I wonder if someone who knows a bit about common mode chokes and their effects on DC circuits could please help me out. I know nothing about them myself, being just a basic hobbyist.

Consider the following schematic which I've drafted up in Multisim. I have used a transformer to denote a common mode choke. The source voltage I am told is from a switching power supply so should be quite stable. The device attached is a 2.5" hard disk drive, typical power draw 2-3W, which like any hard drive will be switching itself between power states frequently.

I am seeing a breakdown in the insulation of the SATA connector. On one example of the SATA connector I measured a short of 100 Ohm, despite there being no physical metallic short, so this was across the plastic. The other unused circuits measure 2-3 Megohms, which is much lower than you'd expect anyway on these connectors.

So to the question..

When the device is being switched on and off could a high voltage spike be produced by the choke as the magnetic field decays that could be tracking away at the insulation in the connector leading to a failure?

Many thanks for your help.

 

A common-mode choke, as the name implies, only has inductance for common-mode signals. It has only leakage inductance, which is normally very small, to normal-mode signals as would be the current going to the SATA drive. And with all those capacitors, I don't think an inductive spike from the leakage inductance is the problem.

 

Your schematic has plenty of capacitance to quench the inductive reaction. A common mode choke is intentionally designed to avoid having much effect on DC current and low frequency AC current. This choke did not eat the insulation.

 

Ok thanks chaps that's very helpful and at least gives me one less thing to concern myself about.