Soft start query

[iansr]

From what I’ve seen most soft start modules comprise an NTC and a relay that bypasses the NTC once the trafo is charged up as it were. My query is why is it preferable to bypass the NTC? I’m sure it will still offer some resistance once up to temperature, but enough to mean the PS caps are not kept topped up? Or is there some other reason for taking the NTC out of the circuit?

[Nick]

Assuming the power supply is doing more than keeping the caps charged up, i.e. something is drawing power and discharging the caps, the power supply will have to supply current, that current will pass trough the NTC and generate heat and unwanted losses in the process. The goal is to keep the power supply impedance as low as possible.

Though if its bipased with a relay then not sure what advantage a NTC will offer. I just use some power resistors and bipass them with a relay after a few tenths of a second.

[Morgan Jones]

NTC stands for Negative Temperature Coefficient. It's a resistor that when it gets hot has much lower resistance than cold. And the key word is hot. NTCs run hot. Which is not good for surrounding components. That's why they're used to take something up to almost working voltage, then are often bypassed by a relay. The relay avoids having that very hot component.

[Paul Barker]

Is another way MOV’s?

[Morgan Jones]

No, MOVs are designed for spike suppression so they're designed to run cool whilst absorbing/clipping the energy of occasional spikes. Warming an MOV irreversibly reduces its clipping voltage - it's why EMC tests allow a one minute cooling time between applying each spike and why MOVs eventually fail when their clipping voltage reaches mains peaks. By contrast, an NTC is designed to operate too hot to touch (where it has low resistance).

[Paul Barker]

I see