EasyCT

EasyCT

Terminology

• Burden Resistor: Also known as sampling resistor

Supported Inputs

EasyCT supports most current-mode current transformers (that is, current transformers without a built-in burden resistor). These will have their outputs listed as currents (typically $\textrm{mA}$ or $\textrm{A}$), not voltages ($\textrm{mV, V}$)

Additionally the PCB footprint has support for the following transformers:

• Three Terminal Jacks (known as TRS, AUX, XLR)
  • YHDC-SCT013-000
• 2-Wire
  • PZCT10020 (generally PZCT-XXXXX, etc)
  • DLXQ26-XXXX
• ZMCT-style
  • ZMCT103C
  • ZMCT118F

Configuration

The burden resistance $R_\text{burden}$ sets the resistance used to create the sense voltage $V_\text{sense}$ using the current from the transformer according to the network shown below, which allows for a wide range of currents. The target $V_\text{sense}$ is about $1\text{V}$, which is doubled by an op-amp amplifier and compared against the trigger voltage, $V_\text{trigger}$, set by the $200kΩ$ potentiometer.

The potentiometer can be tuned to the desired trigger voltage by playing around with the circuit.

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Example Use-Case

I use one of these with a relay to turn on a dust extractor when a large Robland table saw is on. Because the table saw has multiple tools running on separate circuits, and because I don't like to work with mains voltage, it is not possible to directly hook into the mains going to the saw. Further, the saw is 3 phase and additionally draws a standby current for the magnetic clutch for the on-switch, which complicates other potential solutions.

This circuit is very easy to use with any tool that runs on AC voltage and can be used entirely without making electrical contact with mains power.

Notes

Update: after 3 years of operation the circuit started flickering on and off. It had been sitting inside of a table saw, so maybe vibration caused the trim pot to shift, or capacitors started leaking a bit more, or oxide built up in the trim pot causing the $V_\text{trigger}$ to shift. In any case, the fix was simply adjusting the trim pot back into a working position.