Computing stuff tied to the physical world

Inductive spikes and safety

The Electronic Tutorials site has an excellent intro and diagram of a Current transformer:

Trans53

Unlike traditional transformers, the key here is that the primary is placed in series with the load. There’s virtually no voltage across it, but all the current of the load passes through it.

Because of this, a Current Transformer does to current what a Voltage Transformer does to voltage: it reduces (or amplifies) the value from one side to the other. In a CT, the primary “coil” is usually just a single turn, being the wire passing through the ferrite core – once. A CT with a 1:1000 winding ratio will generate a current equivalent to 1000’th of the current passing through its primary winding. A 1 kW load on 230 VAC mains is about 4 Amps, and will come out as about 4 milliamps on the other side.

There is one essential aspect of a CT you have to keep in mind: its secondary should not be left open, just as a voltage transformer’s secondary should not be shorted. Perhaps surprisingly, very high voltages can come out of the CT’s secondary when left open!

The way to explain this is through an analogy: sea waves pounding on a steep vertical coast. As you know, such waves can really pound on those rocks when there is a storm, sending the water sky high. The reason for this is that the energy has nowhere else to go but up! Similarly, a flux change in the CT’s ferrite core moves electric energy in its windings – there must be some change of V and/or I in that winding.

Luckily, while the voltages of an open circuit can and will rise, their energy is limited. The available current is minimal – since it is reduced proportional to that winding ratio.

In our Micro Power Snitch, we’re in fact not using the Current Transformer as intended. Instead of putting a known resistor across it so we can deduce the current going through it, we’re feeding the secondary output to a Delon bridge with reservoir capacitors. In theory, the output voltage could go very high, but magnetic as well as winding losses prevent that.

This is why either a burden resistor or some back-to-back protective zener diodes are normally built right into the CT. That way, its output voltage can’t rise beyond a few volts.

For our energy harvesting purposes, the zeners are considerably more useful than having a burden resistor eating up almost all the energy produced in the secondary winding.

Another safety aspect of Current Transformers to be aware of has to do with failure modes. Equipment can and will fail, and proper engineering is needed to make sure all plausible failure modes are understood and cannot lead to dangerous situations or safety hazards.

What if the primary winding shorts out to the secondary winding? An accidental water leak might one day cause this. It’s ok for our circuit to fail, but it needs to fail in a safe manner.

The usual solution is to ground the circuit attached to the (otherwise floating) secondary. Then, an accidental short will lead to currents leaking to ground, which will then trip the Residual Current Device (RCD) which is part of the mains protection in every house.

In other words: although we can’t prevent this failure, we can still direct its outcome.

If the MPS is fully encapsulated and isolated, grounding will not be needed. But if there is any chance of touching parts of the circuit and you don’t fully trust the CT’s galvanic isolation, then attaching any part of the MPS to “real” earth ground is the way to go.

Note that most modern RCD’s trip at 30 mA, so even if you are in the path from AC mains to ground due to some major fault condition, the RCD specifications are to disconnect the AC mains in a single half cycle – in time to protect you. But it could be a very nasty jolt.

Remember: 230V AC mains is serious stuff!

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