How can I make DIY Isolation Transformer?

[calzap]

This thread prompted me to start a thread on misuse of isolation transformers.  It's https://www.eevblog.com/forum/beginners/how-to-kill-yourself-with-an-isolation-transformer/ .  Be sure to read the follow-up comments.

Mike

[wizard69]

I had this thought. a GFCI is literary supposed to prevent you from getting killed, when wet, by a outlet in the bathroom.

I actually believe that GFCI can provide better protection than an isolation transformer.   This isn't to say that an isolation transformer is always a bad idea, but you can make a mistake wit electronics operating on an isolation transformer that will kill you just a well.   The death comes from current flowing through the heart and any goof that induces that flow will kill.   

I think that is a pretty trust worthy technology compared to a isolation transformer.

Its a globally used well developed technology. It is super mass produced and cheap.

Its even recommended for out door power equipment like pressure washers. And its lighter.

Don't bother with the transformer?

On the flip side there are good reasons to use an isolation transformer especially for sensitive equipment.   Also people need to realize that dim bulb protection circuits are more about protecting old hardware if there are hidden shorts or failures at voltage.   They really have nothing to do with protecting the tech, but are rather low tech current limiting.

[dietert1]

Our isolation transformer has been used for scope work on switched power supplies - to separate circuits normally connected to mains. Yes, this work is dangerous and only for experienced techs aware of the risks involved.
A DR is for a different purpose and working on a mains connected device with difference probes is as dangerous. The light bulb is a auto-reset fuse. It doesn't prevent accidents, like touching metal parts with a loose probe Gnd clip. Better use a separate ground wire and pull the clips from the probes. And avoid probes with a 1:1 - 10;1 switch when working on a mains circuit and don't do this type of work at night when you are getting tired.

An isolation transformer can also help to break ground loops in analog audio. This applies to semi-professional equipment without ground lift. Once more a situation that requires knowledge and experience to maintain safety.

Regards, Dieter

[jonovid]

image 1
an old 700 watt microwave oven transformer
with its primary 240 volt windings. the secondary winding gaps are for another Do it yourself project.
the original high voltage 1kv secondary winding was cut with a hacksaw and pushed out, without disassembling the core.
leaving the low voltage side of the secondary intact. red wires.

image 2
here is an un-cut 700 watt primary 240 volt winding in its varnish collar from another old microwave oven transformer were the core was disassembled.
leaving the wire collar intact.
this is a lot of wire to unwind then poke through a new core to make a mains voltage isolation transformer.

[Gyro]

image 1
an old 700 watt microwave oven transformer
with its primary 240 volt windings. the secondary winding gaps are for another Do it yourself project.
the original high voltage 1kv secondary winding was cut with a hacksaw and pushed out, without disassembling the core.
leaving the low voltage side of the secondary intact. red wires.

image 2
here is an un-cut 700 watt primary 240 volt winding in its varnish collar from another old microwave oven transformer were the core was disassembled.
leaving the wire collar intact.
this is a lot of wire to unwind then poke through a new core to make a mains voltage isolation transformer.

I referenced this in reply #17. A re-wound MOT is absolutely the worst way for somebody to make a safety isolation transformer...

- The windings are probably CCA.
- Never re-use old wire in a transformers - especially a mains winding, and you're uncoiling a varnished coil
- The chances damaging the wire insulation while threading the core are high.
- The secondary-core insulation is nowhere near good enough for a safety isolating transformer
- The average person doesn't have the necessary Hipot test equipment to ensure that the primary-secondary and secondary-core insulation and voltage withstand meet spec.

Don't do it, it's not safe for this application - just don't.

[Ian.M]

+ it would be running on the ragged edge of saturation and overheating if you run it for long, as Microwave oven transformers are designed as cheaply as possible and rely on their limited duty cycle + fan cooling to survive their normal use.  To avoid saturation, you need 10%-20% extra primary turns or the same reduction in input voltage or increase in mains frequency (e.g. a 50Hz transformer in a 60Hz country), if using them to build a normal transformer, and the few who are competent to add to the primary safely, know its not worth the effort as they can buy transformer laminations to assemble a core round a bobbin for less than the labour cost of faffing around with a recycled MOT.