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Thread: Same MIG new problem
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30th Mar 2020, 05:43 PM #61Most Valued Member
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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.
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.Last edited by Stustoys; 30th Mar 2020 at 08:06 PM. Reason: people like pictures
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30th Mar 2020, 08:34 PM #62
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.
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30th Mar 2020, 10:03 PM #63Most Valued Member
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Hi Baron,
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.
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.
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.
Thank you
And thank you to everyone else
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31st Mar 2020, 02:07 AM #64
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.
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31st Mar 2020, 09:01 AM #65Most Valued Member
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Hi Baron,
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
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....)
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)
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.
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
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31st Mar 2020, 02:44 PM #66Most Valued Member
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Primary is overwound to ensure the secondary is in a saturated field. Easy means of upping efficiency a few points.
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31st Mar 2020, 03:24 PM #67Most Valued Member
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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
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.
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31st Mar 2020, 04:04 PM #68Most Valued Member
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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 ,
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31st Mar 2020, 04:29 PM #69Most Valued Member
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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
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31st Mar 2020, 09:22 PM #70Most Valued Member
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