Silly transformer question

[Grauwulf]

I'm pretty sure I already know the answer to this, but I want someone with some actual electronics knowledge to back me up. I have 3 Sanyo 20-ez monitor I need to repair and I finally managed to find a transfomer that will do the step down to 100v from 115v. Now correct me if I'm wrong, but aren't all transfomers isolation transformers by design? All you're doing is inducing a voltage on a secondary winding from the primary windings, and that's all the interactions the two voltages really have. I'm just wondering if I need to pair this step-down transformer with a 1:1 isolation transformer. the transformer I have also does 115->115 and 115->125 as well.

Can someone help my befuddled mind?

[dabone]

Yes you are correct, you can use the stepdown as the isolation transformer.



Later,
dabone

[SirPeale]

You sure about that?  There was a huge thread in this forum regarding iso xformers, and I remember some discussion regarding iso xformers, and regular xformers.

[MonitorGuru]

The OP is correct. Any transformer, by design, consists of a primary and secondary winding.  You feed the incoming power into the primary winding, and then a new, isolated power source is created on the secondary winding.

This power source then is like a battery.... isolated from the originating power and the earth (since neutral AC is carried though the earth)  Just like being able to take a car battery, stand in a puddle, you can touch either terminal (+ or -) (but just ONE of them at a time) and not receive any shock... it's isolated.

The only difference between an "isolation" transformer and a "step-down" transformer is the ratio of loops the secondary winding has to the primary winding.  If they are not the same then the voltage will vary accordingly.

[Ken Layton]

This does not apply to "autotransformers" (a.k.a. variable) since they have no secondary, but rather are a giant variable resistor. An autotransformer is also known by the trademarked name VARIAC.

[Trimoor]

Not quite true.  A variac is a variable transformer, but usually does have isolated windings.
An autotransformer does not isolate.  It has one single loop of wire incorporating all of the windings, making it cheaper but non isolating.

You can however use a 1.1:1 autotransformer with a 1:1 iso transformer, which is likely what i will do.

One of my cabs has a 1:1 iso driving a 100v monitor with no problems.  Probably not a good idea, but something to consider.

[Grauwulf]

Thanks fir the responses everyone. This makes the setting up of my test bench a little simpler, since I will only have to mount and wire the one transformer.