[ARC5] Receiver AC Power Supplies

[Bill Cromwell]

Hi Roy,

Thanks for all of that info. I'll definitely be looking for another 
variac and using it to control the line voltage in my shack as you have 
outlined with a bucking transformer. That will allow me to keep the one 
I already have for use on my workbench where it could also be pressed 
into service as needed to power a boat anchor.

As for the voltmeter I just happen to have an AC volmeter that indicates 
from 50 Volts AC to 150 volts AC. As soon as I saw that scale I knew 
what it was for. I don't have to fuss with diodes or resistors, etc - 
they're already in it. I am building my variac (presently wired for 
"overvoltage") into a wooden cabinet and that meter will be included. 
There will also be an outlet to conveniently plug in the 'load'.

I have a big honking transformer from an automotive battery 
charger/cranking supply that provides something north of 18 volts AC 
with huge secondary wires. That is earmarked for a new life in a "12 
volt" DC supply for those um... sand state radios. <ducking and 
running>. Too bad it won't help me control the entire house.

73,

Bill  KU8H

On 12/13/2014 05:56 PM, Roy Morgan wrote:
> 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 .