I am trying to study the operation of a single plate Induction Hob.

Prolectrix Induction Hob

I wanted to use an oscilloscope to measure some of the low voltage digital signals on the microcontroller inside. So I attached the ground lead of the 'scope to a pin labelled 'GND'.

Inside induction hob

But when I powered up the hob, the fuse inside exploded. I guessed that this is because the GND line is actually bouncing up and down with the AC supply.

Is it possible to use a 'scope to measure signals inside a mains device like this without something exploding or risking damage to the oscilloscope?

I don't have a schematic for this appliance, but it's likely to be pretty similar to most other induction cookers, like this one:

Induction cooker circuit

In the top left you can see the AC come in, then there's a bridge rectifier with the negative terminal defined as ground.


Your 'scope lead is safety (as named in North American standards) what you need to do is to take two probes, one on channel A and the other on channel B and use the A-B function (that is what it is for). I take the ground clips, clip them together and use electrical tape to ensure that they don't come apart and short anything inside.

You can buy a differential probe but that tends to be very expensive and is mean for very sensitive work. Just looking at A-B is good enough for probing around in the device you describe.

  • \$\begingroup\$ All I do is ensure that the scope GND doesn't touch the device? And connect the B probe to the GND line instead? \$\endgroup\$ – Rocketmagnet Jun 6 '13 at 22:50
  • \$\begingroup\$ If you have them, some scope probes have removable ground clips for this purpose. So yes, to your question. \$\endgroup\$ – placeholder Jun 6 '13 at 22:53
  • \$\begingroup\$ Be careful, even if the voltage ratings are sufficient most scopes are only overvoltage category 1 which means they shouldn't be used on direct mains powered gear. As well as letting you put the ground where you want it an isolating transformer will typically take off the worst of the spikes. \$\endgroup\$ – Peter Green Dec 13 '15 at 3:47

The GND of the scope probe is connected to the case of the scope, which is connected to mains earth, so it is impossible to measure a device which is connected directly to mains, unless:

  1. You use a special differential probe (expensive) or
  2. You use an isolating transformer (230V/230V or whatever your mains voltage is) to power the device. I use one when I need to probe the hot side of a power supply. The transformer makes it so that the 230VAC coming to the device is not referenced to ground, then I can connect the ground clip of my probe wherever I want to.

protected by W5VO Jun 6 '13 at 22:32

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