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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