[Amps] NTC thermistors as soft start

[Jim Thomson]

Date: Tue, 2 Feb 2016 18:02:47 +0200
From: "Saandy" <alexeban at gmail.com>
To: "'Conrad PA5Y'" <g0ruz at g0ruz.com>, <amps at contesting.com>
Subject: Re: [Amps] NTC thermistors as soft start

The problem with NTC resistors in a HV power supply is not regulation: when
they're hot their resistance is low and have no effect. The problem is
recovery time. I the power supply is working and you have a momentary
dropout, the NTC,s have no time to revert to their cold state but the filter
capacitors drain down. When power returns you have the bad state: low
resistance-i.e. no current limiting- and empty capacitors; precisely the
state you wish to avoid.....
The best protection would be a dropout detector and a step start relay. A
relay can revert to the start position in a fraction of a second, usually
before the filters dry out, so that it's ready to absorb the inrush
transient when power returns.
Alex 4Z5KS


##  what you are talking about is called..’fail safe’...when the commercial AC
power goes off for say just a few secs, then is back on.   In order for your 
fail safe scheme to work, the TD relay would have to be set for at least 30 secs, 
to allow the NTCs  to cool down  ( where their resistance is high)  and be ready to 
be used again for the inrush job.    Why bother complicating the mess ?  Since you
would requires td relays and all sorts of other parts to make the fails safe function
correctly,  you may as well just use a conventional, old style step start circuit.

##  for single phase, only ONE resistor is required..whether its 120 vac  or  240 vac .....
along with a SPST relay to shunt the resistor.   Ok, on a 3 phase supply, 3 x resistors are required,
along with a 3PST  relay, big deal, so what?   At least when the commercial AC power drops
off for  say  1-1000 secs, the various relays etc, all drop out.     When commercial ac power comes
back on, you are ready to go.  

##  I can see the issue though,   ac power goes off  for say 3-12 secs..... which is long enough to drain the 
B+  way down..... but not long enough for the  NTCs  to  cool down enough to be of any good. 

##  a good place for a NTC might be one leg of a fil xfmr  primary.   Then you would not have to worry
about a fail safe circuit. 

Jim   VE7RF