[Amps] NTC thermistors as soft start

[Conrad PA5Y]

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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