The AM Forum
March 28, 2024, 04:24:11 AM *
Welcome, Guest. Please login or register.

Login with username, password and session length
 
   Home   Help Calendar Links Staff List Gallery Login Register  
Pages: 1 [2]   Go Down
  Print  
Author Topic: Plate Cooler/Caps for big glass tubes? (re:crapped out 3-500Z)  (Read 12945 times)
0 Members and 1 Guest are viewing this topic.
AB2EZ
Member

Offline Offline

Posts: 1722


"Season's Greetings" looks okay to me...


« Reply #25 on: September 26, 2008, 08:40:15 AM »

Bear

The heat flow schematic was helpful to me in thinking through the question of whether a bigger/more effective plate cap heat sink would cause more or less strain around the wire-to-glass seal during turn-on and turn-off.

Looking at the simplified heat flow schematic, and thinking about its implications I now believe that the following is provably true, regardless of the details of the "component" values in the schematic:

1. The sizes of the temperature differentials that form between and across the components during turn-on and turn-off transients (i.e. until steady state is reached) are the same... so the strains that occur on turn-on and the strains that occur on turn-off (whether or not they cause any damage) are the same.

2. If the heat sink is making a good thermal contact to the plate wire (as the simplified schematic implies) then a larger heat sink and/or more air always help to protect the area around the seal by reducing the maximum temperature differentials (and strains) that occur during the turn-on and turn-off transients, and by reducing the maximum temperature rise that occurs in the steady-state when the transmitter is switched from standby to on.

3. If the thermal contact between the plate wire and the plate terminal (or between the plate wire and the heat sink) is not good enough... then one would have to modify the heat flow schematic... as attached below.

4. In the "modified" schematic, even though the larger plate heat sink might reach its steady state temperature more rapidly ... depending upon the "RC" time constant" associated with the larger plate cap's heat capacity: C1 (Joules of energy per degree of temperature rise) and the thermal resistance from the larger plate cap to the air: R2 (degrees of temperature differential per watt of heat flow between the plate cap to the air)... I believe that the schematic implies that a larger plate cap would always improve the situation with respect to:

a. The maximum temperature differentials (strains) that occur around the seal during the turn-on and turn-off transients

b. The steady state temperature rise of each of the components (e.g. glass envelope in the vicinity of the seal, the plate inside the tube, etc.) that occurs then the transmitter is switched from standby to "on".

Best regards (I've learned a lot by reading the posts in this thread and thinking about this problem)

Stu


* Heat flows 2.jpg (93.86 KB, 960x720 - viewed 328 times.)
Logged

Stewart ("Stu") Personick. Pictured: (from The New Yorker) "Season's Greetings" looks OK to me. Let's run it by the legal department
WBear2GCR
Member

Offline Offline

Posts: 4135


Brrrr- it's cold in the shack! Fire up the BIG RIG


WWW
« Reply #26 on: September 27, 2008, 10:19:37 AM »

Stu,

WRT item 4:

It seems to me that a larger heat cap/cooler (ignoring the transfer to air) will have a higher thermal mass, and as you noted it will take more Joules in order to bring it to a given temperature. Similarly, due to the higher thermal mass it will hold its temperature longer (unless it also has an increased ability to transfer heat energy to the air).

Merely having enough thermal mass seems sufficient to sink extra temperature rise from the plate rod/wire, up until the point where that mass reaches a temperature that we could deem "unsafe" (too high). But lacking a means to transfer that added heat energy, it would then try to sink it back down the plate rod/wire once the tube starts to cool...

So, it seems like the optimal solution would take into account both the thermal transfer to the air WRT time and the rate that the thermal mass of the heatsink can change temperature.

How to do that is a good question...

             _-_-bear

Logged

_-_- bear WB2GCR                   http://www.bearlabs.com
Pages: 1 [2]   Go Up
  Print  
 
Jump to:  

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
 AMfone © 2001-2015
Powered by SMF 1.1.21 | SMF © 2015, Simple Machines
Page created in 0.05 seconds with 18 queries.