An isolating transformer AND a variac. To be used together.
With switchmode supplies there are often times you'll want to run (or power up gradually) the supply with some node in the mains side circuits grounded for use as a reference to examine relative waveforms on other nodes. This requires an isolation transformer on the mains input, since with a bridge rectifier early in the mains input, everything from there on is 'moving' relative to true ground. But with the isolation transformer on the mains input you can just pin any circuit node to true ground (assuming it isn't something being driven with a high frequency signal.) A typical point to want to pin, is the negative side of the main storage caps, that may have something like 370VDC charge across them.
The variac is so you can apply a few volts of mains AC while checking for short circuits and other failures in the mains input circuitry, without loud bangs and fuse replacement every time. It also helps to have a dedicated analog dial current meter showing the mains current, and a quick-acting circuit breaker too. I run it like:
RCD breaker --> Mains wall outlet --> standard mains cord --> IEC socket and switch on transformer box -->Isolation transformer --> standard mains outlet on transformer box --> IEC cord --> IEC socket on variac unit --> local circuit breaker, switch and warning light -->Variac --> current meter --> standard switched double power outlet --> IEC cord or special mains cord with alligator clips (very dangerous, hence red warning tape all along it) --> circuit under test.
Do CHECK that the mains ground connection is good through that chain, since when you use it to power some device via a standard IEC mains lead, that's your safety ground.
The variac has a double power outlet, so I can easily plug in something to measure the variac output voltage there, if it's awkward to attach multimeter leads to the circuit under test. It's always good to minimize the number of loose leads and clips carrying mains. Clips can come off, and leads can slide around.
Good idea to put a prominent warning sign on the variac outlets: "WARNING - HV, NOT ISOLATED" to remind yourself and other people.
Another useful thing is a standard male 3-pin mains plug, with only one fat green wire connected from the ground pin. To plug into wall outlets with known CHECKED-good earth, so you can easily safety-ground systems under test, without relying solely on (or blowing up) things like scope probe ground clips. Or potentially accidentally making your instrument cases live, if their ground connection is somehow faulty. Can happen, for instance by a faulty mains cord. This has happened to me, in that instance due to an instrument's cord having spread contacts in the IEC female plug. Fortunately the mains coupling was only via a small cap, so the discovery wasn't fatal. Though quite painful.
A couple of mine:
http://everist.org/NobLog/20151112_planning_vacuum.htm#dusthttp://everist.org/NobLog/20151112_planning_vacuum.htm#54120B_ps
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