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@contesting.com] On Behalf Of Conrad PA5Y
Sent: Friday, January 29, 2016 12:21 PM
To: amps@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@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@contesting.com] On Behalf Of Conrad
PA5Y
Sent: Thursday, January 28, 2016 2:22 PM
To: amps@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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