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thermostat

D

Dejan Durdenic

Jan 1, 1970
0
I need to keep a PCB on the fixed temperature level. I will do it
with Peltier element attached to the aluminum housing which will
hold the PCB. The problem is - what should I use to fill the box
to have optimum heat transfer? It would be nice if the stuff can be
(easily) removed later in the case some service is needed...First I
thought of filling the box with thermal grease used for attaching
heatsinks to transistors/IC's, but it would be impossible to remove
it later... Any suggestions?

Dejan
 
J

John Popelish

Jan 1, 1970
0
Dejan said:
I need to keep a PCB on the fixed temperature level. I will do it
with Peltier element attached to the aluminum housing which will
hold the PCB. The problem is - what should I use to fill the box
to have optimum heat transfer? It would be nice if the stuff can be
(easily) removed later in the case some service is needed...First I
thought of filling the box with thermal grease used for attaching
heatsinks to transistors/IC's, but it would be impossible to remove
it later... Any suggestions?

Dejan

The only material that moves heat efficiently in a large volume is
saturated vapor. (Google ["heat pipe"]. But depending on the volume
and amount of heat to be moved, you may do fine with just a fan to
stir the internal air around (forced convection). Thermal grease is a
lousy conductor compared to copper or aluminum or most metals, and is
only a good conductor compared to the air it replaces in the voids
between a hot object and a metal heat sink attached to it. Tell me
more about the physical details of your box and its contents (heat
load in watts, volume, shape, is it sealed, etc.)
 
D

Dejan Durdenic

Jan 1, 1970
0
John Popelish said:
The only material that moves heat efficiently in a large volume is
saturated vapor. (Google ["heat pipe"]. But depending on the volume
and amount of heat to be moved, you may do fine with just a fan to
stir the internal air around (forced convection). Thermal grease is a
lousy conductor compared to copper or aluminum or most metals, and is
only a good conductor compared to the air it replaces in the voids
between a hot object and a metal heat sink attached to it. Tell me
more about the physical details of your box and its contents (heat
load in watts, volume, shape, is it sealed, etc.)

The PCB is enclosed in the aluminum (milled block) housing. The volume
is approx. cigarette box size and dissipated power is up to 1W. What
I want is to keep the PCB on the constant temperature, due to some
stability issues. I planned to attach Peltier element to the aluminum case
to heat/cool the box, as the power dissipated inside is not constant, as
well as the ambient temperature. I have to fill the box with something, as
the air is not a good thermal conductor...

Dejan
 
J

John Popelish

Jan 1, 1970
0
Dejan said:
The PCB is enclosed in the aluminum (milled block) housing. The volume
is approx. cigarette box size and dissipated power is up to 1W. What
I want is to keep the PCB on the constant temperature, due to some
stability issues. I planned to attach Peltier element to the aluminum case
to heat/cool the box, as the power dissipated inside is not constant, as
well as the ambient temperature. I have to fill the box with something, as
the air is not a good thermal conductor...

Is the 1 watt mostly dumped into a single component that could be
mounted against the wall of the box? Or does it come from many
different components? Are you worried about condensation inside the
box, or will the controlled temperature always be above the local dew
point? Are you worried that all parts of the internals be at a fixed
temperature or only certain parts?
 
J

John Larkin

Jan 1, 1970
0
I need to keep a PCB on the fixed temperature level. I will do it
with Peltier element attached to the aluminum housing which will
hold the PCB. The problem is - what should I use to fill the box
to have optimum heat transfer? It would be nice if the stuff can be
(easily) removed later in the case some service is needed...First I
thought of filling the box with thermal grease used for attaching
heatsinks to transistors/IC's, but it would be impossible to remove
it later... Any suggestions?

Dejan

There's no obvious reason why you should fill it with anything but
air.

The real problem will be to keep the aluminum enclosure isothermal. To
do that, the walls should be as thick as possible, there should be no
gaps that would block heat transfer, and the *outside* of the
enclosure should be insulated with styrafoam of some such.

Why a peltier? It's a lot easier to just use a heater and close the
loop at 65C or somesuch. No condensation problems!

I like to bond a thin surface-mount PCB to a thick aluminum plate and
slap a Minco foil heater on the other side, with feedback from a
surface-mount thermistor close to the most critical parts. That tends
to be very isothermal. I make one crystal oscillator like that, but
with the crystal itself embedded in the aluminum block.

John
 
T

Tim Wescott

Jan 1, 1970
0
Dejan said:
I need to keep a PCB on the fixed temperature level. I will do it
with Peltier element attached to the aluminum housing which will
hold the PCB. The problem is - what should I use to fill the box
to have optimum heat transfer? It would be nice if the stuff can be
(easily) removed later in the case some service is needed...First I
thought of filling the box with thermal grease used for attaching
heatsinks to transistors/IC's, but it would be impossible to remove
it later... Any suggestions?

Dejan

If you fill the box you'll bring your thermally conductive material into
contact with the sides of the box that have been made hot by the other
side of the cooler. If you want this to work you need to insulate the
box from it's contents so that the inside only sees the cold plate of
the Peltier.

Don't ignore John's advise -- for a cigar box sized container with 1W
inside stirring the air inside the box will have a profound effect on
the thermal conductivity. Putting a few itty bitty muffin fans in
there, particularly if the blow onto the hot spots on the board, will do
you a world of good.
 
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