[ARC5] Homemade Isolation Transformer

[Roy Morgan]

> On Tue, Apr 28, 2020 at 2:41 PM Scott Robinson <spr at earthlink.net> wrote:
> Hi Roy,
> 
> Your Variac voltage booster will work, but only at low currents. The 
> Variac current rating is primarily brush current, so if you want to run 
> a Variac as a 10:1 step-up, your rated output current will be 1/10 of 
> the original rating.  10A brush current at 12V will require a 10A Variac 
> to generate 120V at 1A safely.

I may have been unclear:

I am not suggesting any boosting at all. ( I have written a DIATRIBE about variacs in which I describe the situation of a variac meant for 115 volt input and set up for over voltage, supplying up to ONE HUNDRED FIFTY TWO volts to your radio under test.*** - ‘Glad to send this to anyone interested.)

The one amp variac I suggest is feeding the PRIMARY of a 12 volt filament transformer, the 12 volt *secondary* winding of which is in series bucking with the line.  If the filament transformer is rated at, say 12 volts, 10 amps, that’s roughly 120 watts the variac may be required to supply to its primary or about one amp at full output current of the system,  You get line bucking action from the 10 amp *secondary* of the low voltage transformer in series with the line - it can manage up to 10 amps, bucking down the line voltage a variable amount from none to 12 volts.  So 122 volts line voltage can be reduced to 110 volts.

A fine point is that if you want less bucking than the normal output of the low voltage transformer, hook the variac to the OUTPUT of the system and the variac and filament transformer will see reduced voltage as the bucking is adjusted up to maximum.

A further possible fine point is that if the bucking is set at zero, the impedance of the 12 volt secondary may reduce the output voltage somewhat, especially at higher currents. However: the low impedance of the variac set at zero output voltage will reflect through the 12 volt transformer and reduce or negate this possible effect. 

I have suggested that a 400 cycle 120 volt variac can be used safely at up to about 18 volts input.  I remember having a very small physical sized one that is rated at 8 or 10 amps.  Connected with its input from the *low* voltage secondary of a filament transformer, it can be arranged to reduce/buck the incoming line voltage in a variable amount up to the secondary voltage of the low voltage transformer, or more if it can be, and is, connected in over voltage configuration.

***My Varacs diatribe tells how to re-configure your 115 volt variac so its dial reads more correctly for your normal line voltage (you move the input connection to a different turn on the main winding).  A 115 volt variac set to 115 volts on its dial and fed from 122 volt house current will deliver 122 volts to its output, and 152 volts if it is arranged for "130 volt" over voltage.

Other of my diatribes are:

reform.txt  about reforming electrolytic capacitors
powercordsandbypassing.txt  about safe use of line cords and line bypassing. 
capleakagetesting.txt  about testing capacitors for leakage
variacs.txt  about the use and dangers of variacs
line Bucking.txt  about reducing line voltages

Note, the one on line cords and bypassing tells how a FUSED LINE CORD PLUG can make a WIDOW out of your wife.

Roy Morgan
K1LKY since 1958
k1lky68 at gmail.com