Unknown Transformer

[kurtlives]

I got just got this smallish transformer today. I think it is decent but nothing special, fairly new made in the last 5 years.

On the the top it reads...

117V
0V
117V
0V
12V
0V
12V
0V

Anything I can use it for...I think it could be reallt useful but I'm not sure.

Thanks

[brett]

Hi
How many leads does it have?
It seems likely that it is designed for mains power in, and 12 V AC (and maybe 24V AC) out.

Before you use it, you'll need to work out the VA (volts x amps) rating.  For many transformers, the safe power handling (VA, which is more-or-less the rating in watts) is about the square of the square of the width of the laminations in cm.  So for a transformer with 1.5 cm of steel laminations, the power handling is about 1.5 x 1.5 = 2.25 and 2.25 x 2.25 = 5VA.  For 12V that means that you could draw 5/12 amps, = 420 mA.  Remember, these are rough calculations.  Long, thin transformers (non-square cores, looking from "above") handle less power (by as much as half).  Obviously, high quality transformers are better than low quality (a factors of 2 is easily possible).  If a transformer gets more than warm to touch (say, 60 C or 150 F), it is being abused. 

Fortunately, most mains transformers now have thermal fuses on their primaries, which save them from literally going up in smoke.

cheers

[Paul T]

Hi kurtlives,

the transformer you have has two separate primaries and two separate secondaries.

The primary windings can be used in parallel / series (117V / 220V)
The secondary windings can ALSO be used in parallel / series (12V / 24V)

If you don't know what this means, PLEASE, get informed. I don't have the means to give you a schematic right now, but I know what you have will be very useful.Trust me, keep that transformer, I'm in the process of writing/drawing a little article on how to use such transformers (for Good, anyway !!!)....

... think TUBES...

(and don't set fire to your bench, ok ?)

Paul

[kurtlives]

Didn't understand much of that...it's got 8 wires coming from it.

Makes sense that it has two primary and secondary windings...

What could I use this transformer for? And specifeic projects?

[rubix cube]

a power supply

[kurtlives]

Oh nice....How many pedals could I power?

I suppose seeing as it has a 12V output I could use a rheostat and have a variable output control. Hmmm this transformer could be useful.

[rubix cube]

That's where the VA bit comes in.  As for the wiring, lookey here http://www.diyfactory.com/data/transformer_connections.gif.  I suggest stern-faced reading about all this, so you don't die.

[kurtlives]

Alright I think I got that...

Now obviously you can't wire your 12V taps straight to a socket and plug your pedals in....So I guess I need filter caps...and???

I am new to transformers...

[Paul T]

Rubix Cube, thank you for the link to those schematics ! That's what I was talking about (...the tube part is another story...)

Brett: "VA" is actually the definition of Watts : Volts x Amperes = Watts (when a transformer is rated at  120VA, it means you get 120 Watts of power from that transformer, for example)

Kurtlives: you will need diodes (1n4001-7), capacitors (electrolytics and others, like ceramics), and perhaps a regulator (7808/7809/7812/7815/7818 ?)...and an enclosure to put the whole thingamajee in !

Hey, how about doing a cool swirlpaint job ?

Good luck !

[kurtlives]

Tell me how wrong I am

I want to connect the 117V to the live on my AC chord, the 0V to the neutral. No ground I guess?

Then I got the 12V tap going to a bunch of diodes in a diamond, bridge rectifer I belive it's called. Not quite sure what these diodes are doing....Polarity protection?

Then I would have a high value capacitor for power filtering makes sense.

Then I am lost....WHere does or did the AC voltage turn into DC? With 2 117V and 2 12V taps I guess I would not use one of the 12V and 117V taps....Unless I wanted 24V or parraled secondary for more current.

[Paul T]

Kurtlives,
a rectifier bridge followed by a large value electrolytic capacitor turns AC voltage into DC voltage.

These diodes in this "diamond formation" do exactly want you are talking about: converting AC into DC voltage (but you need the big capacitor WITH those diodes to do so).

0V and Neutral/Ground are the same thing...while using a transformer...

I don't want to misguide you as I'm much better explaining things with a paper and pen, and my hands !

Try to find info on the web.

IF ANYBODY HAS ANY LINKS TO SHARE... PLEASE DO SO !

[kurtlives]

Thank you...

That was a nice simple explanation, very easy to understand.

I find it weird that the primary is 117V....why not 120V. Just seems weird.

Also If I was to make a power supply I would want 9V output so how would I take the secondary of 12V to 9V?

sorry for all the questions...

[C.A.E.]

Don't take my word on this, but it seems that you would need a voltage divider after the filter cap.  Try searching that.

[brett]

Hi
Hmmmm....slow down boys and girls.  We don't want anyone fried.

First:

Brett: "VA" is actually the definition of Watts : Volts x Amperes = Watts

Actually, watts is a measure of energy production or consumption per second.  I deliberately avoided equating V x A with watts because Volts x Amperes = W only applies to purely resistive sources and loads (which of course are very common and hence the misconception), but not to complex sources and loads including transformers, chokes and speakers.  It is a common mistake.  I'm fairly sure that RG has commented about this in the past.  I can't, but electrical engineers can explain the bit about loads involving imaginary numbers and the square root of -1.

Concerning rectifying, filtering and reducing the voltage, the usual diodes would be 1N4004s or 1N4007s.  They handle up to 1 A and are rugged.  A full bridge and plenty of filtering (about 2200uF per amp) gives a fairly steady DC voltage of 1.4 times the AC voltage, less 1.5 V lost in the diodes.  Stepping down is best achieved with either diodes (0.7V each), or a regulator (e.g. LM7809).  When using an LM78XX, remember that the input voltage needs to be about 2.5 V higher than the required output voltage.  Also, don't push LM78XX devices beyond about 1W of heat dissipation (V drop x current) unless they have an appropriate heatsink.  If they are way too hot to touch (100 C or 220 F case temp) they are in danger of failing.
cheers

[R.G.]

Kurt, et. al.

Go read "Power supplies basics" at GEO. Then ask questions not answered there.