Soft start query

We all start somewhere
Post Reply
iansr
Old Hand
Posts: 401
Joined: Thu Nov 08, 2007 10:44 pm
Location: West Riding

#1 Soft start query

Post by 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?
"Its good enough for Government work."
User avatar
Nick
Site Admin
Posts: 15707
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

#2 Re: Soft start query

Post by 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.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
Morgan Jones
User
Posts: 56
Joined: Fri Mar 25, 2016 10:58 am

#3 Re: Soft start query

Post by 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.
User avatar
Paul Barker
Social Sevices have been notified
Posts: 8870
Joined: Mon May 21, 2007 9:42 pm

#4 Re: Soft start query

Post by Paul Barker »

Is another way MOV’s?
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
Morgan Jones
User
Posts: 56
Joined: Fri Mar 25, 2016 10:58 am

#5 Re: Soft start query

Post by 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).
User avatar
Paul Barker
Social Sevices have been notified
Posts: 8870
Joined: Mon May 21, 2007 9:42 pm

#6 Re: Soft start query

Post by Paul Barker »

I see
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
Post Reply