Hi guys,

So driving around on a saturday among the garage sales paid off.
I've found a 7500 V , 30ma old school transformer:
http://www.allstarlightbulbs.com/acatalog/Light_Bulbs_T7512W_5903.html

I was planning to use it as a variable HV ac power supply by feeding the input
from my bipolar 100w kepco
(link here for completeness of description: http://www.kepcopower.com/bop.htm)


I've been experimenting with the setup his morning and have observed a few things which i am not sure how to explain. I was hoping for some expert advice, as i am trying to understand the fundamental causes.

My setup is 100w amplifier into the input of the transformer while i feed in the amplifier from a simple audio signal generator.

I'm measuring the output voltage on the transformer by connecting a 500:20 MegaOhm resistor load to it and taking the readout from the bottom 20MegaOhms.

My first question:

As I started cranking up the gain of the amplifier, up to a certain point (50v ac at the input to the transformer),
my transformer was drawing ~ 0.5A of current and no visibile signal was seen at the output. After this critical point, signal appeared and the current drawn dropped to milliamps. I imagine it is something similar to static friction ? you need to go beyond a certain point for things to start moving ?

However when i tried to connect a regular 120VAC:12VAC transformer in reverse for a similar experiment, i didn't observe such a phenomena.

Could you help figuring out - what am i seeing ? is it losses in the iron ? it kind of limits my output voltage from below, but i am ok with that as long as i can understand and predict it.


My second question:

I've connected my oscilloscope to the output of the amplifier (200v peak2peak). When the transformer is not connected, the signal is a beautiful 50hz sine. As soon as i connect the transformer (irregardless of the amp's gain), the sine becomes distorted - it has a high frequency oscillation region for approximately 1/8 of the cycle around one of the zero crossings.

I intend to use this supply for some scientific experimentation so the purity of the signal might be of value, so i am trying to understand if what i am seeing is simply an artifact of my measurements, or is there something else that
causes the distortion ?

another 2 details :

1. the output of the transformer shows the same distortion so i am inclined to think that it is a real distortion, but i cannot identify its source.

2. the amp that i am using is 100w, while the transformer is rated 225Va,
so i think that if i put a heavy load and crank the input of the transformer to its max, i might run out of steam, but the distortion happens way before that - no load at the output of the transformer, and 1/2 the input voltage, so i think it should work.


Thank you for any advice you might have.

Regards,
Lenny.

 

You already have a thread for this project. No need to start a new one:

http://forum.allaboutcircuits.com/showthread.php?t=84023

I expect the mods will probably merge the two threads.

 

Sure thing DerStorm8,
you're right.

L.

 

I don't think you have an old school transformer. You need to find an older iron core type like in your other thread.

 

Brownout, thanks for the message.

what makes me think that the transformer is old school is its weight and size.
It is a rust-covered metal box that weighs close to 15 pounds and is filled with some fiber-glass looking resin.

Also - i remembered that the new transformers are high frequency and this one is Fin = Fout - i played with the input frequency and the same frequency appears at the output.




Lenny

 

Hmmmm... this gets curiouser and curiouser. Maybe an answer will come to me in a dream tonight.

 

Hello,

As said continue in your other thread.

Bertus