Isolation transformer safety for testing?

[SRG]

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

I'm interested in knowing if a 220v - 110v 3.3KVA 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.

 

[CFoley1064]

Subject: Isolation transformer safety for testing?
From: sme@prontomail.com (SRG)
Date: 4/23/2004 10:20 AM Central Standard Time
Message-id: <6d34f1fb.0404230720.3d6771b@posting.google.com

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

I'm interested in knowing if a 220v - 110v 3.3KVA 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.

When working on switchers, isolation is a must. Be sure you actually have an
isolation transformer rather than a step-down autotransformer there. A simple
check from primary to secondary with an ohmmeter (with power disconnected, of
course) should tell you. (View in fixed font or M$ Notepad):

220V-to-110V Autotransformer

L1 In (220V) o-------.
|
C|
C|
C|
L1 Out (110V)o-------C|
C|
C|
C|
|
L2 In/Out (0V)o-------'

created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

I'd like to see a fuse, fast circuit breaker, or other type of interrupting
protection on the primary *and* secondary of the transformer, especially
considering the size of the transformer.

A variable transformer is a good tool for a test bench, especially with a
series ammeter. A variac won't help you with switchers, though -- you
shouldn't apply input voltage below specified minimum (usually 10% below
nominal or so). Doing this may damage the supply you're trying to repair.

Good luck
Chris