[Amps] GRADUAL FILAMENT warm up . . . .

[Gene May]

At the risk of mentioning something that is already familiar to the readers of this thread, but since it has been discussed for a while now (and because making an idiot of myself is not an unfamiliar action or experience . . . ), I point out devices known as "negative temperature coefficient thermistors".  If you "Google" this topic, you will have available more than you need to know from WikiPedia and commercial sources as well.  These inexpensive (less than a few dollars each) devices look like a ceramic capacitor, are about the same size, and behave in a manner opposite from filaments -- they have resistances about twenty to over a hundred times higher when cold than they have when they are hot.  I have put them in the primaries of filament supplies, and they work like a charm to slowly ramp up the filament current.  I suggest putting a bypass switch in parallel with it that can form a direct connection to the filament circuit after a minute or so after the thermistor warms up and filament voltage is about normal.  This will save having extra heat in your chassis from the thermistor and a slight extra voltage drop in the primary circuit of the filament supply transformer.
 
I have never tried them in a plate supply circuit (to slow the current surge that charges the filter caps upon turn-on), and I don't think they would work in that role because they would never get hot enough to have low resistance.  I have put simple resistors (say, five watts, 20 - 100 ohms) with such bypass switches in the primaries of transformers of plate circuits.  Throw the bypass switch after a few seconds, shorting out this resistor, after the filter caps are charged (plate voltage reads normal).
 
Gene May
WB8WKU 
 

> From: amps-request at contesting.com
> Subject: Amps Digest, Vol 108, Issue 32
> To: amps at contesting.com
> Date: Fri, 23 Dec 2011 11:21:22 -0800
> 
Warm up and on time question... (Radio WC6W)

Date: Wednesday, December 14, 2011, 7:38 PM
Speaking of warm up times and filament life got me to thinking about a ramping switch system for the whole power supply.
As I understand it there are devices which will chop up the AC sine wave into pieces like a PWM and slowly add more and more pieces until the full sine wave is present on the load.  I also understand they can be used in reverse. Sort of like having an electronic Variac on the input side of the supply.  Do any of you have a circuit diagram for such a system or could point me in the right direction?? I am not even sure what to call what I am looking for.  It sounds like a ramping soft start system and that would solve a lot of problems with initial turn on of the filaments and the Plate supply.
> > 
> > Thanks in advance.
> > 
> > Mike Baker? K7DD
> > K7dd at cox.net

> End of Amps Digest, Vol 108, Issue 32
> *************************************