isolation transformer theory and practice

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isolation transformer theory and practice

Postby centervolume on Mon Aug 01, 2011 1:30 am

early danelectro leader - death machine on the scene.

no power transformer so I got a triad n68x isolation transformer and wanted to double check my wiring hookup in terms of what is correct, but also how it works.

the triad has 2 wires on the primary and 3 on the secondary (red, red and white).

the hot and neutral from the wall go to the 2 primary leads. The ground from the wall goes to the white/ground of the secondary.

the ungrounded/ 2 conductor power line from the amp goes to the remaining two leads of the secondary.

in terms of function, the transformer is neither a step up or a step down. Instead it introduces ground to a circuit that was not
referenced to ground, namely the amp itself, which is now attached to a grounded power source = the secondary of the iso transformer.

There is no need to tie a wire from the outer frame of the transformer to the white/ground line.

is this the gist of what's going on here? I am about to install this but wanted to check the wiring against my understanding of what is going on!
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Re: isolation transformer theory and practice

Postby Blueracer on Mon Aug 01, 2011 5:02 pm

Here is a link to how to hook up the isolation transformer.

http://www.instructables.com/id/Isolati ... /The-Plan/

I would modify this schematic by reversing the on/off switch and the fuse. The fuse should come first. The rationale is what if the switch fails in some manner that puts power to the chassis. If it were wired as shown the fuse would not protect this sort of problem. Note that the white wire of the transformer is not connected in this schematic. The white wire is to a shield in the transformer which should help prevent electromagnetic coupling into the tranformer. If used it would connect to the earth ground (green primary wire). Try it with or without this connection and see if it makes a difference. I doubt that it will. Same sort of argument for grounding the case of the transformer. I don't see a down side in making these connections however. For your home I doubt that you will be in a significantly high electromagnetic enviroment but you might not always be using the amp at home.

Now to answer the function of this circuit. First these no-power-transformer amps are potentially very dangerous. Here is the problem. One side of the input power is used as the voltage reference for the amp and is tied to the chassis. If it is the white neutral input all is fine. The white wire is at ground potential. However if the wall plug is reversed (or not wired correctly at the wall) then the black wire is used as the voltage reference. This wire is at 120 vac relative to ground. This puts the chassis at this potential relative to ground. So if you are in electrical contact with ground and touch the chasssis your body sees 120 vac. Likewise any other properly grounded piece of equipments has a potential difference of 120 vac between their chassis and the amp chassis. The isolation transformer gets away from this by isolating the ground reference of the input from the amp. Grounding of the amp is through the much safer green wire rather than relying on the white neutral wire. The transformer can also only operate up to a certain current before saturating out. In this manner you are isolated from say Hoover dam which is connected to the wall (a vast over simplification but put here to make a point) which is the other rationale for an isolation transformer. Hope this helps.
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