110 volt AC conundrom.
I needed some power supply of 110 volt ac for a valve device i recently acquired. Its need was 110 volt ac 20 watt. As all valve radio devices have pure resistive loading part . So, they require more current capacity that required in start-up etc.
Well we in india have only 220 AC line. We find lots of auto-transformers for many nintendo and other consumer product. They work for them. i can also use them. My problem is i do not want to have live wire at any part of my instrument. That is because my small kids also loiter around my shack/office room/work room and they touch and help me also. when we are testing less worry we have more we concentrate on the work. 220 ac has very small thevenine resistance that means it can source lots of current.
I once had from junk an auto transformer having 220-110-0 . i did not like because when i put my device on it that ruined my fuze. thank god. you can see that in my vedio. link is here first vedio. part 1
and second vedios is part 2 .
Then i did thought well 110 volt is just half of our line voltage why not put two transformer back to back and get. I thought i got the solution and rushed to the shop. bought two 12-0 , 5 Amp and 12-0-12, 5 amp transformer. With mischievous smile that i conquered my problem urgently wired and the transformer. it cost me 170+170 =340 Rs approximately 5 us$. i said ok as could not get the 110 volt ac transformer. Then made it it gave me around 116 volt ac. i said ok now lets test my American soldering iron of 30 watt. If that work then sure my device with 20 watt power will work. Put my soldering iron and it loaded started to slowly heating up. i said ok, test the voltage and it was around 72 volts.
See my notes. putting here as pictures. Then, i thought that my device is not getting enough source current due to the inherent high resistance. So , i need more current to push. So, on next day i bought another 12-0 , 5 Amp transformer. paralleled it with 6 ohm combiner. Thinking was that it should see small resistance going toward the output 110 volt ac transformers and high resistance path towards other transformer. see 6+6 ohm path and zero to output transformer.
Then i parallel 12 ohm to get 6 ohm as could not get 4.7 ohm. i think this idea is good but you need some small 1 ohm resistance that will be enough.
well what happened, first i said compare my result with only one transformer, 12 ohm. it gave me No-load 93 volt. good enough. Then with 6 ohm resistor i got 107 volts ok. but on loading 30 watt soldering iron it turned down to 43 volt ac. Why this happening because we have finite VA that wattage and we have internal resistance of the transformer so when you need more current then voltage reduces and you do not have enough umphhh. EMF.
Then i paralled two transformer. See the diagram.
i tested and it gave 113 volt ac no load and 57 volt with 30 watt load. As last try removed the resistor by bypassing it. so it hold me around 77 volts and the iron was immediately becoming hot. which means it is getting enough current.
Despite having this 3 transformers i am not getting my result. Why why why. i did everything to reduce the thevenine resistance to get more current ?
i paused, if i had asked someone to make me a transformer then in 510/- that person might have made me a brand new transformer. what i will require is 110 volt act with 500 ma to 1 amp current capacity. it might be big.
I knew that my thinking is right, my problem is thevenine resistance. i do nto want to use 0-18 volt ac in output transformer. As it will increase the no-load voltage and then reduce it to working one still it will be not constant 90 volt, the required minimum. What a conundrum.
What i have not tried. Looked at the setup. Not finding solutions. i gave up. solutions is simple i have more then enought VA as input. Why i am not getting the enough current and voltage as per plan.
Paused. let it go. Then in the night time when i was closing down my room then looked at other transformer. Ohh, the 12-0-12 volt ac has thinner wire compared to my other jumbo 12-0-12 5 amp transformer from different company. Ohh, it seems i found the solutions. I need to replace that big transformer with this tiny transformer.
RULE OF THUMB is that output transformer should be thicker wire then input transformer.
Lets see.
I tried one of my old very meaty transformer having thick wire. it produced me 120 volt ac. No voltage drop. Then on loading 30 watt iron it got me around 82 volts. That means i am nearing the range of 90 volts. I am going on right directions. If i use these two big beefy with large dia-meter wire i am sure that will work out. as 20 volts loads less. and minimum voltage of 90 volts i am near about. If my shop had those tranformers i might have solutions with less cost. Anyway i will be using these power transformers also in other projects. It is not wastage.
Meanwhile, the original source, the friend who got me this GDO has promised that he has a spare power supply transformer and that will work. waiting for it.
I can say it was partial success because, theoretically i was right in getting the voltage, testing it. But, practically i invested 3 tranformers but my choice of half voltage was wrong i should have chosen 18 volt combinations, 0-18 & 18-0-18 vac with thick wire. lower voltage we go the wire gets thinner.
RULE OF THUMB choose higher voltage transformers. Less loss in flux, hysterisis, less internal copper resistance.
RULE OF THUMB Second thing, use both transformer with thicker wire, that will reduce voltage droppings. If you are planning learn from my experience. VA ratings should be atleast 4 times of the load.
problem solved. hope this helps you out. It is possible to use this configurations for low power use. Please always avoid auto-transformers. It has shock hazard as its directly connected to the mains AC line. It can source large current so device if not fuze protected or voltage current sensitive, it will be hard to it.
so, wheel is re-invented but it has puncture. so, waiting for the spare wheel.
