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 warm
s 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@contesting.com
> Subject: Amps Digest, Vol 108, Issue 32
> To: amps@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@cox.net
> End of Amps Digest, Vol 108, Issue 32
> *************************************
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