[Amps] Switching Supplies for High Current Filaments

[TexasRF at aol.com]

The problem with switchers or using any regulated supply is the cold  
resistance of the heater/filament is typically 10% of the hot resistance. That  
issue is automatically handled by the current limitations of a typical 
filament  transformer. The transformer becomes pretty much a constant current 
source until  the heater temperature rises enough to get the transformer out of  
saturation.
 
On big tubes requiring many amps of filament current this can be a real  
problem. If the transformer has to supply say 30 amps, then it will supply 
many  times more current than that for short periods of time. A cold tube can 
be  subjected to very high stress under these circumstances and can even have 
the  filament burned open, just like a 100w light bulb blows out. No 
problem with the  $1 light bulb but loosing a big tube that way is heart breaking. 
(Been there,  done that, have multiple tee shirts).
 
The solution to the over current problem is reducing the startup voltage  
enough that the 10% filament resistance still draws rated current. That takes 
 10% voltage according to ohm's law.
 
So, for any regulated and current limited power supply, there needs to be a 
 method of starting at low voltage and ramp it up as the tube warms up.
 
Being the concept guy, I will bow out at this point and leave the answer to 
 others. I have the answer for transformer filament power sources but no 
clue how  to do it with a switcher or any other regulated/current limited 
supply. If it  was actually a current limited supply, not a current shutdown 
design, all would  be well and the tube could warm up with a constant rated 
current source. A  regulated current design instead of voltage would be 
wonderful.
 
By the way, that 10% resistance figure applies to light bulbs as well. The  
hot resistance can be estimated by r  equals voltage squared divided by  
wattage rating. That is the well known power = e squared divided by  
resistance formula turned around backwards. The cold resistance can be measured  with 
a VOM or DVM.
 
73,
Gerald K5GW
 
 
 
In a message dated 10/5/2010 11:01:14 P.M. Central Daylight Time,  
jim.thom at telus.net writes:

Date:  Tue, 5 Oct 2010 07:34:41 -0700
From: Patrick Barthelow  <apolloeme at live.com>
Subject: [Amps] Switching Supplies for High  Current Filaments



Are there any sensibly priced switching  supplies (even surplus)  that are 
small and light, that could power some  of the QRO tubes
with high current filaments?   Would, say,  a  5v, 30A switcher which could 
power a pair of 3-500Zs, be economically  
competitive with a 5V 30A transformer, and even if it is not directly, it  
might still be worthy of consideration 
if it was small enough, and light  enough to significantly lower size and 
weight in an RF deck? 
Would cold  start surge current be a problem for switcher implementation 
for  filaments?
Best Regards,   
73, de Pat Barthelow AA6EG   apolloeme at live.com 

## This was 1st tried on a  GS35B  6m  amp.  The inrush current was so  
great that the
switcher thought  it was being cro-barred... and went into shut down mode ! 
  A  bigger
switcher was used..and that worked,  but the 2nd one was almost  double the 
capacity
of the 1st one.  Also, I believe my buddy had to  use a Dc-dc  isolation  
converter in their some where, to
solve  yet another problem. 

##  In the end, with numerous other  problems, like heating of the air 
variable load cap, the RF deck was  scrapped,
and  a new one designed from scratch.  2nd version used  2 x GS35B's... and 
this time, 2 x separate conventional
fil xfmr's used,  one per tube.  The problem with  a pair of 3-500Z's 
is.... the B-  return is via the CT  of the conventional
fil xfmr.   We  gave up on the  notion of using switcher's  for fil  
supplies  on oxide tubes.  Some have had better success,
so it can be done   with oxide tubes.  Then you have an adjustable, 
regulated  supply.   They will not work on a directly
heated thoriated  tungsten tube.   On some of the switcher's... hash and 
noise   was across  some of the HF bands...and
a bitch to filter out  completely. 

##  hammond makes a 5V @ 30A  [with CT]   conventional fil xfmr...for 2 x 
3-500Z's.  Runs barely luke warm.   It's   priced 
way below what a dahl is.... and readily available  in the usa, through the 
usual distributor's.   Comes in 2 x  version's ,
with or without bell end housings, IE: bell housings or open  frame.   They 
measure 5.5 V  no load.. and 5.0 V    with a 30A load. 
Last I checked  a few years ago, it was aprx $45.00  .  Their is only one 
primary, 115 vac.   You can also get it in  5v  @  15A . 
They also make a 7.5 vac  @ 21 A.    A friend  in W6 land got his  YC-156 
fil xfmr  from  hammond.    The YC-156 fil xfmr was a 
torroidal type.  

later... Jim    VE7RF.   
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