RE: [AMRadio] Need Variac
Well, if you use a bigger variac and a lot of capacitance in the power supply, its not bad. I have two, both run around 2000 volts at about 400ma, and they don't have much problems. The RF deck is a steady load, so that's no problem, the modulator can peak up to 500ma, but with a swinging choke and 50 uf of capacitance, it does not seem to be a problem. The variacs are mounted on aluminum front panels, and DO generate a nice magnetic field, best kept well away from any audio stuff. I think I use 10 or 12 amp variacs, which gives me a little extra variac head room... The little variacs (2 amps) work great for things like screen voltage in RF decks, bias supplies, ncl supplies, where the load current is very light, and regulation is not important. On audio tube screens, you want something very stiff. I DO like the primary tap/relay setup, works great if you can deal with the fixed voltage steps it gives. I have a ton of 110vac relays pulled from equipment that did not actually operate them except in a power failure (UPS bypass relays), so used them in the 812 rig. I used smaller plug in relays that came with the nice sockets that mount directly to the chassis, with screw down wire terminals, a snap to add to the circuit. DPDT, 10 amps per contact, high quality relays. I parallel up the contacts, for 20 amps. The RCA transformer I use has 3 primary and 3 secondary taps, which gives plenty of range output. I think I have it on maximum power now, 1500, 1700, 1900 volts output. I keep going up, as the 812a rig seems to not mind higher voltage at all. I figure its fine as long as the plates don't show any color. I might use the same relay setup in the 4d32 rig, separate power supplies for rf and modulator, but run things on the low voltage taps. I bet there is a good and easy way to use some sort of solid state components to replace the variac, but I have not looked into it. Brett N2DTS -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Behalf Of Donald Chester Sent: Thursday, February 26, 2004 9:49 PM To: amradio@mailman.qth.net Subject: RE: [AMRadio] Need Variac they work well, but there is loss in the variacs, more sag under load than without one. I think part of the reason is the carbon brush contact. You cannot use a metal contact because it would short out adjacent turns as the variac is rotated and overheat the coil and burn up the contacts as well. The graphite brush acts as a current limiting resistor to protect the winding, but also introduces losses. I also found the voltage regulation to be noticeably better without the variac. The best tapped autotransformer I ever had was the voltage control from an old x-ray machine. It had huge contacts on a rotary switch, and there was no detectable increase in voltage sag with the transformer in line. Unfortunately, I swapped it for some other gear after I had acquired a nice new G-R 20 amp variac. Don K4KYV _ Stay informed on Election 2004 and the race to Super Tuesday. http://special.msn.com/msn/election2004.armx ___ AMRadio mailing list AMRadio@mailman.qth.net http://mailman.qth.net/mailman/listinfo/amradio
RE: [AMRadio] Need Variac
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 ___ AMRadio mailing list AMRadio@mailman.qth.net http://mailman.qth.net/mailman/listinfo/amradio
RE: [AMRadio] Need Variac
At 06:34 AM 2/26/2004 -0600, you wrote:
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
i
:snip:
Wonderful Idea Jim!
Thanks so much for the very astute, and experienced suggestion.
Much, better, less expense and certainly simple way to control and
prevent another {quot}Wreck of the Old 97{quot} I can simply look
long and hard down the track and open 'her' up!nbsp;
Man, there is truly such a {quot}GREAT{quot} bunch of AM ops who are so
willing to keep those of us still climbing the learning curve on the
straight and narrow.nbsp;
After assuming the worst Jim, I discovered with a great deal of joy, that
it was a mechanical problem. The {quot}Disk{quot} holding the
brushes, had slipped backward on the shaft, away from the Windings
therefore, the Variac was fine it was just not able to Pass 'juice'..
BOSEG Now, the {quot}Total Output Performer{quot} is back up and
Rolling out the RF at its usual rate.
Should I ever need to replace, it, your note is now a permanent part of
the 304tl folder! !nbsp; Tnx
73,
Tony/W5OD
River House Radio
Boling,TX
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RE: [AMRadio] Need Variac
At 07:38 PM 2/26/2004 -0500, you wrote:
Now, if you want to change
voltages, there is
another way besides the variac setup.
Brett,
Thanks for yet another 'better' mousetrap! It really is a shame that all
this wealth of knowledge and experience which are the {quot}AM{quot}
community, can't somehow, have their specialities made available other
than those of us that run into a problem and then discover, there was a
better way in the beginning.
Certainly that seems to be a very clean way to have a variable power
level from the transmitter.
Tnx.. again...
73,
Tony/W5OD
River House Radio
Boling,TX
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RE: [AMRadio] Need Variac
they work well, but there is loss in the variacs, more sag under load than without one. I think part of the reason is the carbon brush contact. You cannot use a metal contact because it would short out adjacent turns as the variac is rotated and overheat the coil and burn up the contacts as well. The graphite brush acts as a current limiting resistor to protect the winding, but also introduces losses. I also found the voltage regulation to be noticeably better without the variac. The best tapped autotransformer I ever had was the voltage control from an old x-ray machine. It had huge contacts on a rotary switch, and there was no detectable increase in voltage sag with the transformer in line. Unfortunately, I swapped it for some other gear after I had acquired a nice new G-R 20 amp variac. Don K4KYV _ Stay informed on Election 2004 and the race to Super Tuesday. http://special.msn.com/msn/election2004.armx
