Thank you! for the many replies to my questions pertaining to the
transformer / AC capacitor combination. The output is indeed a square wave
that modifies a bit with changing load. It appears that this transformer's
secondary voltage is very immune to primary voltage fluctuations, however,
secondary regulation didn't seem all that good with higher loads - worse
than expected from a low R winding. Though, I really need to build a better
load bank before I can say for sure how the transformer responds to varying
loads...and repair my scope - accidentally keyed vhf xmitter into one
channel... :-(
I've worked around CVT's and even replaced them in the past, but never took
the time to learn how they provide regulation. Even if these transformers
aren't useful for a bench power supply, they will be helpful from an
educational aspect.
Thank you all for the very useful information and website links!
73,
Jeff - K1LE - CT ><>
----- Original Message -----
From: "Jeffrey Madore" <K1LE@arrl.net>
To: <AMPS@contesting.com>
Sent: Wednesday, May 07, 2003 11:56 AM
Subject: [Amps] Transformer question
I have a transformer question that someone might be able to shed some light
on.
Last fall I picked up 3 similar power supplies at the flea market at MIT. I
believe they were designed to support some type of robotics. The fact that
they were brand new and full of great parts is what really caught my
interest.
One of the components in these power supplies is a power transformer with a
240v primary. On the schematic diagram it shows another winding on the
primary side which is connected to an 8uf 660v oil filled cap. The secondary
is connected to a block bridge rectifier with capacitor filter. The
transformer core measures about 5"x6"x2". My plan was to build a bench
supply using all three transformers / rectifiers / filter caps.
On testing, with the oil filled cap, bridge rectifier and filter C
connected, I obtained the following values using a 75 watt lightbulb for a
load as indicated:
120v pri - 60 vdc @ no load
120v pri - 60 vdc @ loaded
240v pri - 60 vdc @ no load
240v pri - 60 vdc @ loaded
I then disconnected the oil filled cap and repeated the above:
120v pri - 30 vdc @ no load
120v pri - 23 vdc @ loaded
240v pri - 60 vdc @ no load
240v pri - 50 vdc @ loaded
I realize that the load used is small for this transformer but it was
convenient for a quick test. My question is: what is the purpose for the
additional winding and oil filled capacitor. With this capacitor in the
circuit, the secondary voltage seems unaffected by drastic changes in
primary voltage. It will be interesting to see it's behavior at higher
loads. Without the oil filled cap, the voltage regulation is fairly poor at
even a small load. The voltage across the oil filled cap was c. 600vac, and
the DC resistance of the windings is: 2 ohm pri, 0.3 ohm sec, 4.3 ohm oil
filled cap winding. These readings were taken with a DMM thus are
approximate.
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