[Milsurplus] Cold Filament Inrush Current
Brooke Clarke
brooke at pacific.net
Thu Mar 15 13:02:53 EDT 2018
Hi:
When working on the battery adapter for the PRC-25 I discovered that the BA-4386 battery has a length of 30 AWG wire in
the filament circuit that acts as a resistance to limit the cold start current,
http://www.prc68.com/I/2577Battery.shtml#BA-4386
I use an actual 1 Ohm resistor on my 257477BA battery adapter in the filament circuit to do that since modern batteries
have extremely low internal resistance. There's some math at the above link about it's effect.
When the half size BA-5598 battery came out (2 batteries will fit into the battery box on a PRCV-25 or PRC-77 it turned
out that the BA-5598 did NOT include the short 30 AWG wire and so did not limit the inrush current on the tube in the
PRC-25 and burned out some tubes and was banned for use with the PRC-25.
In the days of the No. 6 dry cell the common battery test was to place an ammeter across the terminals and measure the
current (not the voltage). Today that test is called Flash Amps.
http://www.prc68.com/I/No6.shtml#FA
and here are some measurements on modern batteries:
http://www.prc68.com/I/No6.shtml#MFA
I tried to measure the Flash Amps on a modern No. 6 battery and blew the expensive fuse in my Fluke DMM:
http://www.prc68.com/I/MTE.shtml#F87
Note that old wet batteries are capable of supplying some very high currents.
http://www.prc68.com/I/Leclanche.shtml
All of the above is to say that in any tube filament supply I would recommend some way to limit the cold inrush current
to a reasonable value. When I worked in Silicon Valley some of the semiconductor companies left their florescent lights
on all moment. I expect tubes also burn out at cold start but they are typically hidden from view so you don't notice it.
--
Have Fun,
Brooke Clarke, N6GCE
http://www.PRC68.com
http://www.end2partygovernment.com/2012Issues.html
-------- Original Message --------
> Sounds useful. Question: how much more current does a tube filament take when cold?
>
>
> Peter
>
>> On Mar 15, 2018, at 8:37 AM, David Stinson <arc5 at ix.netcom.com> wrote:
>>
>> This DC-DC converter looks really interesting. It
>> inputs 12V and can source a regulated from 1.25V
>> to 12V at 3A for filaments and regulated 150V to
>> 420V at up to 60mA for B+. Gonna get me one of
>> these.
>> https://www.ebay.com/itm/173034019336
>>
>> 73 Dave AB5S
>>
>>
>>
>> ______________________________________________________________
>> ARC5 mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/arc5
>> Help: http://mailman.qth.net/mmfaq.htm
>> Post: mailto:ARC5 at mailman.qth.net
>>
>> This list hosted by: http://www.qsl.net
>> Please help support this email list: http://www.qsl.net/donate.html
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>
More information about the Milsurplus
mailing list