# ~50A 120-18V transformer design help

#### BlueRidge

Joined Nov 13, 2012
12
Hello all, 1st post here.

So I am trying to build a 50A 18V DC power supply (from household supply), and I am a little stuck on exactly what I need in terms of windings for the transformer. Any help or direction pointing would be appreciated.

Thanks

#### bountyhunter

Joined Sep 7, 2009
2,512
Hello all, 1st post here.

So I am trying to build a 50A 18V DC power supply (from household supply), and I am a little stuck on exactly what I need in terms of windings for the transformer. Any help or direction pointing would be appreciated.

Thanks
Probably need to rent a fork lift to be able to lift it. My guess is this transformer is going to weigh about 80 pounds or more.

#### JDT

Joined Feb 12, 2009
657
Over here in the UK there are plenty of companies who will make a transformer for you to your spec at a very reasonable price even for 1 off. Especially if it has a standard primary.

I'm sure the same is true in the US.

Far easier than making one yourself.

#### BlueRidge

Joined Nov 13, 2012
12
I am not opposed to buying a transformer, I just haven't been able to locate what I am hunting. I will look for custom fabricators. Thanks.

It isn't going to be "mobile" (it'll live in my shop, probably with wheels on it) and I could probably get away with ~30A. I have seen 24VDC @ +20A in a compact unit before from 115 @ 7.5, and I know it'll be bigger, but that much? Perhaps I didn't factor in enough loss, I was thinking a 115 @ 15A could supply that within a reasonable cost and size.

If it is <250$it saves me money after a year, and I would enjoy doin it for the fun of it if it didn't But I am more than happy to do more research; I admit I am not very well versed in electronic componant assembly, though I have always "tinkered". I just can't seem to find the solution to my riddle, or the right rabbit hole to go down. Am I missing something about amperage possible through a step transformer? Thanks for the help. #### takao21203 Joined Apr 28, 2012 3,696 Probably need to rent a fork lift to be able to lift it. My guess is this transformer is going to weigh about 80 pounds or more. I have a 500VA toroid here- weight= 4.5 kilograms. #### SgtWookie Joined Jul 17, 2007 22,210 Can we ask what your intended use is (your intended load) for this power supply? 18V * 50A = 900 Watts output, not to mention the losses in the supply itself. If it's a linear supply, you will have a pretty good room heater, as you'll probably be putting out (wild guess) 500W-600W worth of heat, along with the 900W output. Switch-mode power supplies (SMPS) can be much more efficient, using far smaller transformers due to switching at relatively high frequencies, but they are much more complex than a typical linear supply. However, even a linear supply for your current requirement wouldn't be a snap to design/build. Thread Starter #### BlueRidge Joined Nov 13, 2012 12 Sure. I am seeking to duplicate a cordless tool adapter once available that not longer is. I work frequently in my workshop and use cordless tools both there and in the field, but my shop use typically messes up my field use as far as batteries are concerned. It may be cheaper to get a drill for 120, but not a whole set of tools, + it would give me a super light weight tool to use as all but a smaller dc motor would be external. Additionally, a lot of the carpentry work I do requires the smaller size dc tools permit. I also tried the get more batteries approach, and that just leaves me with a bunch of dead batteries. I have a dead one (battery) that I intend to hollow out and attach 30' of cable, or perhaps just a recipticle for ext cord use with aforementioned supply. It wouldn't be a constant high amp draw, 75% of the work would be 10A or less. The rest would peak at a potential 75A or so, but would be functional for all intents and purposes at a 40-50A max. The problem is the important part is that 4 sec climb to 40A. Suggestions? Edit: I was thinking of a simple transformer, full wave rectifier, capacator, output - basically a mini welder/charger by design (not what I will do with it). #### bountyhunter Joined Sep 7, 2009 2,512 Sure. I am seeking to duplicate a cordless tool adapter once available that not longer is ?????????????????? Do you mean some kind of power cable with a plug that goes into where the battery would be on the portable tool and supplies power to it? Or some kind of power cable that plugs into the tool while you use it which helps supply current and keep the battery charged while in use? I also tried the get more batteries approach, and that just leaves me with a bunch of dead batteries. They do die eventually, but you should get many hundreds of charge cycles from a decent quality battery pack. #### takao21203 Joined Apr 28, 2012 3,696 For a hand tool you don't really need a mega-sized transformer. Just check the wattage. Large transformers can be overloaded for some seconds. The voltage also does not have to match exactly. And a hand tool (DC motor) can accept relatively noisy DC. You don't need 150,000 uF or something like that. You can either buy a large toroid, or an electronics transformer with suitable wattage. Or use 12V batteries (if you have one already, that's the cheapest solution). Thread Starter #### BlueRidge Joined Nov 13, 2012 12 The first; this would effectively eliminate the battery, not trickle charge it. Basically a plug in power supply with wires leading to a hollowed out battery casing attaching them to any tool accepting said battery pack & common voltage. I do have more detailed plans if you like, but am stuck on finding a supply or creating the proper transformer. It would greatly simplify my work life to keep batteries charged for field use, and not depreciate them in the shop. I can run them down much faster than they charge back up. I'm sure on the surface it seems illogical, but I feel it is the best solution for my opportunity. My other thought was to build a box utilizing a motor and auto alternator and cap, which would give a fun shop voltage of 15 for all mannor of things, including tool use, at up to 60A. But that needs about 3krpm, and supposes much more to break over time. I thought this would be easier and more reliable at a cost sacrifice. Thread Starter #### BlueRidge Joined Nov 13, 2012 12 For a hand tool you don't really need a mega-sized transformer. Just check the wattage. Large transformers can be overloaded for some seconds. The voltage also does not have to match exactly. And a hand tool (DC motor) can accept relatively noisy DC. You don't need 150,000 uF or something like that. You can either buy a large toroid, or an electronics transformer with suitable wattage. Or use 12V batteries (if you have one already, that's the cheapest solution). I also had the thought to link it to a 12V, but I was not certain it would make the higher draws possibly. I suppose the Ah would overide voltage loss for a few min, but that would also need a supply to keep it charging. So I figured after all that I am still underpowering the motor, might as well make what I want. #### takao21203 Joined Apr 28, 2012 3,696 Starter batteries are not good for high currents. If you have a 90AH battery, you can use it for 1 1/2 hours or so. And the cable...for 50A! Bad. Search for toroids on eBay, there are sometimes cheap one's. You need a small diodes bank +heatsinks, and some 10000uF to 20000uF capacitor. Discount power tools for grid supply might be cheaper altogether. Must be a really big hand tool that draws 50 Amps. Unusual. #### bountyhunter Joined Sep 7, 2009 2,512 The first; this would effectively eliminate the battery, not trickle charge it. Basically a plug in power supply with wires leading to a hollowed out battery casing attaching them to any tool accepting said battery pack & common voltage. I do have more detailed plans if you like, but am stuck on finding a supply or creating the proper transformer. Then you don't need anything resembling a 900VA transformer as you said in the OP, all you probably need is something like 10VAC with maybe 4ARMS run through a FWB with a filter cap to power a hand tool. If the filter cap is a good one with low ESR, it will easily put out the peak currents to run a hand tool. A 40 VA transformer is reasonable size/cost. I can't figure out how any hand tool could draw 40A continuous? It would be too big to lift. Last edited: Thread Starter #### BlueRidge Joined Nov 13, 2012 12 Again, it would not be a constant draw. A momentary (<5sec) draw of 40+A is very possible. That is 720W, a standard drill/saw for 115 is rated at 900W. Under load it will acieve this, and I may be hitting 40A once every 15 sec for 2 min, then off for a few. Even higher with saws. I can put it on the ammeter tomorrow if ya like. Just normal tools. Not constant, but if draw is determined by load why not build for max draw? (Seriously, why?) So would adding a larger capacitor be effective enough at your lower suggested VA rating? Adding a 12V battery with a trickle charger would still leave me dependent on a battery - what I am trying to avoid 7 Thread Starter #### BlueRidge Joined Nov 13, 2012 12 OK, figured out a way to make it happen. I'm just gonna mod a 13.8V charger capable of over 100A, with selections for 55 amongst other amperages. It weighs 33 lbs and uses very simply electronics, + has overprotection circuits already. 95$ US, probably 35\$ more for what I am after - on steroids.

I will retire to amateur comm sites for help with the transformer project, as there are homebrew supplies @40A readily available. Yet here it is implied bulling 40A or more is so ineffecient, so cost prohibitive, so heavy, such a rediculous notion, etc that my application is illogical. Simply put, it is not. If you feel it is, please don't respond to similar posts in the future. Your naysaying is counterproductive, and misleads what is reasonably accomplishable in circuitry.

With the resistance I got in this thread from some on a simple design, coupled with the fact I just saw an interestingly dumb thread by multiple mods on a banned topic meanwhile shutting down a simple lightbar question because it was a violation (as that 12v circuit was connected to wheels, big difference lmfao), I believe I have better places to occupy my time. If those of authority are incapable of selfgoverence, they are not capable of reasonable authority. Beware any time this becomes the case, oppression and tyranny are soon to follow. Yes, even on a little chat board.

Thanks for the help to those who tried. Takao, I will take your suggestions with me. I appreciate the real answers as I feel we are barking up the right tree.

It is my conclusion that many keyboard warriors reside here. I do not wish to be associated with that.

73

BlueRidge

#### Bernard

Joined Aug 7, 2008
5,283
Have you considered a microvave oven transformer? I keep some with all turns removed except for primary & wind as needed, about 3 turns per volt. Should be good for 1 kW intermittant.

#### MrChips

Joined Oct 2, 2009
19,373
Hey, there are many other pastures to graze. 73