Calculating Current From Modified Transformer - Page 1
Forum section: electronics / beginners

author: Lancette2201
author_rank: Newbie
timestamp: November 04, 2024, 04:05:03 pm
content: Greetings.I have created a pair of modified transformers, but I am having trouble selecting a panel ammeter for the output because I cannot identify the secondary's maximum current. The transformers were originally GAL-700E-4 Microwave transformers but have had their secondaries removed and replaced with 10 turns of 10AWG battery cable. The input is 230-240Vac (from a UK plug socket). When plugged in and powered up, they make a real hum and the secondary output reads as 10Vac. My problem is that the transformers do not seem to have a datasheet available and therefore I cannot tell the gauge or number of turns in the primary, which is making it difficult to tell what the maximum current from the secondary would be. Would anyone with any experience with this sort of thing be able to give me some pointers?Many thanks.

author: jmelson
author_rank: Super Contributor
timestamp: November 04, 2024, 04:09:57 pm
content: What was the power rating of the original microwave oven? You can probably compute the power available from that.Jon

author: Lancette2201
author_rank: Newbie
timestamp: November 04, 2024, 04:12:40 pm
content: I never actually saw the microwaves they came from, on account of the fact that these were purchased from a breaker's yard, and were extracted in the shed there.

author: RoGeorge
author_rank: Super Contributor
timestamp: November 04, 2024, 04:14:43 pm
content: Typical power of a microwave oven is 1000W.1000W at 10V would give 1000W/10V = 100A.Beware that microwave transformers are not designed for continuous regime.

author: Lancette2201
author_rank: Newbie
timestamp: November 04, 2024, 04:21:51 pm
content: Quote from: RoGeorge on November 04, 2024, 04:14:43 pm100A is slightly above the mark that I was aiming for, but I can put a solid-state dimmer in front of it and it'll do the job!What exactly is your definition of "Continuous" here?

author: RoGeorge
author_rank: Super Contributor
timestamp: November 04, 2024, 04:32:07 pm
content: The current is dictated by the load attached at the secondary, not by the transformer. 100A is a huge value.If you leave it plugged into mains 24/7, even without any consumer attached, the transformer will heat very much, and eventually fry/burn the isolation of the primary turns.What is the intended use for the modified transformer?

author: Lancette2201
author_rank: Newbie
timestamp: November 04, 2024, 04:36:44 pm
content: Quote from: RoGeorge on November 04, 2024, 04:32:07 pmIt's actually two identical transformers wired into a push-pull format, and the intended use is for an arc welder (of sorts).The insulation on the primary is magnet-wire enamel, with some kind of resin holding it into a block.

author: Lancette2201
author_rank: Newbie
timestamp: November 04, 2024, 04:39:05 pm
content: The basic concept is in the picture (not my diagram)

author: RoGeorge
author_rank: Super Contributor
timestamp: November 04, 2024, 05:03:59 pm
content: A microwave oven goes on/off in pulses of 10s..1minute or so, and in total it doesn't function for more than a few minutes. After that period, it is supposed to rest until the transformer cools down.With 2 transformers in series, the voltage is only half for each transformer, so might even work to keep them 24/7 on. I guess it should work, but I didn't try. No idea how long it takes to weld something. I would keep an eye on the transformers' temperature, at least at the first tests.Another thing, I would put the A-meter in the primary. That way you will only need a 1000W/220V = max 5A full scale.A dimmer in front of a welding transformer seems like a bad idea to me, but again, I know nothing about welding, don't know what are the practices.LATER EDITThe schematic shows the primary connected in parallel, so the continuous working regime problem will remain. Also, in this case the power is 2x1000W, so the amperes values calculate before will double.

author: Lancette2201
author_rank: Newbie
timestamp: November 04, 2024, 05:24:12 pm
content: Quote from: RoGeorge on November 04, 2024, 05:03:59 pmI am so glad you said double, because I saw the 5A figure for the panel meter and had a slight moment because 5A isn't even close enough to weld with. I don't know much about welding (this device is supposed to help me learn to weld) but apparently as a general rule of thumb, for every .001 inches of material thickness, you'll need about 1 amp of output, so 100A would be in the region of 2.5mm steel sheet. (this can also vary depending on the metal and filament).I need a dimmer or some other device to vary the current output from the welder.

author: IanB
author_rank: Super Contributor
timestamp: November 04, 2024, 05:31:50 pm
content: Quote from: Lancette2201 on November 04, 2024, 05:24:12 pmYou should know that a homebrew arrangement won't really weld, it will only "weld".By "weld", I mean that it will make a lot of heat, and melt metal, but it will lack any finesse or control over the welding operation, thereby producing very poor welds.Real welders have various controls and regulation inside them, as well as having rectifiers to produce DC of the appropriate polarity.So you can have fun with it, but do not expect it to be a substitute for a real welder.

author: Lancette2201
author_rank: Newbie
timestamp: November 04, 2024, 05:34:12 pm
content: Quote from: IanB on November 04, 2024, 05:31:50 pmI am entirely aware of this, but the difference between this homebrew and a decent real welder is about 450GBP, and there's no point investing that much until I have the basics down.

author: MrAl
author_rank: Super Contributor
timestamp: November 05, 2024, 03:55:38 am
content: Quote from: Lancette2201 on November 04, 2024, 04:05:03 pmHi,As to the max primary current, if you can measure the wire diameter that would get you going. For a transformer like this I would think around 600 circular mils per amp would be a starting point. The max would be 300 cm/A but that's pushing it for a transformer like this.What that gives you is the max current you should be using for operation for less than many 10 minutes because of the type of transformer. They usually have a high excitation current so the transformer would probably heat up. The fact that they hum a lot means high excitation current.The real key to this is how hot the transformer gets and how fast. You can do some simple tests after you calculate that above. You would run it for the time you think you will need to run it for during use, and if it gets too hot you will just have to reduce the input current.10 volts at 100 amps is 1000 watts, which will be about 4.4 amps input not including the excitation current, which you could measure with current meter. This means you should have a wire diameter of about AWG #16 or better, but with the excitation current maybe #14 which is rather thick wire.Very short use runs however will not heat the transformer up much, as long as there is enough cool down time. Cool down time has to be at least 10 times the run time if you play to use it for a short run and they wait a while and use it for a 2nd, 3rd, etc., run.The heat production comes mostly from the current in the primary and secondary due to the wire resistance, and that happens right away but still takes a while to heat up all that steel. The heat dissipation comes from the entire surface area and the type of cooling. Free air cooling does not work as well as forced air which would come from a high velocity fan. That means a fan could get you a lot more cooling which extends the continuous run capability.The main key here is testing it. The more tests you do, the more you know about how well it will work. That's always the best way anyway, and in a commercial setting for some appliances they would be tested overnight at full load. There is insulation inside the transformers that can melt if it gets too hot, and it will drip out like thick goo. If you see any of that, it's of course no good, but before that you should just be measuring the temperature.Now the output current is usually related to the input current by the turns ratio. If you have 200v in and 20v out that's 10 to 1, so the output current is normally 10 times the input current. Yours sounds like it would be more like 20 to 1. Unfortunately, the excitation current adds to the input current so it's more like Iout=(Iin-Iex)*20 but you can measure all that quite easily.As someone else also said, you'll have to experiment with the welding technique as it may be a little different than when you use an actual welder. You might find some actual welder circuits online though so you can get a feel for what works well.

author: ifrythings
author_rank: Contributor
timestamp: November 05, 2024, 04:05:57 am
content: Quote from: Lancette2201 on November 04, 2024, 05:34:12 pmI’m all down for DIY but you’re walking into failure right off the bat. With no current control your just going to vaporize your rod and blow holes in everything, also a 20v open circuit is very low and a lot of rods will struggle to light and stay going, most welders have up to 70v open circuit to get the rod lit and keep it going. 6010,6011 rods (farmer rod) does better with the lower voltage but isn’t as easy as 7018 rod.My suggestion is to get one of the small inverter welders off of eBay that around $100 and practice with that before buying a bigger unit.

author: Xena E
author_rank: Frequent Contributor
timestamp: November 05, 2024, 07:06:24 am
content: Using a 'dimmer' as a control on the input for output current control isn't going to work very well as the core of the transformer will tend to have to magnetise on each mains cycle.In line frequency welders, the normal method is to use tappings on the primary, an adjustable magnetic shunt in the transformer, or more expensive versions used a saturable reactor, which will be beyond your skill set to produce.The microwave transformer will have had a fixed shunt fitted at manufacture. If this has been removed during modification, the transformer is probably going to go into saturation (lots of input current off load, lots of noise).This modification of microwave transformers is only really successful to create small spot welders as the output is only required very briefly.I have a small inverter MMA welder at home that was bought from Lidl, it was les than GBP50 and welds 3mm very well with 2.5mm rods.It's an interesting project but ...Regards X

author: IanB
author_rank: Super Contributor
timestamp: November 05, 2024, 02:27:20 pm
content: Quote from: Xena E on November 05, 2024, 07:06:24 amI am having trouble processing "a small MMA welder that was bought from Lidl"If you said you bought a small welder from Tesco, my head would explode. Lidl must be a different kind of supermarket

author: DavidAlfa
author_rank: Super Contributor
timestamp: November 05, 2024, 02:50:01 pm
content: Do you remember the iron plates present on the original transformers?They're field limiters, their purpose is to limit the energy going to the secondary, lots of welders use this same method to control the arc current.If you go full power you'll burn through small parts, or convert thin electrodes into sparklers.Most microwaves can run for 1h or more, so same here, but will need some cooling, so attach a fan, or even better, add a 60-70°C thermostat switch to trigger it.

author: Xena E
author_rank: Frequent Contributor
timestamp: November 05, 2024, 05:24:26 pm
content: Quote from: IanB on November 05, 2024, 02:27:20 pmOK. MMA (manual metal arc) stick welder.Lidl/Aldi often have limited special offers on cheap tools, multimeters, soldering irons, welders, plasma cutters, angle grinders... usually decent enough quality considering the budget prices.If you've not encountered them before then yes they do stock more variety of items than the average grocery megastoreX

author: IanB
author_rank: Super Contributor
timestamp: November 05, 2024, 05:54:59 pm
content: Quote from: Xena E on November 05, 2024, 05:24:26 pmYes, I have heard about this with basic things like cordless power tools. But a stick welder seemed rather astonishing.Here in the USA, I think Lidl is not the same. I think it only sells grocery items.On the other hand, we have Harbor Freight, which is like an Aladdin's Cave of cheap tools and gizmos.Honestly, shopping for tools in the UK can be very poor experience, with the likes of Homebase and B&Q having rather limited selections compared to say Home Depot or Lowes.

author: Xena E
author_rank: Frequent Contributor
timestamp: November 05, 2024, 06:17:04 pm
content: Quote from: IanB on November 05, 2024, 05:54:59 pmJust totally at random I've picked out a review on YouTube for an example of what's sometimes available: this item, a plasma cutter:-And here's the welder...Edit: reread and noticed this comment:-QuoteHomebase and B&Q are for home DIY.Instead Try...Tool Station, Machine Mart, Halfords, Wilco, Screwfix, Cromwell... ... ... ...X