Isolation transformers safety?

[SRG]

Dear Group it's been a while....

I interest in knowing if a 110v isolation transformer such as those
used in the building trade (yellow boxes in the UK) would provide
adequate safety when working on SMPS's?

I was also thinking of adding a variac to allow variable ac voltage
input thus enabling testing at a much lower voltage.

Any advice would be much appreciated.
TIA
Steve.

 

[Walter Harley]

I interest in knowing if a 110v isolation transformer such as those
used in the building trade (yellow boxes in the UK) would provide
adequate safety when working on SMPS's?

I don't mean this to sound disrespectful, but: safety comes from your brain,
not from a device. If you don't understand the reasons behind the shock
risk of working on a mains-powered device and how an isolation transformer
helps address them, then an isolation transformer by itself is not going to
keep you safe.

An SMPS powered by an isolation transformer still contains deadly voltages.
However, they are no longer ground-referenced, until you attach a ground via
your test equipment or load.

I was also thinking of adding a variac to allow variable ac voltage
input thus enabling testing at a much lower voltage.

Testing at a low line voltage is rarely a useful thing to do when working on
SMPS's, I think. It just changes the duty cycle. It's not like a linear
power supply which you can bring up gradually - it won't work at all till
there's enough juice to power the logic part, and then it'll run at full
steam trying to make the output voltage with minimal input. So, it's useful
if you're trying to see how the SMPS works on low line voltage, but it's not
the "safe harbor" that it can be for linear supplies.

The "light bulb in series" trick works fine, though.

 

[Genome]

| Dear Group it's been a while....
|
| I interest in knowing if a 110v isolation transformer such as those
| used in the building trade (yellow boxes in the UK) would provide
| adequate safety when working on SMPS's?
|
| I was also thinking of adding a variac to allow variable ac voltage
| input thus enabling testing at a much lower voltage.
|
| Any advice would be much appreciated.
| TIA
| Steve.

An isolation transformer is an excellent idea. As are RCD trips.

You can also get 1:1 (220/220) ones as well.

Without it you'd probably do something....... silly like disconnecting
the ground lead on your scope.... then someone else borrows it and goes
snuff just after it goes spark.

Even more fun when you forget to switch off your supply and land your
own soldering iron on it to change R86

It also stops the cleaning lady going snuff when you go home and forget
to switch off your bench.

Obviously after a hard days slog with your squealing piece of dirt in
front of you you switch your bench off and unplug the snuff bits.

You just have to if you want to avoid having to climb over snuffed
bodies the next morning.

Variacs are also totally way cool. You can float in an auxilliary supply
to get the control stage up and running and then slowly wind up the
volts whilst checking various bits and pieces are performing the way you
expect them to.

Add in a rectifier and some smoothing capacitance for PFC stages and
you'll be able to see more sensible waveforms.

Have fun

DNA