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Flyback Transformer secondary current

Discussion in 'High Voltage' started by killivolt, Dec 14, 2012.

  1. killivolt

    killivolt Well-Known Member

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    If I measure the voltage on the output of a flyback Transformer which is 20k.

    How do I know how much current is being used ? I can't just clamp an amp meter around it.

    kv
     
  2. Grossel

    Grossel Member

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    Hi.

    If you can measure input current, then the relationship between input and output current is oposite propotional as the turn ratio.

    How accurate do you need the measurement to be? If very accurate, you also need to know the effiency of the transformer at a given load.
     
  3. dougy83

    dougy83 Well-Known Member

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    If it's AC you can use a current transformer, if it's DC you can use a hall-effect sensor... depending on the current magnitude.

    Grossel's method is much easier to apply however.
     
    Last edited: Dec 14, 2012
  4. dave

    Dave New Member

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  5. killivolt

    killivolt Well-Known Member

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    I don't think it needs to be that accurate. So, if I measure the positive rail on the input side of the flyback. I should have what I need then?


    Thanks. kv

    Edit: Thanks, dougy83
     
    Last edited: Dec 14, 2012
  6. dougy83

    dougy83 Well-Known Member

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    The way to work it out is:


    where
    is the efficiency


    So you need the input current and voltage and the output voltage (and an estimate for the efficiency).

    But anyway, if this is a DC supply, then what's stopping you putting the ammeter in-line with the load?
     
  7. killivolt

    killivolt Well-Known Member

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    I'm not an engineer. Just a Novice, I only have a clamp ammeter, electronic ammeter. I do have a DMM I could use to get the input current and voltage on the rail feeding the Transformer.

    I do have a part# for the Trans but, I'm not sure if that's enough for the estimate of efficiency.

    A friend of mine, owns a TV repair shop. He did the measurement on the HV output. I don't know if he has something that could measure the current on the HV side.

    As far as putting DMM inline with the load, I didn't think you could do that and if so, I don't have the knowledge. Any help on that would be greatly appreciated.

    Thank you.

    kv
     
    Last edited: Dec 14, 2012
  8. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    The DC current is only to support the output load and to make up for loss. This can be measured with a meter.
    The AC current has to do with the primary inductance, time and voltage. I have current probes the clip around a wire.

    In the horizontal section of a TV the AC current is much larger than the DC current. If you are using the flyback transformer with out the yoke the AC current will be much lower.

    Now that I reread the question, you want to know the secondary current. Some transformers has a ground lead (ground for the 20kv). You can put a current meter there. One problem is that the transformer may be very complicated inside and the ground current may be a combination of many different circuits. Many transformers have multiple outputs. 20kv, 2kv, 1kv etc. You don't want to hang a meter on the 20kv wire. A current probe around the wire will work. The secondary is probably not a simple winding. Most transformers I know of use many secondary windings. It is common to have each winding output less than 5kv. You could have four (winding and diode) that adds up to 20kv.

    If there is no output load then there should be no current in the secondary. There will be primary current with or without secondary load.

    In better TV sets and CRT monitors there is often a load resistor hidden in the transformer. This load helps regulate the output voltage. In this case there will be secondary current but you will not be able to measure it.

    If you are trying to measure the current out the 20kv wire then you can measure the current in the bottom of the load.
     
  9. killivolt

    killivolt Well-Known Member

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    So, right now I'm just jumping a gap between the Anode, and the clip that was used for the Degaussing coil. If I can clamp their I'm not sure but it looks as if it feeds back into the flyback?

    The weird thing is I can run it this way, none stop over 24hr period and I don't understand how or why it doesn't just burn up or fry the Driver? It's not connected to the tube or Yoke, and the chassis is out of the set.
     
    Last edited: Dec 15, 2012
  10. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    Why you have not burned something up yet:

    Because you unconnected the yoke the LC resonant is very different then designed. I think you high voltage is about 1/3 what you think it is. If the yoke is 100uH and the flyback is 1mH then, as designed, the total inductance is 91uH. When you unplug the yoke the total inductance is now only 1mH. The AC current is much lower than designed. (1/10) The transistor is is not running in resonant mode.

    Flyback transformers have inside a hidden current limit resistor inside.

    Many big sets have a current sense resistor so the horizontal supply voltage will be reduces if the HV-current is too high.

    The air gap is limiting current. If the gap starts conducting at 10kV it may stop conducting at 5kv. The output is oscillating between those two voltages.
     
  11. killivolt

    killivolt Well-Known Member

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    Here is the data sheet on the Driver ........ http://www.futurlec.com/Transistors/2SC1413A.shtml

    It's an old Zenith Chassis, there is a small transformer connected to the Driver. Where should I test the input voltage and measure the amperage?

    Edit: What I mean is Collector, Emitter, Base?
     
    Last edited: Dec 15, 2012
  12. killivolt

    killivolt Well-Known Member

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    I measured the primary of a small Transformer stepping up the voltage for the Driver.

    On the primary side of the transformer measures ( 00.1ma at 4vac ) with my clamp on ammeter. The same Transformer secondary is 162v DC going to the H.O.T

    I have no idea what the efficiency of the LOPT is to do the calculation. But, I would highly doubt the output current of the flyback would be over 3-5ma.

    kv
     
    Last edited: Jan 20, 2013

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