Tony,
A variac like what you are asking for is large, somewhat lossy, and
usually
expensive. Variacs tend to be a little lossy as the voltage in/out ratio
increases. I don't know of what your application is, but consider an
alternative that I'm going to propose. Lets say you just need the variac to
vary the line voltage +/- 10 volts to maintain 120 vac to your rig. You
could use a 10Volt CT 25 ampere filament transformer as a boost / buck
transformer. Using a DPTD switch on the primary to switch the phase of the
AC into the primary, and another switch to select the secondary voltage from
0, 5, 10volts. The secondary circuit is put in series with whatever your
load is (like mega 304tl rig). This way, you can vary the AC line +/- 0, 5,
10 volts with two switches, and do it efficiently.
A variation on this idea is to put a much smaller variac to control the
primary of the filament transformer. This would eliminate the need for the
25 ampere rated secondary tap switch. You still need the DPDT primary phase
(buck / boost) switch. Since the filament transformer in this example is a
[EMAIL PROTECTED] device, this works out to 250 va. The variac then needs to
only
supply 250 va, so a 3 amp 120 volt variac (360 va) would do the job nicely.
This would provide +/- 0-10vac ac line correction depending on the DPDT
phase switch setting, and the variac setting. For us poor guys, this allows
a 25 ampere load to be controlled (limited voltage range to +/- 10 volts)
with a 3 amp variac. Using the little variac eliminates the need for a large
25 ampere rated tap switch in the first example. In either case, the DPDT
phasing switch only needs to switch 250 va, so a > 3 ampere rated switch is
all that is needed.
If a 25 amp variac doesn't fall on your lap (ouch!), you might want to
consider the boost/buck idea as an alternative. You can vary the concept to
a different voltage range depending on your needs, and what kind of surplus
iron you have in your junk box.
Good luck with your project.
Regards,
Jim Candela
WD5JKO
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] Behalf Of RoadKing
Sent: Wednesday, February 25, 2004 7:41 PM
To: amradio@mailman.qth.net
Subject: [AMRadio] Need Variac
Looking for a Variac 120v input, 25 amp or greater rated. if you have
something that you are not using, would love to talk with you about it.
73,
Tony/W5OD
River House Radio
Boling,TX
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