[ARC5] Receiver AC Power Supplies

[Roy Morgan]

On Dec 13, 2014, at 10:25 AM, Bill Cromwell <wrcromwell at gmail.com> wrote:

> ... Maybe I should just buy a big honking 12 volt transformer and buck the power line for my entire radio shack. That should keep me closer to 115 to 117 volts. 

Bill.

Good idea.  Consider getting a 12 volt 20 amp transformer (maybe 10 amps is enough) and running it in bucking mode from a 2 amp variac.  Add a voltmeter and current meter and you have a very handy bench accessory or total-shack voltage control system.

There is no need to have a variac big enough to run the entire shack.  The small variac running the line input to the low voltage bucking transformer gives you plenty of current capability and  the ability to set the output voltage where you want, even if the house line voltage changes with time of day or season.  A 7.5 volt filament transformer from the 1940’s days of 7.5 volt transmitting tubes is pretty much ideal, especially if you run the variac in over-voltage mode to get more than 7.5 volts maximum bucking voltage.

The ultimate gadget for this situation is the General Radio 1570A Automatic Line Voltage Regulator.  It handles up to 50 amps (for the low range of input voltage) and 25 amps for larger range of variations. It’s automatic (has a thyratron-driven motor in there) and weighs nearly 60  pounds in the rack mount case.  If you ever see one at a fest, negotiate with a sympathetic approach.  It may be offered with an “I *really* don’t want to take this home again” attitude.  To encourage the Radio Gods arrange for you to find one, bring your wheeled luggage carrier with you to all hamfests.  This thing will solve all your line voltage adjustment problems.  Get spare 2D21’s when you can.

Note that a normal variac meant for 115 volts used in over-voltage mode (most are set up that way) can put out 145 volts if turned all the way up.  There are two ways to solve his danger:  move the input tap in the variac to apply the 120 volt line to the correct turn on the core, or set it up with the input to the end of the winding to avoid over-voltage altogether. 

The sola regulator approach is also valid.  The larger of these things can be noisy and make a lot of heat, even at low current loading.  I suspect that if the output voltage is not low enough for you, a bucking transformer, or perhaps a few turns of wire on the transformer itself in bucking mode would work fine.

If you want to take any of these approaches to regulating the 220/240 volt feed to your linear, be cautious.  If your transmitter uses 120 volts from the neutral to one side of the line, you can’t use a simple variac or Sola.  I have here a monster three-unit 400 cycle Variac that would manage lots of power if adapted to the 240 volt situation.  A 120 volt, 400 cycle variac will run ok up to its rated current on (60/400)x120 volts or 18 volts.  Any one in  western Mass have some 18 volt 20 amp transformers?

As to simply measuring the line voltage:

I have a Weston Model 433 AC Voltmeter (0 to 150 volts AC) here that tells me the line voltage.  It’s nice to have a  mirrored scale moving needle instrument of rather ancient vintage to watch.  I have a soft spot in my attitude toward these 400 series mirrored scale Weston meters.

Short of that, consider a zener diode with rectifier to establish a hundred volt reference and a DC meter with scale 0 to 25  (or 0 to 50) measuring the line voltage referenced to the 100 volts.  This gets you a nice easy to read 100 to 125 volt indication.  You’d need to mess with the resistors depending on your meter to keep the zener happy and to get the indication just right.

See this link:
http://sound.westhost.com/articles/meters.htm#5
the section "5.0 - Expanded Scale Voltmeter” for some useful details.  The car battery voltage version described might not be too accurate at voltages down near 10, but you really don’t need that.  If you scale the parts to run on line voltage, and add a rectifier, this will not be a problem (the rectified voltage will be much above the 100 volt zener reference level).

I have had one of those RCA WV-120A "Power Line Monitor” meters.   It has a moving vane movement and an expanded scale that displays the 100 to 140 volts range, but I tore my hair out trying to get it calibrated.  The thing would change its calibration as it heated up and drove me nuts.  When I got it just right and put it back into its case, it went out of calibration again.  They *look* like a very neat gadget but you may want to pass it up unless you see one at a fest that’s really cheap.   If it’s cheap enough and you are willing to calibrate it and leave it running, one can be useful though. For a picture, details, and calibration procedure, see: 
< http://www.indianaradios.com/RCA%20WV-120A%20Power%20Line%20Monitor.htm>
It could be that my example had some problem causing the trouble.

Of course you can order a $25 plug-in gadget to be mailed to you from the Far East that gives you a lighted digital readout, but they are not boat-anchor-ish in any way.

Happy voltage adjusting to all!

Roy


Roy Morgan
RoyMorgan at alum.mit.edu
K1LKY Since 1958