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  1. #61
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    Evening All,

    Well it would seem we have a result.
    Perhaps not the result I was after but you cant win them all.
    I will check my numbers again tomorrow(though it wont come as a surprise when you see them that I have checked a few times already.
    Quote Originally Posted by racingtadpole View Post
    Without having gone back to check the circuit diagram again, I can’t see a need to power both secondary windings, all you are after is a comparative reading across the primary windings to find out if anything is astray there, you should be able to see that just applying AC to one winding.
    That is what I did.

    Doesn't everyone have three reasonably large 150ohm resistors in their junk box?
    No load V of my 9V supply is 11.11V.
    With the resistors in circuit its 10.69V at 120mA.
    Still numbering primary taps same as last time, left to right 1 to 5, 5 being neutral.
    1-5 = 28.5mV
    2-5 = 21.9mV
    3-5 = 16.3mV
    4-5 = 15.8mV

    1-2 = 5.3mV
    2-3 = 4.4mV
    3-4 = 0.0mV

    So am I right in guessing the shorted 3-4 tap explains why when powering 4-5 tap it draws so much current?

    Assuming these figures are correct of course.

    Have I done something wrong?
    Is it worth trying any other test I have missed?
    Haven't found a 240V bulb of required W as yet, but I haven't given up hope on that yet.
    Attached Images Attached Images
    Last edited by Stustoys; 30th Mar 2020 at 08:06 PM. Reason: people like pictures

  2. #62
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    Hi Stuart,

    Seems reasonable.

    An observation ! The welder transformer is a centre tapped single winding ! Not two parallel ones.

    Assuming that you have used a 12 volt battery charger transformer, using bulb as a series resistor its resistance would not matter because if the welder transformer secondary was a dead short the bulb would light up just as it would putting it across the car battery.

    A shorted turn on the transformer primary would cause a large current to flow, regardless of the tap setting.

    It seems that you have your smoking gun. Well done.
    Best Regards:
    Baron J.

  3. #63
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    Hi Baron,

    Quote Originally Posted by BaronJ View Post
    An observation ! The welder transformer is a centre tapped single winding ! Not two parallel ones.
    Well only when the windings are connected at the Inductor . I guess atm they can be either as there is nothing connected.
    The good news for today is a "quick count" at the welder would suggest my guess of 50 secondary turns(or 25 as the primary see is) is on the high side.

    Quote Originally Posted by BaronJ View Post
    Assuming that you have used a 12 volt battery charger transformer, using bulb as a series resistor its resistance would not matter because if the welder transformer secondary was a dead short the bulb would light up just as it would putting it across the car battery.
    Well yes as long as the supply can supply the full current to light the bulb, then its all good. While I have a battery charger I didn't feel like opening it up to bypass rectifiers(no idea what a transformer would do with rectified AC, I don't even recall if its half or full, but it is capable of about 200amps.... so I left it alone). The bulbs only became an issue when the first DC supply I came across was 9V 1amp.


    Quote Originally Posted by BaronJ View Post
    A shorted turn on the transformer primary would cause a large current to flow, regardless of the tap setting.

    It seems that you have your smoking gun.
    Well that's not what I want to hear. You do realise I'm hopping to be proved wrong don't you? lol
    Still it is what it is.

    Quote Originally Posted by BaronJ View Post
    Well done.
    Thank you
    And thank you to everyone else

  4. #64
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    Hi Stuart,

    It might be a good idea to grab a small transformer or two before they all disappear. I have kept several old valve heater transformers, 6 and 12 volt ones along with a couple of the 6 amp 6/12 volt battery charger ones.

    The use of a bulb for short circuit protection is that whilst the bulb is effectively a short circuit when cold, it almost instantly rises in resistance when it lights up.

    Well I'm very sorry to have confirmed your worst fears However I might have saved you spending some loot on new capacitors, since I priced some up, they are not cheap at all. Between £15 and £70 each in the UK.

    The better news is if the shorted turn/turns are near the tap point it may be practical to just unwind a few turns towards the next lower tap.

    Have you recalculated the turns ratio based on your new secondary count ?
    Best Regards:
    Baron J.

  5. #65
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    Hi Baron,
    Quote Originally Posted by BaronJ View Post
    It might be a good idea to grab a small transformer or two before they all disappear. I have kept several old valve heater transformers, 6 and 12 volt ones along with a couple of the 6 amp 6/12 volt battery charger ones.
    oh I have a few kicking about. Not knowing where is an issue. Sadly my best ones are 32VAC. In "a former life" I could have had pretty much anything I wanted

    Quote Originally Posted by BaronJ View Post
    The use of a bulb for short circuit protection is that whilst the bulb is effectively a short circuit when cold, it almost instantly rises in resistance when it lights up.
    Yes,but I wasn't convinced that a 12V 6W bulb in going to get hot enough before it overloaded my 9V supply(but 9V 1Amp is 9W so I guess it has to....)

    Quote Originally Posted by BaronJ View Post
    Well I'm very sorry to have confirmed your worst fears However I might have saved you spending some loot on new capacitors, since I priced some up, they are not cheap at all. Between £15 and £70 each in the UK.
    Yes it may well be a cheap repair. Just lots of labor. I couldn't even find caps that wouldn't require a resigned(though surely they must be out there)

    Quote Originally Posted by BaronJ View Post
    The better news is if the shorted turn/turns are near the tap point it may be practical to just unwind a few turns towards the next lower tap.
    Not sure I understand this.(ok maybe I think I get it, you mean fixing a shorted tap by leaving it a few turns short? hehe see what I did there?)

    My guess is its the second tap on the winding. so it will pretty much all have to go. I say this because, the thermal overload is between the windings(the bobbin is open at the back) and the core, so I'm guessing this will be the 4-5 tap(highest amp and always used), would seem silly to wind the low amp taps first and there by bury the most used tap, but who knows? I don't know why they bring the secondary over the top of the primary either.

    Quote Originally Posted by BaronJ View Post
    Have you recalculated the turns ratio based on your new secondary count ?
    No just a rough count(which I don't even recall). I'll take some measuring gear today so I can get a "best guess" on turns

    There was something else I was going to ask, but for the minute it escapes me.

    Thank you

  6. #66
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    Primary is overwound to ensure the secondary is in a saturated field. Easy means of upping efficiency a few points.

  7. #67
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    Quote Originally Posted by Stustoys View Post
    Yes,but I wasn't convinced that a 12V 6W bulb in going to get hot enough before it overloaded my 9V supply(but 9V 1Amp is 9W so I guess it has to....)
    Ok I was "worried" about nothing, the bulb draws 370mA
    repeated tests
    dropping 29.7mV across the secondary(is that really enough? The V drop and the V on 4-5 are the same. Although I see the argument "its not enough to tell you much about the number of turns but the 0.0mV on 3-4 sure tells you something)
    1-5 = 52.3mV
    2-5 = 40.6mV
    3-5 = 30.3mV
    4-5 = 29.7mV

    1-2 = 11.1mV
    2-3 = 9.6mV
    3-4 = 0.0mV

    There is a typo in the last set of figures, the current should have read 210mA

    Quote Originally Posted by Stustoys View Post
    No just a rough count(which I don't even recall). I'll take some measuring gear today so I can get a "best guess" on turns
    Seems I confused myself, I was counting in pairs and was happy when I got a number less than 50
    The secondary as best I can measure has 58 turns. 29 pairs.

  8. #68
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    I should add, only 12 of those pairs are over the primary. If they can be unwound that's about 5m of wire in each.
    Last edited by Stustoys; 31st Mar 2020 at 09:18 PM. Reason: moved the ,

  9. #69
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    Your numbers are consistent, the winding between taps 3-4 appears short. You aren’t seeing any voltage induced between those two taps, and the voltage between both those taps and common is near enough identical.

    There’s a cheap BossWeld 186 in the market place section at the moment

  10. #70
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    Quote Originally Posted by racingtadpole View Post
    Your numbers are consistent, the winding between taps 3-4 appears short. You aren’t seeing any voltage induced between those two taps, and the voltage between both those taps and common is near enough identical.


    Quote Originally Posted by racingtadpole View Post
    There’s a cheap BossWeld 186 in the market place section at the moment
    Thanks but one nonfunctional MIG is more than enough for me hehe

    Time to clear some space in the shed. On the up side at least I wont have to bend over this thing again for awhile.

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