isolating transformers, construction and use

[ghostman11]

hi guys,
i was having a conversation about the repair of an old oscilloscope and it was sugested that it would be safer to use an isolating transformer while working on it.
it was also sugested that one coulod be built cheaply and easily. so my question is what exactly is an isolation transformer and what is it used for? also how would you go about making one?
many thanks jason

 

[Reloadron]

I don't know about building cheaply or why working on it connected to an isolation transformer makes things any safer with the exception of ground isolation but all an isolation transformer would be is a transformer with a 1:1 turns ratio capable of handling the applied load sans any ground connection.

Ron

 

[RCinFLA]

Isolation transformer is used when working on a system that allows you to float the AC so there is no earth ground neutral. Most switching power supplies have AC side isolation so unless you are working on that section there is not much need for isolation transformer. The AC grid neutral side (larger prong on 120vac plug) is earth grounded at your service entrance breaker box. I believe UK has effectively the same situation, just 220 vac.

An example of need for isolation transformer would be trouble shooting a flourescent electronic ballast. Most of these are not AC grid isolated so their ground reference for the circuit is not the same as grid neutral ground reference. You can get shocked from a scope probe ground, or worse blowing out something when scope ground lead is connected.

If you have to work on the AC input side of a switching power supply then you need an isolation transformer.

Buy one if you need it. Making one is difficult and you probably will not get the core assembled tight enough to avoid a lot of leakage inductance that degrades power transfer coupling.

 

[ghostman11]

thanks for the info guys thats realy helpful, actualy i am working on the line side of the PSU! not normaly one to mess with greater than 12v! but thought i would have a go at attempting an repair on a old scope i have, might be safest to leave it alone thanks for the useful info tho

 

[squishy36]

If you're going to service stuff regularly or make lots of measurements, an isolation transformer is not a bad idea. Used ones can be found for reasonable prices on ebay (I got a 1 kW medical-grade one delivered for about $75 a number of years ago). You can also "make" one from two identical transformers: connect the secondaries together, apply line power to one primary and get an isolated voltage from the other transformer's primary. Don't exceed the power rating of one transformer though.

 

[debe]

My isolation T/F is a salvaged 150Watt 240vac/240vac transformer. It was out of an old colour TV, mounted it in a suitable enclosure. Used a lot when repairing tvs a such with SWMPS.

 

[KeepItSimpleStupid]

The common mistake made is that people isolate the scope when repairing an SMPS etc. You isolate the SMPS or the item you are repairing, not the scope.

Differential measurements ie, using the A-B node of the scope is the way to repair something attached to one side of the line without an isolation transformer or if you need one that would be too big such as for a 30 kW electron beam power supply.

 

[ghostman11]

what i dont understand is how this isolation makes it 'safer'?? and how much safer is it to use one? i am not questioning the need to use one but rather what a isolation transformer does that makes working on such equipment safer to do, BTW for me at present seeing as i have to ask such questions i wiont be messing with the repair untill i am safer to do so
by the way all the info recieved so far is much appreciated i love learning this stuff

 

[Reloadron]

In normal typical wiring for example residential where 3 wires are used we have the line (hot or high side), neutral and earth ground with neutral and earth ground tied together at the service entrance. While working on an energized mains powered circuit there are potentially dangerous voltages between line (hot) and neutral or ground. Using a 1:1 transformer affords isolation. Additionally here is a WIKI explanation.

Personally I don't bother and have updated my benches to GFIC outlets. Also here in the US the mains voltage is 120 VAC. Not to say 120 VAC won't hurt but is less likely to fry me than 240 or higher.

Ron

 

[ghostman11]

thanks loads ron (ok pun intended )

from wiki "In electronics testing and servicing an isolation transformer is a 1:1 (under load) power transformer used for safety. Without it, exposed live metal in a device under test is at a hazardous voltage relative to grounded objects such as a heating radiator or oscilloscope ground lead (a particular hazard with some old vacuum-tube equipment with live chassis). With the transformer, as there is no conductive connection between transformer secondary and earth, there is no danger in touching a live part of the circuit while another part of the body is earthed."

thats exactly the kind of explantion i was after!! it makes sense to my tiny brain now

 

[KeepItSimpleStupid]

Near the end of the tube TV era there was a filament string that started with 38 V filaments and worked their way down so the sum of the tubes filaments were 120 V. This also happened with Radio.

There were warnings that the chassis was "HOT". It made nice sparks when probes ended up in the wrong place.