Well, I recently recieved my boards back from ap circuits in Calgary and I’m pleased with the result. I kind of fucked up by ordering 4 of the same board and not 2 of the control board and 2 power boards. Oh well! I can use the current boards for the new ones. Anyways, here are some images of the board.
After being apprised of an issue with heat transfer through PCB traces, being the stubborn idiot I am, I had to try and solve the problem. The issue is with a particular PCB that has a cutout section with a thermistor placed in a partially isolated section within the enclosure, unfortunately the traces themselves are serving as a heat transfer conduit especially so since copper is a particularly good conductor of heat.
After thinking about it for a while I came up with a couple of ideas like calibrating the calculated output to accommodate for the temperature discrepancy, however that probably wouldn’t work without a second thermistor closer to the heat source, accounting for the difference. Another idea I had was to use an infrared thermopile but they’re kind of large and expensive and would require a heat channel mounted above it to prevent measuring the temperature of the case.
After thinking for a while about it, the simplest answer I could come up with would be a heatsink to mitigate the heat transferred to the thermistor. While it is theoretically impossible to totally remove all heat difference, I believe that a large portion of the heat transfer can be eliminated. So I wanted to see if heatsinks actually help equalize heat much at all so I devised a bit of a rough experiment.
I decided the quickest way to test this would be with materials I already had. I have a selection of thermistors so I used a glass axial thermistor.
The thermistor used is a glass 10kOhm NTC Thermistor in series with a 1k resistor in order to detect current/divide voltage. With the setup above, the voltage detected from GND to the divider is 438mv. the temperature in my basement is pretty steady since the furnace isn’t running. So now I attach the leads.
I used Teflon coated wire to prevent conduction between the two sides and used my soldering iron as a heat source since it’s temperature controlled. I let the heat soak up through the lines for 10 minutes to ensure that the heat stabilizes at the thermistor. I also performed a test with the soldering iron closer to the sensor.
Finally, I decided to place a heatsink on the line. I was going to solder aluminum shims to the leads in order to provide for heat sink. That turned out to be difficult at best, so I used coiled up copper wire and soldered it to the leads. I performed the same two tests with the position of the soldering iron. Anyways, here are the results of the test.
No leads, ambient
With Leads, ambient
Pressing thumb on thermistor ~33.1c
Heat with long leads, No sink
Heat with short leads, No sink
Heat w/ long leads, w/ sink
Heat w/ short leads, w/ sink
It should be said that this test is far from perfect and doesn’t prove anything quantitatively with any real degree of accuracy, however I wanted to see how effective even a rudimentary heat sink would be in a situation like this. It should be noted that while thermistors are inherently non-linear, we have voltage drops of 43% and 34% with the short and long leads respectively. I simply thought it was an interesting experiment. I have a few ideas on how to sink the heat or account for it but I think the heatsink is the easiest plan.
Here’s a beautifully rendered image of the idea to reduce thermal linkage between the PCB and the thermistor ;).
If there were a heatsink over the exposed traces and the traces made as long as possible, it should be possible to bring the traces fairly close to ambient. Like the experiment showed, even a rough heatsink was able to reduce the heat going to the thermistor by a substantial degree.
I thought I would have nothing to post but I do I guess. I’ve felt the need to really expedite this project now that I am starting a business. I have done a few of the mechanical things associated with it after a major redesign. Today I have completed the design of the board and despite it’s sloppiness, I’m happy it’s done. Now to get it made.
Like I state in the description, I used the autorouter on this image, I may revise many of the traces so that ripple can be eliminated from IC’s by bringing the caps closer electrically to Gnd and Vcc. This week I hope to finish the machining side of the device, I’ll have to wait for bolts from Fastenal to arrive but it’ll be worth the wait. Here are some shots of the parts so far, note that the main block is made of phenolic, I love this material since it looks kind of like wood but is reasonably machinable.
Just for the record, here is the list of, well electrical features:
16 key Keypad for data entry, mostly for calibration.
16×2 LCD display for seeing alarms and viscometer output.
24V motor, PWM driven with TIP102
Light interrupted sensors for top and bottom
provisions for temperature sensing
Provision for an external RS-232 board, if needed
TVS’ed to the hilt, hopefully this will prevent funny stuff from happening.
Well, For the last week or so I’ve been developing a new viscometer board. Sloppy as it is, it’s a jump ahead from the last board in terms of overall inputs allowed. I used the 68PLCC package PIC18F6680 in this one because it had a sufficient number of IO and because it looks cool in it’s socket. 🙂
Anyways, this one has the following features:
Several analog channels for temperature measurement.
Several analog channels for use in a torque sensor set-up.
Some analog channels.
built in RS-232 Output.
built in RS-485 Output.
Provisions for PICKit2 hookup.
TVS’s on all input lines
Separate PIC12F683 for PWM output
Debug Serial Output.
All in all, it’s gone smoothly, I’ve double checked a number of the traces, let’s hope all of them are well when they come back from AP Circuits later on. 🙂
Yet another iteration of the stormer viscometer board
Well, as far as viscometers go, I could be considered an expert by now (no, not really). Today my new board arrived and I assembled it, I’m kind of proud of it, it works real slick just like the last one. This one has the following changes
ICSP provisions so that I can program it in place.
a PIC18F2620. Has 10 times the program memory and RAM.
diode protection in case the power is hooked up wrong.
Fast recovery diode for motor induction absorption.
Larger traces for the motor.
Fixed resistor array.
Anyways, here is a comparison shot. Old on the left, new on the right.