Agreed! Ever try to hook a headlamp across an Astron power supply? Pretty
tough to do..
73
Jim W7RY
--------------------------------------------------
From: <TexasRF@aol.com>
Sent: Wednesday, October 06, 2010 7:40 PM
To: <jim.thom@telus.net>; <amps@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@telus.net writes:
>
> Date: Tue, 5 Oct 2010 07:34:41 -0700
> From: Patrick Barthelow <apolloeme@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@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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