## 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@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@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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