The CT to BNC board is a cascaded Current Transformer (CT) that can be connected directly to a shielded BNC cable. Cascaded current transformers are used to step-down large AC currents to manageable levels that can be measured or used as feedback and other control signals in a resonant system driver. This can be useful for high power electronics applications such as:

• DRSSTCs, high power flyback transformers and X-ray transformers, and other high power inverter systems.
• Feedback and over-current detection (OCD)
• Real time current sensing and diagnostics using an oscilloscope connected via the BNC

The board comes with two N77 type toroidal ferrite cores for the CTs, a PCB, and a BNC connector. This configuration has been successfully tested to over 1000A. The boards have holes so they can be mounted to a chassis and/or stacked to have multiple outputs (for a DRSSTC, I like to use 3 - Feedback, OCD, and real time oscilloscope measurement with a 3 meter BNC cable. Make sure to add burden resistor to the scope measurement transformer!!).

Example configurations: (if you're not interested in the theory or want a quick solution)

1. 0 - 250 A(peak) 1:16:16 = 1:256 5R 2W burden = ~2V out / 100A (22 - 24G)
2. 250 - 500 A(peak) 1:23:22 = 1:506 5R 2W burden = ~1V out / 100A (22 - 24G)
3. 500 - 1000 A(peak) 1:32:32 = 1:1024 5R 2W burden = ~0.5V out / 100A (24G)

As a rule of thumb it is best to keep currents around 1A at the max voltage you plan to see through your primary. For simplicity in 100V increments, these are my go-to's. Accuracy of the output voltage will depend on burden resistor tolerance.

The theory:

The transformer equation tells us that the ratio of the number of turns in the primary to the number of turns in the secondary is proportional to the voltage and current change across the coils when the transformer is running.

        Np/Ns = Vp/Vs = Ip/Is

So a step-down current transformer such as this is intended to get these huge currents and step them down to a much smaller level that we can measure. By running a high current wire through a toroid transformer wound with many turns, we are stepping down the current through the wire by the ratio of the number of turns around the toroid. Since the wire running through is acting as a primary coil with 1 turn, if the secondary of the transformer has 32 turns, we will see a current that is 1/32 the magnitude of that going through the wire. However if we are trying to measure 1,000A and have about 1A on the output, we start to run into issues with having to wrap something 1,000 times. So if we want to measure 250A, 500A or even over 1000A, we have to get a little creative.

The "cascading" of the transformers allows us to step the current down much further by wrapping the output of one transformer around another step down transformer. In the example above of a single wrap to a 32 wrap being a 1/32 ratio, and we wrap the secondary of that one around a second transformer to create another 1/32 step down, we are now stepping down 1/32*32, or 1:1024 times reduction in current.

Now how do we ACTUALLY measure this? Well, we use a "burden resistor" across the output to create a voltage across the output of the transformer. Then, we can use our old friend ohms law to calculate the current across the output of the known resistor value. So if we have 1,000A, reduced by 1:1024 = 0.9765A on the output. If we add a 5R resistor (capable of handling that current) across the output and use V = I * R, we see that we get 4.88V (~5V) on the output at 1,000V, which is 0.48V (~0.5V) for every 100A through the high current primary.

Quick Note: NEVER USE A CURRENT TRANSFORMER WITHOUT A BURDEN RESISTOR !!!... Seriously. The voltages seen are unbelievably high if you don't, which will certainly destroy anything you connect it too and could easily give you a nice zap if you're not careful.