about Transformers

[Ziddik]

today i took two 12v 5 amps transformers and powered the 1st transformer as usual and connected its two of 3 AC outputs ("12v" and "0v)" to the secondary windings of the 2nd transformer! And connected a 2v LED with a 560ohm resistor to primary windings of the 2nd transformer to indicate that power flows through it! And "Baaang", the resistor fried!!! So what the heck is that??? i really need some help regarding this phenomenon!!!

 

[BrownOut]

Stop doing thigs like that. Until you know enough to be safe, stick to safe, low-voltage experiments.

 

[Ziddik]

Stop doing thigs like that. Until you know enough to be safe, stick to safe, low-voltage experiments.

i want to know about that n what happended and how can i use that on my circuits.. Hope someone will help me

 

[BrownOut]

There is no use for connecting transformers that way in circuits. The purpose of a line transformer is to reduce the voltage from the lethal line voltage to a safe voltage for your electronic circuit. Never try to connect the primary to an AC voltage and use the secondary. That's not what transformers were made to do.

 

[Ziddik]

so how much voltage and amperes the 2nd transformer (please see post #1) will produce if i connected them in such a way? I just want to know..

 

[carbonzit]

Wait, not so fast there on the "ixnay on connecting transformers back-to-back".

There's at least one reputable person out there who powers his circuits from back-to-back power transformers. Check out this page from Fred Nachbaur's vacuum-tube web site.

Of course, what he does is a little different: he wires the secondaries together, essentially creating a 1:1 (approximately) isolation transformer, with an output close to the incoming line voltage. Perfectly legitimate use of transformers.

I'm a little puzzled by the flash-bang that the O.P. got here. I would have thought that connecting the primary of a second transformer should have worked, giving a small step-down voltage at the 2nd transformer's secondary (~0.6 V, since the transformers are ~20:1, assuming the O.P. lives in 230-240-volt land).

To the O.P.: are you sure you connected the transformers as you described? No chance you connected the 2nd transformer's secondary to the 1st transformer's secondary? or something equally wrong?

 

[BrownOut]

It will be whatever line voltage you connected the 1st transformer to. ( assuming I understand exactly what you did in post #1 )

 

[crutschow]

.................Never try to connect the primary to an AC voltage and use the secondary. That's not what transformers were made to do.

You have it backward. Connecting the primary to the AC supply and using the secondary as the output is the normal way to connect a transformer.

 

[crutschow]

today i took two 12v 5 amps transformers and powered the 1st transformer as usual and connected its two of 3 AC outputs ("12v" and "0v)" to the Primary windings of the 2nd transformer! And connected a 2v LED with a 560ohm resistor to secondary windings of the 2nd transformer to indicate that power flows through it! And "Baaang", the resistor fried instantly!!! So what the heck is that??? i really need some help regarding this phenomenon!!!

Sounds like you connected the secondary of the first transformer to the secondary (output) of the second transformer. That would generate the line voltage at the primary output of the second transformer, which would blow the resistor.

 

[BrownOut]

Wait, not so fast there on the "ixnay on connecting transformers back-to-back".

You need to understand what he's doing here before recommending it to anyone else. Also, carefully read his warning about working with high voltage. Nobody should be fooling around with these voltages unless they know exactly what they are doing. Few, if any, projects require these high voltages, except valve ( tube ) projects. If anyone is going to make such a proejct, he must become well versed in high-voltage safety. Anyone who is arbitrarily connecting things to high voltage just to see what happens isn't ready to fool around with it.

 

[BrownOut]

You have it backward. Connecting the primary to the AC supply and using the secondary as the output is the normal way to connect a transformer.

Blah! Time for me to go home.

 

[Ziddik]

To the O.P.: are you sure you connected the transformers as you described? No chance you connected the 2nd transformer's secondary to the 1st transformer's secondary? or something equally wrong?

sorry! I connected them exactly what i said in post #1(Post #1 has been edited) and the resistor started burning and blowed(LED not blown) within 5-7 seconds! I think it did't pass the same line voltage, or it would have blown quickly!(not sure, and i have not dared to measure them as the mains voltage is 230v here in india moreova i dnt want fry my other multimeter again!!) i think the 2nd transformer passed around 60-70 volts and the mains power here is 230V

 

[Ziddik]

this is how i connected them!(pic is from the site carbonzit pointed out)

**broken link removed**

 

[carbonzit]

That's not what you told us originally:

today i took two 12v 5 amps transformers and powered the 1st transformer as usual and connected its two of 3 AC outputs ("12v" and "0v)" to the Primary windings of the 2nd transformer!

Which I take to mean this:

**broken link removed**

Is that correct? If not, can you show us a diagram of how you actually connected your transformers?

As I said before, what I showed above should have worked (that is, no smoke and resistors blowing up), although you would have gotten a voltage too small to be useful out of the 2nd transformer (less than 1 volt).

 

[BrownOut]

this is how i connected them!(pic is from the site carbonzit pointed out)

**broken link removed**

Even though you said you connected the primary of the 2nd transformer to the secondary of the 1st one, I made an automatic assumption that you meant you connected the secondary of the 2nd transformer to the secondary of the 1st. Strangly, my assumption was correct. Thus you connected your LED and resistor across 240V. An LED can only tolerate a few volts in the reverse direction. You had the full 240V across the 330 ohm resistor, with 727mA flowing. That's 175 Watts through a tiny reisitor. No surprise it would fry instantly.

 

[Ziddik]

i was pretty sure that the LED and resistor would blow, but i was a needy for something to indicate lol, so are u all saying that the second transformer's primary windings produced the same voltage that is present on mains?.. If then, cant i use it to save some electric bill?!!

 

[BrownOut]

What were you trying to do, exactly.

 

[carbonzit]

i was pretty sure that the LED and resistor would blow, but i was a needy for something to indicate lol, so are u all saying that the second transformer's primary windings produced the same voltage that is present on mains?.. If then, cant i use it to save some electric bill?!!

So I think we've established that you connected the two transformers as shown in Fred's power supply, not as you had originally described it. In other words, you connected them back-to-back (i.e., tied the secondaries together).

You should be able to figure out how this would work, keeping in mind that a transformer works in both directions.

You basically built a 1:1 isolation transformer consisting of 2 xfrmrs.

Why would you think you'd be able to use this to save electricity? You can't create something out of nothing.

Fred's circuit uses the transformers to isolate his power supply from the power line for safety reasons. Otherwise, he'd just connect his bridge rectifier directly across the incoming line ("mains").

And yes, I'd be curious to know what you were trying to accomplish too.

 

[Ziddik]

What were you trying to do, exactly.

was experimenting with them lol.. I had 7 12v 5amps transformers in my junkbox.. I used 1 of them for my 5.1 amp and anotha1 for an adjustable voltage regulator, 5 are left now.. Its quite expensive here

 

[carbonzit]

Well, at least you didn't blow the transformer! Be grateful: you learned a lesson (hopefully) and all it cost you was one little bitty resistor. I'd consider that money well spent.