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  1. #46
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    Hi Stuart,

    Do not under any circumstances use a variac !

    Use a transformer, say a 6 volt ideally or a 12 battery charger transformer. Measure the voltage output from it first and then when connected measure the primary voltage from the welder transformer. You shouldn't have to limit the current at all but it is worth checking the loaded to unloaded voltage drop.

    Since the capacitors in the welder are rated at 40 volts and I would expect them to be over rated by, say 25%. So the output voltage from the welder transformer would be around 30 volts. Since the mains voltage into the transformer would be 240 volts.

    Divide 240 by 30 gives a transformer ratio of about 8. So 6 volts AC in to the welder transformer secondary should produce 6 X 8 = 48 volts on the primary side, with 12 volts you can expect double that.

    Please take care !
    Best Regards:
    Baron J.

  2. #47
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    Hi Baron,
    Quote Originally Posted by BaronJ View Post
    Do not under any circumstances use a variac !
    Well that is disappointing, I know where that it is.
    I'm bound to have an 6 or 12V transformer around some place, I'll find one, but could you please explain why I cant use variac?

    Quote Originally Posted by BaronJ View Post
    Divide 240 by 30 gives a transformer ratio of about 8. So 6 volts AC in to the welder transformer secondary should produce 6 X 8 = 48 volts on the primary side, with 12 volts you can expect double that.
    All makes sense. About the transformer ratio, that would apply to the turns of the windings wouldn't it? So for example 50 secondary turns would be 400 primary.(now the secondary is 2 parallel windings so that would mean 200 turns) I'm sure its not that simple but I'm just trying to work out a ball park. If its a smallish number trying to unwind the secondary in place maybe worth it. If its a large number I might have to try cutting the core* which would at least make unwinding the secondary easy.

    Quote Originally Posted by BaronJ View Post
    Please take care !
    Certainly will

    Thank you.

    *I don't mean cutting through the laminations. The core is welded together, so to separate I would have to cut the welds out. Cutting it open brings issues with assembly of which I am sure I am only aware of a few, like air gapes and excitation currents. Don't confuse being aware of with knowing a great deal about them lol

  3. #48
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    You will need to put a current limiting resistor in line with the supply from the test transformer. The secondary of a welding transformer will present to the test supply as something akin to a dead short. Failure to do so will result in the smoke being released from your test supply.

    Personally, Iím not convinced the issue is the transformer at this point.

  4. #49
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    Hi,
    Quote Originally Posted by racingtadpole View Post
    You will need to put a current limiting resistor in line with the supply from the test transformer. The secondary of a welding transformer will present to the test supply as something akin to a dead short. Failure to do so will result in the smoke being released from your test supply.
    Got a number in mind for a 9VAC 1amp supply?

    Quote Originally Posted by racingtadpole View Post
    Personally, I’m not convinced the issue is the transformer at this point.
    Damn I hope you are right!

  5. #50
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    Anything bigger than 9ohms. 9V/9ohm=1A

    47ohm will supply around 180mA that should be plenty for what you are doing.
    Best not to lick the primary contacts either, assuming around 8:1 step you’ll be looking at around 70V give or take across the full side of the primary, that will be a rather unpleasant sensation. 47ohms will limit to 180 on the secondary side and you’ll see it at around 25mA ish on the primary.. not lethal but still painful

  6. #51
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    Hi Stuart, Russ,

    I'm bound to have an 6 or 12V transformer around some place, I'll find one, but could you please explain why I cant use variac?
    OK. A variac does not provide isolation from the mains supply ! Main point !

    They are also not guaranteed to provide a smooth voltage rise, usually just little steps as the wiper crosses over the windings. The windings can also move and you can sometimes get spots where the wiper doesn't make contact at all, sometimes arcing, particularly if the variac is old and well used. Some variacs used a carbon brush wiper, which wears leaving conductive dust between the windings causing partial shorts and overheating, particularly at low voltages, other types use a folded brass or copper brush which smears the copper windings. Again causing some problems.

    In their day they were the bees knees for theatre and cinema lighting !

    The "Ohms Wheel" can be used to work out what resistance and current values are given any two values. As far as a current limiting resistor is concerned, just use a 12 volt side or headlight bulb.

    EDITED to add:

    the secondary is 2 parallel winding
    Paralleled turns are equivalent to a single wire !
    Best Regards:
    Baron J.

  7. #52
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    Evening guys,

    Quote Originally Posted by racingtadpole View Post
    Best not to lick the primary contacts either,
    I will try my very best to avoid the temptation.

    Quote Originally Posted by BaronJ View Post
    They are also not guaranteed to provide a smooth voltage rise, usually just little steps as the wiper crosses over the windings. The windings can also move and you can sometimes get spots where the wiper doesn't make contact at all, sometimes arcing, particularly if the variac is old and well used. Some variacs used a carbon brush wiper, which wears leaving conductive dust between the windings causing partial shorts and overheating, particularly at low voltages, other types use a folded brass or copper brush which smears the copper windings. Again causing some problems.
    thank you.
    Quote Originally Posted by BaronJ View Post
    As far as a current limiting resistor is concerned, just use a 12 volt side or headlight bulb.
    My maths doesn't like that. But I've already dug out my resistance box.

    Quote Originally Posted by BaronJ View Post
    Paralleled turns are equivalent to a single wire !
    Yes. The thing I was trying to get at is, guessing from looking at the picture there are 50 turns on the secondary. As its two parallel windings that's really 25 turns. So if my ball park number of 50 is about right and your ratio of 8 is about right that would mean 200 primary turns would be about right. Thats all.

  8. #53
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    In testing the transformer have you thought about disconnecting everything from the secondary side of the transformer, then connect the primary directly to the mains through a 240v incandescent globe ie the globe is in series with one of the mains leads, before you power it up make sure everything is insulated and securely connected with terminal blocks etc, not crocodile clips or wires twisted together. About 75W or 100W globe should do it, it may come on dimly even if the transformer is good, check each of the taps this way, if the globe lights up full brightness then I'd say the transformer is toast, if the transformer is good you should be able to measure the welding voltage on the secondary side.

  9. #54
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    Hi Stuart, I keep forgetting that its late evening down there.

    Quote Originally Posted by Stustoys View Post
    Evening guys,


    I will try my very best to avoid the temptation.


    thank you.

    My maths doesn't like that. But I've already dug out my resistance box.


    Yes. The thing I was trying to get at is, guessing from looking at the picture there are 50 turns on the secondary. As its two parallel windings that's really 25 turns. So if my ball park number of 50 is about right and your ratio of 8 is about right that would mean 200 primary turns would be about right. Thats all.
    Ah I see so yes you might be close !

    Don't destroy your resistance box, the power rating of it will only be a 1/4 or 1/2 watt ! Use a bulb !

    Remember, 1 amp 1 ohm 1 watt. Use a bulb.
    Best Regards:
    Baron J.

  10. #55
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    Hi Familyguy,

    Quote Originally Posted by familyguy View Post
    In testing the transformer have you thought about disconnecting everything from the secondary side of the transformer, then connect the primary directly to the mains through a 240v incandescent globe ie the globe is in series with one of the mains leads, before you power it up make sure everything is insulated and securely connected with terminal blocks etc, not crocodile clips or wires twisted together. About 75W or 100W globe should do it, it may come on dimly even if the transformer is good, check each of the taps this way, if the globe lights up full brightness then I'd say the transformer is toast, if the transformer is good you should be able to measure the welding voltage on the secondary side.
    Yes I agree ! But I was trying to avoid Stuart doing anything that might be risky or dangerous.
    Best Regards:
    Baron J.

  11. #56
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    Morning
    Quote Originally Posted by familyguy View Post
    In testing the transformer have you thought about disconnecting everything from the secondary side of the transformer, then connect the primary directly to the mains through a 240v incandescent globe ie the globe is in series with one of the mains leads,
    It was considered, and I can see it working on smaller transformers but wasn't so sure it would show anything on a transformer of this power. Having said that I'm happy enough to try it if you think it will show something, providing 1. I can find a bayonet fitting. 2. I can find a 75W/100W bulb.....haven't seen one of them for some time. I do have some 25W ones.

    Quote Originally Posted by BaronJ View Post
    Don't destroy your resistance box, the power rating of it will only be a 1/4 or 1/2 watt ! Use a bulb !

    Remember, 1 amp 1 ohm 1 watt. Use a bulb.
    First let me say, I love using headlight bulbs as loads as they have built in spade connectors.
    But a 55W headlight bulb is showing about 0.3ohm, now that is not going to increase much until its well after a 9V 1amp supply is maxed out surely? Even a 6W bulb is only showing 1.3ohm. Now if it was a 9V bulb that wouldn't matter as when it heated up it would draw about .666Amp(well in a DC circuit). But its a 12V bulb so its not going to heat up nearly as much....I'm missing something

    I wont get to try another until late this afternoon at best.

    Thank you guys

  12. #57
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    If you are using mains AC voltage to power up the primary then you need to use a mains rated light globe ie 240v, I think 25W may be too small assuming the transformer is ok there will be a small amount of magnetising current that will light up a 25w globe a bit more than dim, any incandescent globe is ok even a halogen type. For testing amplifiers I generally use a 60W globe it does not light up even dimly when connected to a 300w transformer that has no load ie everything disconnected from the secondary. If the transformer seems ok then perhaps start connecting things on the secondary side one at a time the capacitors will cause the globe to light momentarily due to inrush charge current but it the globe should dim back down one the capacitors are charged, the bleed resistor needs to be disconnected.

  13. #58
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    55w bulb is going to draw 6.1A at 9v
    6W bulb will draw 660mA at 9V
    Keep in mind:
    There is no need to run the the little plug pack at full noise, you just need enough to overcome losses and produce meaningful numbers, a couple of hundred mA will be adequate in that regard
    You can use a variac to test a transformer from the primary side if necessary

  14. #59
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    Quote Originally Posted by familyguy View Post
    any incandescent globe is ok even a halogen type..
    Yes but I'm not even sure I have one of those. Mostly down lights here.

    Quote Originally Posted by racingtadpole View Post
    55w bulb is going to draw 6.1A at 9v
    6W bulb will draw 660mA at 9V
    Are you sure about those numbers?
    Sure a 9V 55W bulb is going to draw 6.1A, but we are talking about 12V rated bulb. Its not not going to get as hot as it should so (as I understand it) the current will go up. But 6W is 6W lol I dont know. Going to have to test it and see.



    About connecting low V to the secondary. Do I need to connect the parallel secondary windings? (I can't see why I would, but thought I would ask)

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