Building an Isolation Transformer

[cdmarion]

Hey Guys,

I want to build an isolation transformer to protect myself from some of the AC/DC sets i'm working on and have a question.

I understand how to build an IT using two similar transformers. But i just found this IT from WWW.Mouser.com part# 553-N51X. Here is some info from the data sheet,



[QUOTE][Electrical Specifications (@25°C) 1. Maximum Power: 35 VA 2. Input Voltage: 115 V, 50 / 60 Hz 3. Output Voltage: 115V + 5% 4. Full Secondary Load: 0.30 Amps RMS 5. Voltage Regulation: 15 % TYP @ full load to no load
Description: The N-51X is power transformer for isolating equipment from direct connection to the power line. It is constructed with nonconcentrically wound coils. The primary and secondary are wound on separate arbors, then assembled on a laminate core side-by-side separated by insulation. This prevents electrical connection under normal or overload conditions between the primary and secondary windings. WHTBLKBLKAREDRED.187 DIA (REF)2 HOLESDB Safety: These units are designed with 1500V isolation between winding to winding and between winding and core. Materials and construction are rated for Class B insulation system./QUOTE]

If i understand this rite i can use a single IT like the N-51X above instead of a pair of power transformers to build an IT. Am i on the rite track or do i need a pair of these also? Any thoughts or opinions?

Chris

[electroking]

Hello,

ONE transformer of this type is just what you need to work on a 5- or 6-tube
AC/DC radio. The 35 VA rating would however be too low for anything
bigger, such as a series-string TV.

[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

[Tom Bavis]

50 VA is enough for the typical AM or AM/FM radio - though I have a couple of stereo ones that would need maybe 60-70... I have a 500VA transformer (computer power conditioner), so I'm ready for anything...

[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.

[louisiii]

In the 60s an Isolation transformer was simply a 1:1 transformer with no physical tap off the primary. We used them as a basic protection from the mains. One other practice we used when mounting more than one transformer on a chassis was to to mount them 90 degrees to each other to avoid inductive coupling.