Hi John,
I think that an "iron vane" type meter to read the voltage will give the
true rms voltage no matter what the wave form is.
73
Gary K4FMX
John E. Coleman wrote:
I haven't seen any post about the saturated core constant
voltage XFMRs. I use them a lot. They keep the OSCs on the RCVR and
XMTR stable. I have never used one on a filament source for a directly
heated cathode tube but since the output wave form is not a sine wave
but somewhat squared due to the saturation of the core, I would assume
the voltage would need to be reduced to arrive at the same heat on the
filaments. They were easy to make. A lot of the hybrid TVs of old use
power XFMRs with saturated core technology to keep the output voltages
constant so that the picture didn't shrink or jump when the air
conditioners or heaters would kick on. There was a 2 or 4 uF oil filled
cap across part of a secondary winding that some what resonated and
saturated the core. The input voltage could vary for 100 - 130 with
very little change in the DC from the rectifiers. I would use the
filament winds to buck or add to the output to get it where I wanted it
to be. With one manufacture, I actually came up with 115 volts as read
on a RMS meter. The scope showed it to be clipped about 10-15 percent.
A light bulb test was made and found to be brighter on the constant
voltage XFMT than on a variac delivering the same RMS measured voltage.
This could be compensated for I would think.
John, WA5BXO
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Donald Chester
Sent: Friday, January 17, 2003 11:53 AM
To: [email protected]
Subject: Re: [AMRadio] Today's AC Mains voltage too high for many BA
Rigs
I have the problem here. I have seen the voltage as low as 108v and as
high
is 125. My main transmitter has a rheostat on the tube filament
transformer
line feeding several transformers, but I have the same problem, having
to
keep a constant eye on the thing and frequent adjustment.
Incorrect voltage won't burn out tubes right away, but the manufacturers
have warned for years that this will shorten useful tube life.
One solution I have thought of, and probably have all the parts for,
would
be an automatic voltage regulator using a variac and reversible motor.
I
have a relay that looks like a meter, but the pointer has two sets of
contacts, one of which will engage wenever the pointer is above or below
the
set value which is adjustable with a knob near the zeroing screw. It's
just
a matter of figuring out exactly how to build the thing and putting it
together. Minus the special relay, I have thought about using a VR tube
and
comparing its voltage output to that of an unregulated power supply with
the
same nominal output voltage. Of course, it could also be done using
solid
state components.
One thought about using a variac. With most of the ones I have used,
even
the HD ones, the regulation sucks. A tapped autoformer with a heavy
duty
switch is better. Instead of using the variac to drive the equipment
direcly, it would be better to use a large, high current, low voltage
transformer@ about 15-24 vac/ 30 amps as a bucking transformer and use
the
variac in the primaryof the transformer. That way you get better
resolution
of voltage and better regulation. The problem with variacs is the
carbon
brush contact. It must have a certain amount of resistance so that
shorted
adjacent turns don't overheat, since the wiper contact must make before
break to assure uninterrupted output as the voltage is varied.
Don K4KYV
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