[Amps] NTC thermistors as soft start

[Conrad PA5Y]

I have changed them out for a traditional soft start, in fact the one 
shown on the GM3SEK triode board. I have used this circuit many times 
and trust it totally.

Regards

Conrad PA5Y

On 02/02/2016 17:02, Saandy wrote:
> 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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