[Amps] Switching Supplies for High Current Filaments

Wed Oct 6 20:01:36 PDT 2010

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

73
Jim W7RY

--------------------------------------------------
From: <TexasRF at aol.com>
Sent: Wednesday, October 06, 2010 7:40 PM
To: <jim.thom at telus.net>; <amps at contesting.com>
Subject: Re: [Amps] Switching Supplies for High Current Filaments

> 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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