power transformer question

[rascalseven]

I have a torriod power transformer with dual 115V primaries and dual 18V secondaries, each currently wired in parallel. I was expecting to measure around 18V from the transformer, but instead I'm getting 27V (the power at the wall measures 122V). Does this sound right?

I do not yet know much about power supplies and transformers, so I don't know if it would drop to around 18V under 'load' (when I hook it to the unit to power it), but this doesn't make sense, and I'm not willing to toast my work with one stupid move like that.

BTW, I also connected the secondaries in series (watching for polarity) with the primaries in parallel and got 54V -- twice the 27V that I'm getting with the secondaries in parallel, which seems consistent, but it's just not what I was expecting from a transformer that says its secondaries are 18V. Do I just have a goofy transformer that needs to be swapped out for another?

Help??

Thanks so much!

[cjenrick]

Put a 200 ohm resistor or close across one of the secondaries. That voltage will drop like a led baloon! Use a 2 watt resistor or work very quickly! Pre connect the voltmeter and turn on and off quickly if using a 1/4 watt.

[Kev]

Yep,

you are about learn something. [img]graemlins/up.gif[/img]


diy, diy.
happy, happy
diy.

:D

[rascalseven]

Yep, the 1/4-watt toasted out QUICKLY (ooh, that smell!!). The voltage only dropped to 24V. If I were to use a larger 2-watt resister to get it down to 18V, are you suggesting that I leave it attached and put it into the unit this way?

Just wanting to be sure that this is meant as a permanent fix for my problem rather than just an excercise in learning.

Thanks so much!

Joel

[Kev]

Originally posted by Joel Cameron:

If I were to use a larger 2-watt resister to get it down to 18V, are you suggesting that I leave it attached and put it into the unit this way?

Just wanting to be sure that this is meant as a permanent fix for my problem rather than just an excercise in learning.

The Wattage is just going to have a baring on how long the resistor will last under heat.

The ohms will set the current flow and the voltage will drop as you draw more current. The smaller the TX the more voltage drop with load. Plug Packs and small PCB mount TX can be very lossy.

NO do not leave the resistor in place. It is just being used as a load to demonstrate the affect of loading.


The TX's voltage may vary with load and this is brought under control later with the Voltage regulator.
TX out is AC
Into Rectifier and out as DC
DC may still vary.
DC into regulator and Controlled DC out.