the chassis. The chassis is SUPPOSED to be grounded or at least cold, but there are many ways that the chassis can wind up hot. Especially in amps without a three wire mains cord, I ALWAYS measure for AC volts on the chassis with respect to the earth ground on my bench. If it is present then I reverse the polarity switch, reverse the mains plug, or connect the thing to an isolation transformer. Point being I don't assume, I find out. Then deal accordingly.

    I don't know why you'd disconnect the ground, but things can be without. There aer plenty of occasions where I might disconnect the transformer windings and have the wires pointing into the air. For example when I suspect it is bad and I disconnect the secomdary loads for current draw tests.

    To make a safe voltage reading, the probes must make good and stable electrical contact with the point in the circuit of interest. The pointed tip tends to grip into the solder and keep the probe from slipping, but MANY places you cannot rely on this. Your probe slips off a HV pin of a tube socket and you can wind up touching the HV AND ground at teh same time - shorting out the high voltage supply. SO we have to be careful.

    Experience helps a lot, but so do proper tools. Most meters come with pointed probes, but they also make probe leads with clips on the end, or clips that slip on the probe tip. A clip is a much more reliable and stabl;e connection that just probing.

    I keep a selection of clip leads on my bench, and when I want a secire connection, I clip one to my probe and lay the probe on the bench. Then I clip the other end of the clip lead to the circuit node of interest.

    It is real simple and safe to connect clip leads to the circuitry while the power is OFF. Then you can power it up with the power switch and not even touch your meter.

    Read up on electrical safety.
    Education is what you're left with after you have forgotten what you have learned.


    • #3
      A good article to read.