[Amps] Switching Supplies for High Current Filaments

Wed Oct 6 21:26:00 PDT 2010

##  The typ cold resistance of a light bulb is 10% of the hot resistance. 
On tubes, it's
even worse, typ 12%.   On the few switcher's we experimented with.. all had 
a built in ramp up
feature on start up.   IE:  with no load on the switcher's output,  the 
output Vdc  would ramp up
anyway.  With a load  ALREADY connected [oxide tube only]... the switcher is 
then turned on... and it goes
through it's ramp up feature.   Between that and the current limiting 
feature... the switcher will
not cro-bar itself.    But this only worked if the switcher's output current 
rating was aprx double the
tubes rating.   IE:  tube fil was 3.6A.... a 8A switcher will just work...a 
5 A switcher would not work.
--------------------------------------------------

> Agreed! Ever try to hook a headlamp across an Astron power supply?  Pretty 
> tough to do..

##  was the astron turned on 1st..... then the headlamp connected ?    Or 
did you
hook the headlamp to the astron 1st.... then  turn on the astron ?  Is the 
astron a
switcher type ?

> 73
> Jim W7RY
>
> > --------------------------------------------------
> From: <TexasRF at aol.com>
>>
>>>> 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.
>>
 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.

##  On my little  lab type switcher supply, the current limit can be 
adjusted to any value.
If set  for normal rated tube current [ + maybe 1%]  it will perform just 
like you wanted.
You can cro-bar the output all you want, and all it does it put out the max 
current you dialed
in.   So this type of switcher will work as well.

## None of these switcher's  will work on a thoriated tungsten  tube like a 
3-500.  The B-
return for any thoriated tungsten tube is via the CT  of the conventionl fil 
xfmr sec.

Jim  VE7RF

>>
>> 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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