[Amps] NTC thermistors as soft start

[Saandy]

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

-----Original Message-----
From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Conrad PA5Y
Sent: Friday, January 29, 2016 12:21 PM
To: amps at contesting.com
Subject: Re: [Amps] NTC thermistors as soft start

Yes for the heater transformer I will leave them in but for the HV supply I
am uncomfortable with them. The resistance varies with load so how can this
be good for anode supply regulation? The parts for a GM3SEK style soft start
using Omron GL7 25 amp relays arrived from Farnell Electronics this morning.
I will shunt the thermistors after 1 second or so.

Regards Conrad PA5Y 

On 29 January 2016 10:35:58 CET, Branko <branko.cehner at guest.arnes.si>
wrote:
>Hi,
>
>I think NTC's are the simpliest solution for providing soft start. I 
>use them in the tube heating power supplay on the secondary of 
>transformer as the heating current is 3 to 4 A. For higher heating 
>current it is wise to put them on the primary side of heater 
>transformer
>
>Yes they getting hot, but they provide current limiting to the heater 
>filnament of the tube which is well advised for QRO tubes.
>
>Best regards, Branko, S52V
>
>-----Original Message-----
>From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Conrad 
>PA5Y
>Sent: Thursday, January 28, 2016 2:22 PM
>To: amps at contesting.com
>Subject: [Amps] NTC thermistors as soft start
>
>I have just acquired a GS35B PSU. It is 3 phase and the schematics show
>3 x
>NTC thermistors in series with each primary. I presume that these are 
>some kind of soft start. I am not familiar with these devices but they 
>make me uncomfortable, I have always used a series resistor that is 
>shunted by a relay after a few seconds in the past. I do not have the 
>model number of the devices used as I will need to open the PSU to find 
>out what they are but I assume that they are similar to this device.
>
>http://www.ametherm.com/datasheetspdf/MS325R020.pdf
>
>The resistance varies depending on the load current and they get hot in 
>order to operate, this really puts me off. They have quite a long time 
>constant so I am uneasy about them altogether.
>
>Should I be concerned and replacing them with a soft start for each 
>phase?
>
>Apart from that the PSU is very nicely made and will provide up to 
>1.6A@ 3600V.
>
>Regards
>
>Conrad PA5Y
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