# What defines the maximum primary current of a (sensor-only) current transformer?

Take the Coilcraft CS1200L for example, which states the sensed current maximum is 35 A.

This limit doesn't seem to be defined by the wire size which can fit through the hole (5 mm) as you can pass > 100A of current through a 5 mm wire.

It also doesn't appear to be limited by saturation in the transformer core. Using the calculator provided by coilcraft with 35 A sensed current, 25 kHz input frequency, 50% duty cycle and 1 V output, it calculates $$\B_{max}=67\$$ Gauss. Coilcraft state to stay below 2000 Gauss to avoid saturation so we've got a way to go before hitting that limit.

I found a reference discussing the 53200C which states "The maximum current of 10A within the rated bandwidth of this product is a thermal limit". This implies power dissipation is the limiting factor, at least for the 53200C.

I calculated is heating of the secondary from the DC resistance. Using a primary current of 35 A and DCR max of 4.5 Ohms, the losses are $$\P=I^2R=\left(\frac{35}{200}\right)^2\cdot4.5=0.138\mathrm{W}\$$. That doesn't seem like that much power. There's also hysteresis losses to consider, which should also be low due to the core material and low maximum magnetic flux density. I assume there must be more losses that I'm missing?

On the topic, what allows the CST206-2A to handle 80A RMS rather than the 35A of the CS1200L? It appears to have a slightly lower inductance and DCR max, and a much higher volt-time product.

• I'm not sure there's anything in the Coilcraft datasheet to say that 35A is a maximum, it seems to be used as a reference current at which the RT and Vout are calculated. The higher current in the Triad datasheet says it gives 40C rise, which would be too hot to hold at a 25C ambient. – Neil_UK Jul 19 at 13:59
• Isn't that what the "Sensed current Iin max (A)" spec is? – intecher Jul 19 at 14:46
• lin=linear. it would mean that beyond 35A the response starts to become significantly non-linear. – dlatikay Jul 19 at 15:24
• What would cause the current transformer to operate in a non-linear way if it isn't saturating? Is it possible that magnetic hysteresis is can no longer be neglected? – intecher Jul 19 at 15:33
• doh! I missed the word max, even when looking for it. A bend in the BH characteristic can happen way short of saturation, but will cause a small change in gain, as will a rise in secondary temperature which increases the output voltage and hence core flux. – Neil_UK Jul 19 at 15:35