What is an isolation transformer? How do you scope the mains side?

An SMPS repairer's most expensive mistake: clipping the scope ground onto the bridge's negative. A spark, a vaporised trace and often a dead scope input. The reason is simple: the clip is earth, and neutral is earth too.

What does it do? In the grid, neutral is bonded to earth in the panel; a bench scope's probe clip is earthed through its plug too. The 'hot GND' on an SMPS primary (the bridge's negative) connects to L through a diode during one half of the mains cycle. Clipping onto it shorts L straight to earth. A 1:1 isolation transformer feeds the device from a winding that has no link to earth: even if the clip ties one point to earth, there's no path for current to return. A differential probe measures the difference between two points without connecting to earth at all. Remember: an isolation transformer doesn't protect you from touching two points at once, and it makes any RCD behind it useless.

Tap the dashed parts for what they do, how to measure them and how they usually fail. Numbered points are scope measurements.

Phase 1/3: No isolation: the clip hits the mains

In the panel the neutral is bonded to earth (N–PE link), and the scope's clip is earthed through its plug. The moment you clip onto the bridge's negative (hot GND): when L is in its negative half, the bridge diode connects hot GND to L and current flows L → bridge → clip → scope → PE → panel → N with nothing to limit it. The clip melts in a spark, the probe lead burns, and the bridge diode or the scope's input dies.

1. Hot GND relative to earth (no isolation)

0 V during one mains half-cycle, down to −311 V during the other. So the primary-side 'negative' is not earth, while the scope clip is.

On the scope

Multimeterbetween 0 and −311 V (!)

🔍 How to spot it on a board

•A heavy transformer box on the bench with 230 V in and 230 V out (300 VA–1 kVA), often labelled 'isolation'
•The output socket either has no earth contact or its earth isn't tied to the secondary
•Differential probe: a boxed probe with two red/black leads, a 1:50 / 1:500 selector and a voltage rating (e.g. 1300 V CAT II) printed on it

🔧 How to measure it

•Megger between the isolation transformer's secondary terminals and PE: it should read megohms (no connection).
•Before measuring the primary side: with the device off, clip to hot GND and put the probe on the test point, then power up.
•One-clip rule: all the scope channels' clips are common, so put them on the same point or use only one.
•A multimeter floats anyway; you don't need isolation to measure primary voltages against hot GND with a meter (still mind the CAT rating).

⚠ Common faults

•Clipping onto hot GND without isolation: the bridge, a trace, the probe and the scope input burn; a fuse or RCD trips.
•Defeating the scope's earth (an unearthed extension, a cheater adapter) is not a fix: the scope chassis rises to mains voltage.
•Trusting the isolation transformer and working two-handed: there's still 230 V between the secondary's two ends, and no RCD will protect you.

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Go further

Concept: series bulb (use it alongside)Concept: bootstrap (a floating measurement example)Concept: earthing and the RCD A primary side you'll measure: flyback Component: transformer