# Why would a current transformer act like a high pass filter, how can I estimate the low frequency cutoff?

I am struggling to wrap my head around this. I understand that a transformer is not going to pass DC, and that every current transformer (CT) has some pass band. But what properties drive the high pass behavior, and how can I estimate what this corner frequency would be? I understand that it has something to do with the inductance of the primary, core material, etc. But if I know my turns ratio and core material is there some rule of thumb to estimate where the low frequency cutoff is? I read this article on CT design but I am still trying to grasp this. Perhaps if someone could draw an equivalent circuit for me showing whatever parasitic RLC components are in play.

• This doesn't answer the question well either but it appears to be core saturation that causes the minimum core frequency: electronics.stackexchange.com/questions/100968/… Commented Aug 6, 2014 at 19:24
• Hmm, I did a quick search before posting this but didn't see that post. Thanks for linking it. I will review.
– Nick
Commented Aug 6, 2014 at 19:26
• Another couple links that're related but not answering your specific question: electronics.stackexchange.com/questions/117281/… electronics.stackexchange.com/questions/83072/… It sounds like it's an experimentally found thing if it's not on a manufacturer's datasheet. Otherwise you have to work through the equations for core saturation and then determine at what frequency you'll saturate. Commented Aug 6, 2014 at 19:27
• My issue is I have no data sheet. I've been told that thing was basically wound by some guy in his garage. I know the turns ratio and the materials but don't have any good experimental data. I was trying to see if I could ballpark this to try explain some other stuff going on in my system w/o having to actually go and measure it just yet! But if I am reading your links right it sounds like my burden resistance might be a factor too. That makes things interesting because that's not fixed.
– Nick
Commented Aug 6, 2014 at 19:37
• Even if you found the correct equations for your calculations, I wouldn't be surprised if they're an order of magnitude off. That's just the way the real world works. Why is your burden resistor changing? Commented Aug 6, 2014 at 19:42