Need Help with Chamber Temperature Control

Discussion in 'The Projects Forum' started by SoulBlade, Sep 20, 2012.

Sep 17, 2012
37
0
Hi
I'm working a project to detect some gases which enter a small gas chamber, LxWxH= 75x66x25mm, i'm using is made out of stainless steel ( thermal conductivity of about 0.17 ). I'm using 4 gas sensors which operate at 340 oC but dissipate power of 96mW each (i.e. 384mW of heat . Worst case scenario i assumed is 400mW )

So the overall power dissipation within the enclosure is about 0.4W. Since there will be a certain temperature rise, in the chamber's ambient temperature (25oC), due to this heat, i need to remove the heat in order to maintain the temperature of the enclosure constant. I am uncertain of the exact rate of rise, are there equations to calculate this ?

I intend to make use of a peltier (coupled with a small flat heat sink;since a peltier isn't recommended to function properly with at least some kind of heatsink !!) to do this removal.
The peltier module has the following specifications:

Tellurex C2-55-2708R

LxWxH = 55x50x3.5mm

(at T(hot) = 25 oC ; which is the temperature area I'm working around.)
delta T = 68 oC
Vmax = 26.4v
Imax = 8.5A
R = 2.70ohms

I am uncertain of the rating of the heatsink to use. Might you highlight what equations i can use to calculate this. I'm aware this is an issue of thermal design, but I'm more of a software guy, thermal stuff isn't really my thing, and my electronics knowledge is mostly theory.

I intend to use the peltier to not just remove this 0.4W amount of heat, but to maintain/control the temperature of the enclosure at a set point/steady-state. I'm aware this can be done by implementing a P (or PI) controller via the microcontroller I'll be using (a TI MSP430F6638 ) in conjunction with an n-channel MOSFET H-bridge driver to control the proportional direction of current flow to the module (for heating/cooling to do the temperature controlling).

PWM of a high frequency (>1kHZ) is outputed from the MCU and used to drive the Peltier through the H-bridge. The PWM is smoothed out with an RC low-pass filter to reduce the voltage ripple below 10%.

I intend to power the peltier from a regulated power supply, but since i don't know the exact voltage the peltier would run at; i can't specify what regulated voltage to use. Most likely this would be a switch-mode regulator.

Already the chamber has a Sensirion SHT20P temperature sensor inside to measure the ambient temperature of the enclosure and give feedback to the MCU to complete the closed-loop control.

The gases enter the chamber through an inlet air micro-pump and outlet one. It's a 3Vdc diaphragm pump (http://ph.parker.com/webapp/wcs/sto..._-1_14099_14099_14097_ProductDisplayErrorView )
The pump is operated with a logic-gate MOSFET irlml6246Pbf ( www.irf.com/product-info/datasheets/data/irlml6246pbf.pdf )

What i'm stuck on is:
- how to calculate the temperature rise, in the ambient temp within this chamber enclosure, due to this 0.4W heat ?
- how to calculate the required heatsink given a peltier module of the above specifications ?
- the schematic for interfacing the H-bridge driver to the MCU and also to the Peltier with its power source.
- the value of series resistor to use between the MCU and the MOSFET, as well as the gate-to-source resistor value ?

Thank you so much for all replies

2. Wendy Moderator

Mar 24, 2008
20,772
2,540
I found this thread automoderated by the forum software. I have released it.

3. THE_RB AAC Fanatic!

Feb 11, 2008
5,435
1,305
Don't worry about the calcs. You said "the overall dissipation into the chamber is 0.4W" so just make sure the peltier can remove that amount of heat plus a decent safety margin, then regulate the temperature using a closed loop system that adjusts the power supplied to the peltier device.

You may not need a h-bridge unless you want to reverse the peltier voltage to make it heat the chamber as well? I would just use a resistor for that, either heating all the time (probably better for precision temp control) or switching between heating and cooling depending on the ambient temp (better for reducing energy usage).

Re the resistor values etc maybe you should post a schematic so people can help you better regarding options there.

4. Dodgydave AAC Fanatic!

Jun 22, 2012
5,143
768
Heating up an object uses Specific Heat formula, ie how much Watts for how long Weight of material,etc

So is the thermal conductivity of Stainless steel same as specific heat?

Last edited: Sep 20, 2012
5. wayneh Expert

Sep 9, 2010
12,377
3,231
+1
It's certainly possible to make some reasonable estimates of heat loss to ambient by the various mechanisms of conduction, convection and radiation. But that really won't help you control the temperature. For that you need to be able to remove heat at a rate higher than it is being generated.

You might enjoy my calorimeter project. Perhaps a few ideas in there, for instance the heat sinks.

Sep 17, 2012
37
0
Thanks alot for all the replies. I decided to stick to cooling instead. I determine an estimate for the temperature rise using this guide http://www.summit.com/products/Hoffman/catalogs/Thermal-Management/Design-Guide.pdf

Since i'm sticking to cooling-only, i won't be in need of an H-bridge, and would want to preferably do something similar to what was done here with the peltier: http://electro-geek.blogspot.com/2009/05/thermoelectric-temperature-controller.html

^ would that circuit be applicable to what i want to do ??

I figure I'd only need to take gas readings while the sensors are working, (during which time they are dissipating); the peltier would hence be used the counter the temperature rise (i'd implement a proportional controller in the MCU) to create a psuedo-isothermal state for the chamber; where i can then take my readings as the test gas is pumped through. Although at the same time i will still make use of the temperature readings not just for P-controller feedback, from the two temp-sensors present in the chamber, back to also compensate, in software, for any variations.

With regards the pumps, the schematic is closely similar to the one i attached below. The MCU there is replace with a MSP430F6638, the Diode's are peak 1A rated rectifiers (with 100-200mA thereabout continuous current). The mosfets are the one i mentioned earlier. The 5ohms series resistor would be to limit the current from the MCU, while the 100kohms would be to protect against gate-floating resulting from the mcu during startup. The power supplies to pumps, i intend to use a ( http://www.linear.com/product/LTC3103 ) buck-regulator (whose input voltage is 12V) to supple 3Vdc to the two pumps (though they have a maximum rating of 4.5V).

• Mock_setup.jpg
File size:
53.2 KB
Views:
13
Last edited: Sep 20, 2012

Sep 17, 2012
37
0
I'm still yet to calculate the heatsink for that peltier though...

8. THE_RB AAC Fanatic!

Feb 11, 2008
5,435
1,305
BIG. With a fan. That's your peltier heatsink calculation.

Also re the sensors, it is much easier to regulate an exact temp if it has had lots of time to stabilise, so it might be best to leave the sensors ON and let the temps all stabilise and remain there.