Questions?

[Philipc]

I’ve just recently started playing AC voltage, and been have some good old fun in the lab(well just the old tool shed out back) So I’ve been trying to build my own transformer, haven’t taken too many theory classes yet, so using the trial and error method, so far many errors! After blowing the breaker, I have gotten much smarter about the way I’m doing it now, with a variac and homemade fuse box that blows at 3.6amps(just a single strand of wire from braided wire)
The questions I have,
Well I would like to build a 12v power supply with an out put of around 5+ amps. Just for kicks I would like to wind my own transformer from scratch. Any good ideas of where to start?
Second, what 12v regulator that can handle 5+ amps would you use?
Third, I built a current shunt, but my El Cheapo multimeter only reads milivolts in DC. So I’m in the market for a good all around meter, would like a Fluke, but wouldn’t mind finding a cheaper version, to do me over till I graduate
Thanks
Philip

 

[Klaus]

Your statement "playing with AC" is a bit worrying. I am sure you know that mains voltage AC can kill you very easily and horribly painfully too, so, be very very careful. Home made fuse boxes for mains AC are really a no no. Do yourself a favour and get the real thing as there is a very good chance your 'piece of wire' will not blow fast enough for some things you connect to your power supply.

Regarding winding transformers, I'm sure the exact info is to be found on the net if you do a search. What I did was to take an existing transformer apart, leaving the primary winding undisturbed. If you had measured the secondary voltage and the maximum current you could get out of it you have all the info to rewind the secondary for a different voltage if you count the No. of turns while taking it apart.
Keep in mind that you CANNOT increase the power output from a given core size, you can increase the voltage with a consequent smaller current or vice versa.
Building transformers from scratch is not easy, first, where will you get the laminations from? Then you have to do all the calculations, make a bobbin, find the right winding wire and layer insulations etc. etc., much easier to rewind the secondary of an existing transformer if you find one that comes apart easily (not all do).

A word of warning about variacs: usually they are auto transformers, which means the secondary has a direct path to the mains - a lethal device in the hands of a novice. Take care!
I suggest you do not use it directly on the mains but use a double wound transformer with its output fed to the variac instead. It does not matter if the double wound transformer steps the secondary voltage down low, the variac can step it up again, this time 'isolated' from the mains. You can still get zapped by the output voltage but you are no longer getting zapped directly from the AC mains. Which means, if you are handling some earthed equipment while playing with the AC from the isolated variac, you will not get an instant shock if you touched just one wire, only if you touch BOTH wires from the variac.
Klaus

 

[Philipc]

Klaus,
Thanks for the words of advice I will look for a 120v transformer. Guess the phrase "Playing with AC" didn't look so good. Guess what I've really meant, is that I have been building strictly DC circuits up till now, and now getting a taste of AC theory. I built my first DC motor nearly 20 years ago, so I'm not totally brand new to the world of electricity(know enough to keep my tongue of the two prongs of a 9volt), but I have just recently started an EE degree, so I am new to theory, which makes playing with electricity much more fun!
Philip
Any advice to finding a 120v transformer?

 

[stevez]

Much of what I've learned about the construction of various components comes from taking apart discarded radios, televisions, etc. I am not sure where you live. In the US it's not uncomon to find old microwave ovens out by the curb - they contain a transformer that's fairly easy to liberate from the chassis. Other electronic stuff contains components though transformers are hard to find in solid state equipment - computer power supplies being an exception.

There are several ham radio articles that describe how to rewind microwave oven transformers for high current, low voltage power supplies. Several people who post here have experience in rewinding. In the ARRL Handbook for Radio Amateurs there is quite a bit of info and at least one project with a do-it-yourself power transformer. There's lots of info on RF transformers too but they are somewhat different aminals.

Ditto the comments on safety. I'd be inclined to start with a bell transformer as a source of AC (still observing precautions) and experiment with that first. As a rule the transformer is designed so that it won't deliver a lot of current, won't overheat, etc. The variac is dangerous in that you might forget the setting.

 

[Philipc]

Steve,
Thanks for the info, I will be looking out for an old Microwave now, I also get a kick out of taking old things apart, sometimes to the point where I take new things apart just to look inside. I even have an old transformer from the old AC unit that would work, but I just wanted to try and wind my own, just for the heck of it. I tent to get a better grasp of things when I try it myself, therefore personally knowing how bad 120 hurts :shock:
Philip