Manfred, are there computer repair shops there? Many of the thermal grease
products are targeted for CPU cooling applications.
73,
Gerald K5GW
In a message dated 8/24/2011 2:19:19 P.M. Central Daylight Time,
manfred@ludens.cl writes:
Roger,
> Agreed. I use Artic Silver when I can not solder.
That's good. I would like to buy some Arctic Silver, but need to find a
store that will sell me a small amount, and export it! Here in Chile, of
course, that stuff is unobtainable from the local distributors. I can
easily get the usual white ceramic thermal compound, packaged in little
jars with no label, so I don't know what's really inside! And with
ceramic thermal compound ranging from 0.7 to 3.8 W/mK in thermal
conductivity, I would like to know what I have... So I guess that I will
include a small jar of labeled, specified thermal compound in my next
order from Digikey, but that's still only 3.something thermal
conductivity, while Arctic Silver is rated at 8.something. Digikey has a
silver loaded thermal compound too, but with no specs. Any advice, anyone?
> The problem with solder is the temperature compared to the critical
> temp for the device
I used to believe the same, but since starting to build SMD equipment by
reflow soldering in the kitchen oven, and never killing a component, my
beliefs in this regard have changed! Nowadays I believe that it's pretty
hard to fry a component by soldering.
Normal 60/40 solder melts at 189 degrees Celsius. Silicon devices can
operate continuously at 150 degrees junction temperature, and some
manufacturers claim 175, 200 or even 225 degrees for some devices. And
what they can survive for a few minutes is even higher. I have
oven-soldered many devices so far, both to circuit boards and to heat
sinks. I heat the oven to 200 degrees, wait for the solder to melt, wait
a little more to make sure it really has molten well at every place, and
then switch off the oven. That has worked great so far.
> and the need to make sure the entire area between
> the device and heat sink is wetted.
As long as everything is clean, that's no problem. And even if you get
some air bubbles trapped in, it's still better than 100% filling with
thermal grease!
> OTOH there
> are "solder pastes" available that can be applied to the mating
> surfaces
That's what I use. I bought a 10 gram syringe of 63/37 no-clean solder
paste, and it still lasts. A little of that stuff goes a long way,
because the layer formed between the part and the PCB or heat sink is
very thin. When you apply the proper amount, almost none of the solder
flows out from under the part.
> and then melted by a brief induction pulse
I don't know about that technology. I would be scared to fry sensitive
parts by inducing such a high current in teh metal as to melt the
solder! I prefer to use the kitchen oven. That takes about 5 minutes for
a PCB, and half an hour to solder a large transistor to a thick copper
plate, but is electrically safe!
> A bucket under the desk would be a disaster for me the fist time I
> put my feet under the desk. <:-))
Either bolt down the bucket and put a lid on it, or tie down your feet! ;-)
> What you need are two 1CM square plates, separated by 1 CM which
> should give a close approximation of the resistivity in ohms per
> CM^3.
I just did that test, and got 280 kiloohms. But it seems that the
resistance from the electrodes into the water is more important than the
bulk resistivity of the water, because changing the distance between the
electrodes only produced a small difference in the resistance, while
inmersing more or less area of them changed the resistance in inverse
proportion.
In any case it looks like the resistance from side to side of my
amplifier will be on the order of tens of kiloohms, at least, and that
should be fine. So I can use spring water!
> I think the limit we set was on the order of 3000 ohms/CM^3.
Well, what I got is nearly 100 times better!
> Of
> course that was with 6 to 8KV, but we used about 12 to 15 feet of
> hose for isolation.
I will only have 140V. In principle I would have only about 20cm of hose
between the sides, but if necessary I can use a longer hose and coil it
up, to increase the resistance.
> IF you only have water running through, or in the device when it is
> in operation (IOW it drains back into the bucket when not running)
> eating metal ions will take a very long time to become a problem as
> in many years..
That's a good info!
> Here's a thought...put the coolant exhaust 6" to a foot above the
> device or higher (what ever is handy) with a quart container (or what
> ever is handy) in the line. Take the return from the top of the
> container. When power fails to the pump, you will have a quart (or
> what ever size container you use) for a safety reserve that will
> drain back through the device, giving plenty of time to shut down
> before lack of cooling becomes a problem.
I have a much better plan: I intend to build a flow sensor into the
system. No flow, no power to the amp. Digikey has a flow sensor for 18
dollars that seems to fill the bill for the flow level I expect to use.
The output is simply a reed switch. I don't like the very wide
hysteresis that sensor has, and maybe I can find (or build) something
better. But even this simple flow sensor should provide pretty good
protection. Digikey also has better flow sensors, but at several hundred
dollars apiece, they are out of all consideration.
I plan to switch off the pump during prolonged RX, but keep the circuit
primed. That doesn't play well with the safety reservoir system.
> I'd really like to build a SS 160 through 6 meter amp capable of
> 1.5KW CCS with enough overhead to comfortably run that legal limit.
What for? I can see the need to run 1.5kW key down for a few minutes,
such as for RTTY and SSTV. But CCS? Hardly. At least I don't see why a
ham could ever need 1.5kW CCS! Maybe you can give me some example!
> Many commercial tube amps advertised as a legal limit amp just don't
> have the power supply or components for sustained operation at the
> legal limit in SSB or CW let alone digital.
Yes. They are aimed at hams who WANT CCS specs, but don't NEED them, and
will never actually use the amp that way! ;-)
> Some of them even push
> the tubes way beyond their ratings to get the power they do and
> that's at a greatly reduced tube life.
That's usual practice in ICAS, and has a long tradition.
> IOW I'd want an amp capable
> of at least 2KW continuous and preferably 2.5KW that should loaf
> along at 1500 watts without breaking into a sweat.
Not a good idea! A 2.5kW amp running at 1.5kW will necessarily be less
efficient, as it will be operating at part load. If you really want to
broadcast in RTTY around the clock at 1.5kW, then you should build a
1.5kW amp with generous thermal capability, and not a 2.5kW amp! IOW,
the electrical design should be optimized for 1.5kW, but using
components, cooling, etc, designed to run significantly below the
temperature limit of each part, at that power.
> The current project is a 4CX3000A7 that I want to run class A.
Don't allow promoters of energy efficiency to hear that. In this world
where energy is getting ever more expensive, and energy wasting devices
draw the scorn of many people, we should be looking at class D, class E,
and class F, with envelope elimination and restoration. Not really at
class C, much less class AB, let alone class A!
> The
> problem with the higher power tubes is their size. Plus they use more
> filament power than the average ham station uses in total power.
That filament power, even for a class AB amp, is one of the reasons why
I want to replace my tube amp. I run my house on my own microhydro
turbine, and in winter I have all the power I want, but in summer it can
get really scarce. I use battery backup in summer, and drawing 2kW
pulsed for high power SSB transmission is OK, for short transmissions,
but running filaments and bleeder resistors and a blower even while in
RX is unacceptable.
> (Bout ~375 watts) One amp on HF and one for 6-meters. It' d be nice
> to have one for 2-mters as well. I have the tubes and sockets but
> not enough components to build complete amps.
I got two 4CX1500B tubes some time ago, for free. Broadcast pulls. The
guy said they were still in perfect shape, but I have no means to test
their emission. Maybe some day I make a cavity amp for two meters
moonbounce with one of them. But more likely not...
I have no sockets for them, and would probably have to make them. But
really a MOSFET amp for HF is a more attractive project to me!
Manfred.
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