Building an Isolation Transformer

[cdmarion]

Originally Posted by electroking

Hello,

The 35 VA rating would however be too low for anything
bigger, such as a series-string TV.

What VA rating would you suggest for a series-string TV? Or how about a good do all VA rating? They make different models with higher VA ratings.


Thanks

[Chad Hauris]

You would need probably from 90-150 watts for a B/W series string TV and probably 250-325 for a color series string set. Not sure of the exact difference in VA and watts but volts * amperes=watts so I would think for these purposes VA and watt rating should be similar...a 350 VA unit should cover all types of hot chassis equipment you will likely encounter.

[RetroHacker]

One place that might be good to look for would be an arcade supplier. The monitors used in old video game machines were hot chassis. To connect them to the game circuit boards, they had to be isolated. So all the older arcade machines had an isolation transformer in the bottom that was connected to only the monitor. Arcade monitors only draw about 80 watts, but the transformer typically had a bit of margin of safety - they would be good for smaller sets and radios anyway. Newer arcade monitors aren't hot chassis so don't require these anymore. You can typically buy arcade isolation transformers for $15 or so online - but if you have a local arcade operator, try asking there. Ask for the biggest, oldest isolation transformer he can find. Some games like Pac-Man have a combination isolation transformer/power transformer that puts out low voltage for the game board as well as isolation for the monitor. So if you wind up with one of those, be sure you know how to connect it. Also, watch out for Nintendo (Donkey Kong, etc) isolation transformers. They are set up for 100V output, since that what Japan uses - not 120V. It's not quite a 1:1 transformer.

-Ian

[RetroHacker]

Also, forgot to mention - you can also run two transformers in parallel for higher current handling. So, if you get two old arcade machine transformers that are the same 1:1, you can wire the primaries in parallel and the secondaries in parallel - but on the secondary, watch out, because they need to be phased the same. If they're backward, you won't get any output!

-Ian

[Old1625]

Originally Posted by RetroHacker

Also, forgot to mention - you can also run two transformers in parallel for higher current handling. So, if you get two old arcade machine transformers that are the same 1:1, you can wire the primaries in parallel and the secondaries in parallel - but on the secondary, watch out, because they need to be phased the same. If they're backward, you won't get any output!

-Ian

Best if the transformers are identical (same make and pt#) to ensure that the voltages are exactly the same. And I would also certainly recommend that the secondaries be phased properly.