An isolation transformer is a transformer used to decouple two circuits. It allows an AC signal to be taken from one device and sent to another device without electrically connecting the two circuits. They allow AC to pass but not DC. They also block interference caused by ground loops.

In radio repair, an isolation transformer is a 1 : 1 power transformer used as a safety precaution. Since the neutral wire of a wall outlet is directly connected to ground, grounded objects near the radio being repaired may be at a hazardous potential difference with respect to the radio. 

AC/DC radios of this era are a SHOCK HAZARD.. One side of the AC line is grounded to the chassis. If the power cord is plugged into the wall properly, the chassis is grounded. HOWEVER, the did NOT use "polarized" cords. So you could get the plug in the wall socket backwards. You now have 110 VOLTS on the chassis...If your test gear is properly grounded, you could get shocked by touching both grounds at the same time.

By using an isolation transformer, the bonding is eliminated, and the shock hazard is entirely contained with-in the radio itself.

          --) || (--
            ( || )
115 Volts   ) || (   115 Volts
            ( || )
          --) || (--

Isolation transformers are readily available from many sources.

ANTIQUE RADIO SUPPLY

ACTION ELECTRONICS

You may also use two identical transformers that will handle the total load, A power cord is attached to one primary and an outlet connected to the other primary. You then connect the 2 secondary together

Here is an example

					
         +--) ||                  || (--+
            ( || (--------------) || )
115 Volts   ) || )   35 volts   ( || (    115 Volts
            ( || (--------------) || )
         +--) ||                  || (--+

POWER LINE                        TO RADIO

You may also use a VARIAC to control the input voltage. An AC meter would also be a good idea

     COM -----) ||      
              ( ||      
              ) ||      
      OUT  -->) ||
              ) ||
              ( ||
      IN -----) ||

Now that we understand the basics, lets put one together:

Here is the proto type Isolation Transformers. I used 2 power transformers out of some old broadcast audio consoles

We have a meter to measure the output voltage, 2 outlets, power on/off, a varic to control the voltage, voltage lights and a fuse.

The perf board inside the unit is a meter calibration circuit. I used a 12 volt xmfr, rectified the voltage and used a trim pot to the meter. (see schematic) There are some other ways to do the same thing, it all depends on what you have in your junk box of parts.

Here is the schematic for the unit

I cannot be held responsible for any damages that could occur to you or your equipment while following the procedures present on this page. Also, I GIVE ABSOLUTELY NO WARRANTY on the correctness and usability of the information on this page. Please note, however, that the procedures above have worked in my case without any damages or problems.

Now you cannot say that I did not warn you

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9/25/2007