Questions on building an Isolation Transformer

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Questions on building an Isolation Transformer

Post by Steve Myers on Sun Jul 01, 2012 2:23 pm

I bought two 120 60 Hz Sec. 6.3-0-6.3 transformers to build a isolation transformer. I understand to wire the sec. together to get 120 out. Question: I will wire the 3rd prong ground to the frame of the 1st transformer, but do I "bond" it to the 2nd transformer and onto the 3rd pin (ground) of the receptacle? Does not this "defeat" the purpose of the isolation? Maybe I am missing somthing here.

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Re: Questions on building an Isolation Transformer

Post by Resistance is Futile on Mon Jul 02, 2012 12:31 pm

If you have a radio that has a ground wire, you still need to Isolate. I have an isolation transformer that has an output 2 prong receptacle on one side and a three prong input from household power. If you happen to get a short in the windings to transformer shell, Then I could see the need for connecting a wire from the transformers shells to ground on the inlet side only. No ground on the outlet side however. I could be wrong, but that would be my reasoning. And a fuse in-line on the input side of course which would be on the hot side, which is the smaller blade on a three prong plug.

The house wiring is set up to have three prong plugs and receptacle's with the smaller prong being the Hot wire and the return being the bigger prong. The reason being if a large current is put on a smaller of the two prongs it will limit the current by heating up faster and melting before the larger prong which can withstand more current before melting for safety. The return wire is supposed to be at ground potential when no power is used in a circuit, but with improper grounding practices such as loose connections there would be some resistance on the return side and a potential shock hazard. That's why there is a third wire called a ground wire, it is supposed to be electrically at earth ground (0 volts) in-case the return wire shorts. If the Hot side shorts it will be shunted to earth ground. That's one reason why two wire appliances such as a toaster are insulated from the metal shell so in-case of a short it safely goes to ground and wont shock you. The same principle applies to isolation transformers.
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