Current Sense Transformer guidance

[danners430]

Hey folks,

I was wondering if anyone knew where to find some guidance regarding current sense transformers?

I am considering them as one method of detecting model trains in a block section, since they'd be using DCC (AC square wave). My only issue is that in a short circuit condition, there is a potential for a short pulse of 5A or more, where a normal current might be in the order of mA magnitude...

Cheers guys

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[Zero999]

Presumably the normal current is hundreds of mA? That's smaller than most current transformers are designed for. You'll need to wrap several turns through the middle of a higher current rated unit.

The surge shouldn't be a problem, so long as it isn't prolonged.

How about a Hall effect current sensor? Again the nominal current is on the low side, but they're not damaged by overloads.

[David Hess]

High current which saturates the current transformer is not a problem especially if the output is clamped like with diodes unless you manage to drive so much power into the secondary resistance that it overheats.  Usually saturation is avoided to preserve linear measurement but that does not matter in your application.

Current transformers are sometimes used to drive a bridge rectifier producing a DC output for a sensed AC input.  The forward voltage drop of the diodes increases the required volt-second product for linear operation but could make your detection circuit easier.  Do *not* half wave rectify the output of a current transformer; it requires a low output impedance under all conditions.  If the secondary is ever open, then the current transformer becomes a voltage step up transformer and may produce damaging voltages across its secondary.