DSLR cooling project #5

[rcheshire]

Out of interest - another DSLR cooling project - #5, I think. This should be entertaining reading...

It's PWM using a Teensy 2.0 with Arduino code. The Teensy native PWM frequency is much higher than the Arduino and more suitable, I think.

The idea was to cram all the electronics for cooling, dew heater and camera sensor defogger into one relatively small box (which hinges as shown for battery replacement - yet to fit the velcro strap), to make temperature control accurate and human friendly - a single switch and LED provide all the control and feedback necessary.

Cooling capacity is 20C max differential - cooling is zoned within this range to operate at 7, 5, 0 and -5C (7 is a bit arbitrary at the moment) depending on air temp. Once setpoint is assigned it doesn't change (based on temperature reducing at night). I'm not going to use it at > 25C.

Within a few degrees it will accommodate an increase in air temperature. A new session can be started within 5 -10 minutes of shutdown, giving the cold finger time to warm up to ambient allowing setpoint to be recalculated with the push of a button.

The push button on the front of the box is depressed during power-on, disabling cooling, while registering ambient temperature and assigning setpoint temperature - using a single temperature sensor attached to the cold finger. The red LED flashes morse code to report the selected setpoint - I kid you not. ... ..-. --.. -- S F Z M ??? you guessed it Seven, Five, Zero and Minus (5 only):D. Releasing the button activates cooling - on-temperature is indicated by the LED.

Now I want to try it out. I have fiddled with the snubber network to get the smoothest MOSFET switching possible, based on oscilloscope output. Virtual design tools are amazing.

FYI - high power, logic level MOSFET, Vgs 4.5 and very low Rds(on) - IRF2804 from utsource. I was using a NXP PSNM1R1-PL30, but these are difficult to source and expensive to ship. Both run cool without a heatsink - barely warm to touch. However, I've used a Freetronics SMD MOSFET in this project, with similar characteristics.

I know that there are differing views on the use of PWM, but I like it. Linear is naturally noise free but, having spent many hours working with it, it's a pain to manage in a nights imaging, where consistency is needed, particularly for calibration libraries.

It's been fun...

[nebulosity.]

Super nice work there Rowland :thumbsup: Looks great.

Lot nicer than my cooling job :lol: need to update me thread.

Awesome work
Jo

[rcheshire]

Thanks Jo. A few refinements and hopefully this package can be used across several platforms. When I'm happy with it, I'll move it across to a cheap second hand 450D I bought off ebay.

Next job is a custom light weight heatsink to meld into the black anodized look. I'm not sure how to go about this - machining an aluminium block or creating a mold and casting. A machined block might be easier. I cant find a heatsink with the right specs on-line.

The breadboard will go eventually.

[ZeroID]

How about a PC CPU heatsink ? I've got heaps of small weird shaped alum\copper slugged, coolant piped critters with radiators I've rescued from old PC's.
Have you got any specs of your requirements ? Even to take something already made and mod it to fit might be an option.

[rcheshire]

I'm using an old Phenom cooler which does a pretty good job, but no aesthetics.

Rule of thumb is twice the power rating of the TEC - 140w. I have a 200watt Alpha Novotec, but it's a brick.

Lightweight, compact, lots of surface area and high volume, silent, vibration free airflow is the criteria. I think thats why the Phenom cooler works so well - but it could be flatter.

[rcheshire]

Jo, I think yours is superior in several ways. I am not skilled in machining and I don't have a workshop - stuck with a small bench, vice, hacksaw, file, drill and soldering iron - sort of have to adapt.

I tried out a new cooling algorithm last night. It's more dynamic than the previous. Setpoint didn't budge, except for the usual momentary spikes in readings - very happy with it.

[rcheshire]

Just playing around I programmed the LED on-temperature indication to illuminate at set point +/- 0.2C, and, it remained on and steady. I don't know, but that doesn't seem too bad for a cooled DSLR. I suspect it will do better with aref 3.3v - a bit more accurate than the present 5v.

I have sourced a heatsink to replace the ugly looking thing, that hangs off the coldfinger. Lower profile and will take 2 x 50 or 60mm fans.

Code calculates a target pwm value to maintain setpoint and then drives TEC 100% duty cycle to setpoint. After that pwm value is recalibrated based on any temperature differential from setpoint and dithered either side of setpoint.

[rcheshire]

New heatsink arrived today. It has good fin density, is light and low profile compared to the cpu cooler. It looks better too. Flat side is nicely machined. Balance is much improved.

Differential increased by 4C to 24C with a 70mm fan. The tape is temporary until I make a shroud to prevent light leaks.

[nebulosity.]

Very nice Rowland :thumbsup: The heat sink looks to be exactly the same as mine, great minds must think alike :lol: :rofl:

Jo

[rcheshire]

Well this has progressed and you may have read about my agonizing efforts to find a quiet switching solution to eliminate EMI and get rid of those nasty lines in images. Digital cameras are very sensitive to EMI.

I have tried 4 or 5 different logic level n-channel very low rds on mosfets driving direct from the PWM pin and an MCP1415/16 gate driver with mixed results. Having come up with all kinds of snubber networks, poor, fair and bad, rigorous testing revealed flaws in all .... back to square one.

In a nutshell the easy method, producing the cleanest images is a 22uf Tantalum capacitor Gate to Source. This does not send the mosfet linear from what I can see. The system is slow and temperature control sloppy, although useable, which is to be expected. The NXP PSMN1R1-30PL runs cool - unlike others. It's smd cousin the 1R3-30LY runs in liquid solder - don't be distracted when testing these things - it stopped working when I tilted the board and it slid off. So much for home brew smd, for now.

The Android SPICE based program I been using seems OK and tends to verify practical observations. Images below. Time base is 20us. 12v side is 10mv resolution and offset ~5mv ripple. Low side is 1v resolution. The other Android simulator had a bug producing incorrect results. The developers appear to have fixed it.

I understand that a 22uf cap gate to source may not be optimal as far as switching losses are concerned, however, I am at a loss to find another workable solution. I thought I had EMI solved on several occasions, but there was always anomolies. I will be less hasty to cry Eureka in the future - DRAIN turn-off produces some nasty voltage spikes which are easily eliminated with a high speed diode source to drain, or so I have found.

If I have some spare time today, I will play with one or two notions that come to mind, based around the 22uf Tantulum capacitor, to improve the system response.

[rcheshire]

Changed direction and abandoned any hope of a snubber solution. Instead, in a moment of unusual electronic insight I decided to use ground planes to isolate analog and digital ground and went a step further adding a second copper plate for the mosfet with planes for each pin - source is connected to digital ground.

Analog and digital ground are connectd by a 100R resistor - added a power plane as well?

To be honest, I'm not certain which part of this system is most effective, but temp control is rasor sharp and images show no traces of switching events.

NO CAPACITORS... NO NOISE...:thumbsup: Even driving directly from the pwm pin.

Sadly, my beloved NXP mosfet broke a leg and had to switch to a StrongIRFET, which runs in the palm of your hand - no heatsink.

Thanks to everyone who helped.

[rcheshire]

First of all, I apologise for the hand drawn diagrams in this age of Eagle CAD and other software, but it is a matter of time and opportunity.

I've attached the general layout of the system. Other services could be added, expanding on the concept of keeping all functions in a single box. Shutter control can be moved to the camera as well.

At this stage sensor cooling, defogging and dew heater are in place, with plans to implement automatic control to these functions - having resolved the EMI problems producing a workable flexible self regulating system.

Control and indicating is minimalist. 1 button and 1 LED with setpoint and on-temperature indications - the addition of temperature sensor failure indication with automatic cessation of cooling and other services is in the pipeline.

EDIT: Following a little maintenance the camera chassis was unintentionally isolated from the cold finger, which generated EMI in live view. Bonding the cold finger to analog ground resolved the interference. However, both camera chassis and cold finger should be connected to analog ground for silent operation.

[rcheshire]

This is the general layout/principle for grounding the camera chassis and cold finger. Absolutely essential.

EDIT: I noticed an error in the diagram, which would have been obvious to the trained eye - I missed it. Corrected and new image uploaded. The fan power was connected to the heat sink and looked confusing - removed for clarity.

[nebulosity.]

Gee whizz Rowland your really perfecting this camera cooling :) nice work :thumbsup:

Love your hand drawn diagrams :thumbsup: the grounding layout is very helpful for my camera.

What arduino module are you using? I don't know anything about them but they certainly look interesting and I was thinking about getting one to experiment with and hopefully gain a better understanding of how they all work. I'm hoping someday to make an electronic filter wheel.

My camera cooling is going well, getting around 35C below ambient regulated cooling, unfortunately I wrecked the sensor while removing some leftover CFA so it's out of action for a bit.

Cheers
Jo

[rcheshire]

You've done a really well considered job on your camera, Jo. It's very good. I have some spare TMP36 temp sensors. Your welcome to one if it will help. No charge.

It's taken a while to acquire the learning to get anywhere near the results to date. It fell into place one day - I am tempted to do an EE course at Uni. I think it would be fascinating.

I'm using a Teensy 2.0 - it's not Arduino, but it is compatible and it was necessary to install the Teensyduino environment alongside the Arduino environment to use Arduino code - a bit easier for me than C++. Teensy can be run with C++ or the Arduino Wiring program.

The Teensy probably has better hardware, but that's a matter of opinion. It's not so easy to configure for external voltage sources as the Arduino. Handy if you want 3.3v for analog sensors and 5v for other stuff - motors, pwm, leds etc.

I would go with the Arduino for now of similar size such as the Leonardo and Nano, which is a bit smaller. Maybe grab a bigger model such as the Uno, which might be easier to handle first up. Jaycar sell the Freetronics version along with a bunch of convenient ancillaries, break out boards etc.

Some of the guys on the forum are real whizzes with programming. Mine is very basic - nothing flash, but it does what I want.

Next will probably be a double sided PCB. I will do the artwork by hand - roughed it out already. I'm looking at dual redundancy power and mosfets.

[rcheshire]

I am thinking of load sharing the TEC current with two mosfets, as well as single mosfet operation (one failed) and pwm source redundancy.

Two pwm sources are hard wired to the mosfet gates a primary and secondary. The secondary pwm source is shorted through a transistor. On failure of the primary pwm source, the secondary circuit is closed, kicking it into action.

Conceivably, in a nights imaging, loss of one or the other mosfet or the primary pwm source should ensure, uninterruped operation, replacing failed components in the hangar, instead of in the field.

It needs failure indications and I am not sure about the correct values for the (sense?) resistors. But it works in simulation.

[rcheshire]

Made a few changes to the auto pwm switching because there is no easy way of switching the ground side of the pin. Added switches to test the loss of pwm or mosfet.

EDIT: This is just being clever. In reality a mosfet is likely to fail before a pwm pin, in my experience, and mosfet failure is very unlikely. However, from what I can see, the mosfet remains in the state at which pwm failure occured, either on or off. If on it is linear and will most likely overheat. Although it is at 5V in this case, so maybe not.

[rcheshire]

This is the rough artwork for the Arduino compatible (Teensy 2.0), PWM, MOSFET driver board.

The idea of the multiple rails is to provide 5V, analog ground or digital ground points for various sensors and devices. Not too restrictive if I want to add new features.

It makes use of ground and voltage planes, which has proved successful in reducing mosfet switching EMI to visibly indiscernable levels.

The board will fit to the left of the battery door attached to the tripod mount (through AGND), which is part of the camera chassis. The cold finger is bonded to the chassis.

I want to see whether a bigger board or an additional board is required to increase the size and effectiveness of the ground planes.

The Teensy also has its own 5V supply - a miniature smps regulator, same size as a 7805.

[rcheshire]

Found these at LED sales. SMPS regulator, trimmable with logic on/off. There is a 5V version - 3 pin replacement for 7805 linear regulators.

http://www.ledsales.com.au/pdf/MDC_OKR-T-1.5-W12-C.A01.D5-214787.pdf

I have integrated 3 of these into the cooling circuit. The logic pin allows activation of dew heaters in response to temperature or humidity conditions. Rated at 1.5A, maximum current to each heater will be 0.7A - 1 unit per heater, plus a spare.

[rcheshire]

This is the Beta board. A very crappy job of etching. The production board will be factory made This will do for now. Past the experimenting stage, it needs to be tested in situ.

I have incorporated the ground planes for the MOSFET pins as well as separating digital and analog ground sources adding 3 adjustable power sources for dew heaters and an experimental 3.3V external supply for the Teensy/Arduino, as well as a 5V supply. Cooling and heating off the same board.

One side of the Teensy has all the analog pins with a third hole for decoupling capacitors to analog ground for sensor inputs.

All ground connections are grouped on the ground planes according to digital or analog inputs.

The board and case clear the battery door. The case will be clamped to the camera base through the tripod mount hole and fixed to the heatsink for rigidity with double sided tape. The MOSFET is located so that it can contact the main heatsink.

Eventually the case will accommodate camera and dithering control - unguided widefield setup.

[rcheshire]

Here is the populated board - hand drilled - with some learning about component spacing.

I abandoned the idea of dual mosfets for now to simplify things.

This will run sensor cooling, sensor defogging and lens dew heater. A second board will drive the camera shutter and dithering and perhaps camera power - for now it's batteries.

A little more design work on the pcb , a little bit of code to drive the heater supplies at the right time.

EDIT: Amazing!!! - found an error in the board. Disturbingly, it was the digital ground to the Teensy +5. Anyway all fixed - cut the trace and use a jumper to the correct pin.

[rcheshire]

Latest artwork. Comments, suggestions? The basis is ground planes. Some respacing required. SMD pads are a substitute for an unavailable part with the required 1.7mm in-line pin spacing - SIP. The terminal block at ground is a convenience measure to avoid soldering a heavy ground wire. Same for the Drain terminal block

[rcheshire]

Getting close. More generous spacing and gaps. Some small adjustments required, on second look, but this is the RC board, as far as I'm concerned. It fits clear of the battery door on the camera base with the MOSFET in contact with the main cooling heatsink.

A smaller version demonstrated the same perceptibly noise free performance, as the original huge ground planes - confident that this will do the same.

It was easier and less time consuming to draw it up on the board editor (Eagle CAD). Consequently, there is no schematic. It's not complex and should be easy to follow.

As mentioned earlier it is all based around more thorough and careful separation of ground sources from analog and digital devices and sensors, leading back to a common ground at the microprocessor board.

The planes, in simple terms, produce low inductance and low impedance pathways to a common ground, with a high impedance connection between analog and ground planes to balance potential difference - that's the theory, anyway. Not new by any means, but a departure from the use of snubber networks for mosfets.

On to an electronics compartment-to-heatsink sub-frame, to stablise the otherwise flexible copper cold finger. I want this to be as neat and compact as possible with basic/convential tools. Anyone should be able to build this - that's the goal. "Some say he's a dreamer, but..."

[rcheshire]

Well, I'm not happy with the results. I think there is need for isolation of some components which seem to interfere with the microprocessor board. Puzzled:question:

EDIT: OK I worked it out. Time for a rethink.

[rcheshire]

Nope! Wired the pushbutton incorrectly and shorted the whole thing? All working now... attached below.

Small change - low side switching the dew heater only. As this rig will image at 0C, over a wide range of ambient temperatures sensor deffoging is on while cooling is operating

The dew heater is low side switched through an N-channel MOSFET below 5C for now. Easy enough to use a humidity module as a reference.

There's not much more to this and it's time to start imaging with it.

Summarizing this rather drawn out escaped. Thermoelectric cooling of a DSLR sensor is not too difficult, providing the camera is easily adapted. Regulating setpoint temperature has several advantages, such as consistent and reliable image calibration results. Control using PWM and a MOSFET to switch current through the TEC module presents one or two challenges. Switching of the MOSFET shows up as lines in images, if EMI is not reduced to insignificant levels. Ground planes and careful separation of analog and digital ground sources and camera chassis and cold finger bonding has produced best results.

I will post the Eagle .brd file, although it hasn't changed much, along with the cold finger dimensions. There is a link to the heatsink in an earlier post to this thread - heatsink performance is critical. Cooling performance is related to heatsink and fan performance, TEC rating and supply voltage - if the heatsink/fan cannot keep up the TEC will expend energy heating the system - indiscrimately.

Test the system at various PWM settings note the max differential and PWM setting. If the best differential is at a PWM value less than 100% then the heatsink is probably too small or the fan is not moving enough air or both.

The system will shut down automatically on the predicted failure of certain components - that is; anything that is getting outside desired temperature levels.

EDIT: Uploaded edited circuit diagram - added the latest PCB version..

[alistairsam]

Nice work Rowland, obviously a lot of thought went into this.
Do you use set point cooling and how do you set what temperature you want to regulate at? LCD interface?

should be straightforward with the arduino to add a serial LCD module.
this one is via I2C http://www.ebay.com/itm/Arduino-IIC-I2C-TWI-1602-Serial-LCD-Module-Display-/190573003243

if noise was an issue with switching the mosfet's, would it help if you isolated the controls from the TEC and had it outside on a lead?
do you have a "warm up" sequence as well where you reduce power in steps over a minute to slowly withdraw cooling as most CCD's have?

I've been into hobby electronics for over a decade and most of your theory was "overhead transmission" for me, but interesting nonetheless.

if you wanted to etch PCB's, i've got everything including toner transfer, blank PCB's, ammonium persulphate.
else once you have the eagle files done, I'd suggest using OSH park for the prototype boards as they're professional quality and low cost.

what exposure lengths are you aiming at now that you're cooling the sensor?

Cheers
Alistair

Edit: I use these bits for drilling holes on the PCB using a drill press
http://australia.rs-online.com/web/p/pcb-drill-bits/6665650/

[alistairsam]

ha ha just read your first post, set point via morse code??
its like the martian rovers leaving tyre tracks in morse code reading "JPL".

just add a serial red LCD with two push buttons, add code to display ambient at startup, and accept input for target setpoint and away it goes. dead easy.

the project for automated dew heater controller in the projects section should give you some ideas on the code required with the arduino for display and input.

Cheers
Alistair

[rcheshire]

Thanks Alistair.

Setpoint is currently determined by ambient temperature range, taking into account the capability of the system on the night, rounded to 5, 0, -5C and is automatic. An LCD would be a nice addition. A previous mod used two pushbuttons to set temperature by degree, but I opted for auto setpoint on this one. Depressing the pushbutton disables PWM, reads ambient temperature and calculates setpoint - release pushbutton to start cooling.

Basically what you describe is the arrangement. I have attempted to keep the footprint as small as possible without going down the SMD path, just yet - that's to come.

A warm up sequence is a good idea and easy to implement.

The theory is interesting and I'm happy to say the application has very quick switching response, while maintaining setpoint. I've had a bit of help here and there from one or two members.

I have etching gear, onto board #5. I will try OSH park. I'm over the transfer thing. It's driving me nuts.

Exposure times will be around 3 - 3.5 minutes at iso 800 - 400 as a rule. I find this produces the best quality subs depending on sky conditions. The lower iso is to prevent star bloat in widefield images. There's not much space between stars at 200mm and 1.6 crop factor.

Exposure time is a trade off with widefield to avoid splodgy groups of stars. Just need more subs - I take about 40.

Drill bits - I break lots of those.

I like minimalist controls, cables and so on. The Morse Code thing was a gimmick. The led blinks at a different rate for each setpoint while the button is depressed and lights up again at setpoint.

[rcheshire]

Cooling Conversion Part 1 (http://www.synergous.com/flatpress/fp-content/attachs/canon-1000d-cooling-conversion-part-1.pdf) is an attempt to document in one place. 3 Parts and 2 appendices.

[nebulosity.]

Rowland, this it awesome :thumbsup::thumbsup::thumbsup:

Thanks so much for writing this up, I'm sure it will benefit many people. I look forward to part two, I'm very tempted to have another go at cooling following your instructions :D

Would you know if the 1100D is very different from the 1000D? I might have to pull mine apart and see.

Cheers
Jo

[rcheshire]

Thanks Jo. It is different. Will have a look at Gary Honis instructions and see if I can work out what's involved.

[rcheshire]

3 parts in one place, with extras to come.

DSLR Cooling Conversion Overview (http://www.synergous.com/flatpress/fp-content/attachs/canon-1000d-dslr-regulated-thermoelectric-cooling-conversion.pdf)

DSLR Cooling Conversion Part 1 (http://www.synergous.com/flatpress/fp-content/attachs/canon-1000d-cooling-conversion-part-1.pdf)

DSLR Cooling Conversion Part 2 (http://www.synergous.com/flatpress/fp-content/attachs/canon-1000d-cooling-conversion-part-2.pdf)

DSLR Cooling Conversion Part 3 (http://www.synergous.com/flatpress/fp-content/attachs/canon-1000d-cooling-conversion-part-3.pdf)

Appendix 1 Canon 1000D Thermoelectric Cooling Conversion - PCB etching (http://www.synergous.com/flatpress/fp-content/attachs/mosfetgatedriver7.pdf) (Basic board)

Appendix 2 Canon 1000D Thermoelecfric Cooling Conversion - Arduino/Teensyduino code (http://www.synergous.com/flatpress/fp-content/attachs/dslrcoolingv5.txt) and .ino file (http://www.synergous.com/flatpress/fp-content/attachs/dslrcoolingv5.ino)

Canon 1000D Thermoelectric Cooling Conversion - Drawings and Notes (http://www.synergous.com/flatpress/fp-content/attachs/canon-1000d-dslr-cooling-drawings-and-notes.pdf)

First image test (http://www.synergous.com/flatpress/fp-content/attachs/first-image-test.pdf)

EDIT: Minor changes

[rcheshire]

Edit: amended some sections

CMOS Heater - aka "defogger" (http://www.synergous.com/flatpress/fp-content/attachs/cmos-heater.pdf)

I have added as much information as is necessary, hopefully, with some order to it. None of this is set in concrete and there is always room for individuality. The idea has been to set out the basics and at the same time present a working model. I hope it's useful.

The code works well and no doubt can be improved by better programmers than me. It's a low level approach. Pretty much where my programming skills sit at the moment.

The setup was tested at the March 2014 Snake Valley Astro Camp, and it ran nicely at -5C for several hours, between 20 and 15 degrees differential. I will post the image when it is processed.

There are some minor improvements and adjustments to be made. If you don't like the analog sensor readings in the program as control points, just convert and use degrees Celcius.

I did notice, on the second night, while focussing with a mask, what appeared to be diffraction due to a small amount of condensation on the sensor face. This did not reappear once evaporated by the sensor defogger. I suspect that the defogger had not caught up with the cooling, due to low air temperature.

Camera has a full spectrum mod with an Astronomik clip-in UV/IR filter from BINTEL. Probably the easiest and less costly option for hobbyists - allows IR as well, with a filter.

[rcheshire]

Finally got it completed - here is the test image... http://www.astrobin.com/82477/F/

[scottholio]

Wow, this is really cool. Nice job

[Garbz]

I feel a bit guilty chiming in with suggestions after you got it working but I only just noticed this thread. Sorry. But these few notes are just something to consider when you do your next circuit:

- D1 and D2 can often be omitted through careful selection of Q1 and Q2. Many MOSFETs now ship with reverse protection diodes in the package itself. It's more common than not these days especially to meet electrostatic discharge requirements.
- What's the purpose of the mini trace connecting the big trace to digital ground?
- How are your two grounds connected? Not the 100ohm resistor I hope? If so what's the purpose of that connection?
- For home manufacture especially but also if you start hitting the tolerances of the manufacturer of your circuit board (i.e. 6mil traces) use right angles. For routed boards it prevents overshoot on the routing bit from cutting into your trace, and for home made boards it prevents etchant from pooling at a corner and cutting in. Use 45degree angles where possible making a turn and if you can afford the space use 45degree bevels on T-junctions on the board as well. No electrical reason for it, it just helps ensure that you or someone else won't stuff up the manufacture.

[rcheshire]

Thanks Scotty. It was fun...

No problem Chris. Good to get a critical eye over it. Responses below comments. many thanks.

[Garbz]

Yeah toner transfer drove me up the bend.

Just a note on grounding, a ground must never have a high-impedance connection, whichever paper said that stop reading it. I have a bit of history in mixed signal and low noise design.

The principles of high-impedance links are designed to prevent signals from flowing between different grounds. Unfortunately what happens is if a signal does flow over this link it raises the potential of the ground from the side it's coming from. Your digital ground may suddenly float a few volts above the analogue ground leading to very screwed up responses in mixed signal devices like digital to analogue converters.

Where high *resistance* connections are useful is if the ground current will only flow in case of a fault condition and the fault condition needs to be detected. e.g. in my power amp I have a 10ohm resistor connecting my signal ground to my chassis as there should not be any current flowing unless something goes bang, but none the less I need them connected to prevent the floating ground from causing it's own noise, and if something goes bang I want it to trip the earth fault detector in the house.

The solution is to filter out the signal of interest at the point where grounds connect. In this case you could for instance put capacitor banks on either side of the resistor, or substitute the resistor with an inductor (high impedance at higher frequencies, but low impedance at DC).


In mixed signal design routing the ground plane creatively is the best way of keeping signals and noises separate. It is all 100% about looking at the impedance loops. Return signals travel along the path of least inductance, which is often as close as possible to the original signal which went out. The picture I've attached below is a classic mixed signal device like a DAC with digital on one side, and analogue on the other.

The idea is to route slots down to the point where the signals get mixed (straight through the chip) and connect the grounds there, and also connect the powersupplies there. Keep all the digital signals on the digital side and the analogue on the analogue side. If ever they need to cross via some device they should either be isolated or cross at the one point (kind of like your star ground).

If you do this you effectively limit any mixing of the signals on the plane as you can see by the cream lines. If you do however cross then the signals need to travel back via the wrong side of the plane which is bad and will lead to contamination for want of a better word. When circuits get large it becomes very hard to route, even harder if you can't dedicate an entire plane to ground.

It's more of a black art than anything else. But if all else fails just remember the word star ground. Return all your grounds to one big very low impedance point and you're about 90% of the way there. Chasing that last 10% will drive you mad.

[rcheshire]

That is excellent feedback Chris. Thank you very much. Amazingly, you didn't loose me at all through your explanation. Simple fix, I think - remove the resistor. Might explain the noisy temp sensor analog inputs. I'll have a closer look at my circuit, per your diagram as well.

[Garbz]

Or replace the resistor with a suitably sized inductor if you're trying to eliminate noise. As a matter of interest how is the noise presenting? I assume you don't need lighting fast response. Maybe an 8 point moving average filter implemented in the code may fix the issues too?

http://arduino.cc/en/Tutorial/Smoothing

[rcheshire]

Noise is presenting as spikes in analog sensor input values during setpoint modulation - switching - and occasional chirping through the 12v supply - I will refer to your diagram on this one.

I think the connection DGND to GND is necessary, but could that also be through an inductor?

On the mosfet gate side, I'm thinking inductor in series with the current limiting 100R.

Simple enough with an on-line aircore inductor calculator to wind my own.

I've used that smoothing algorithm before, but I think first, sort out the hardware. As you rightly point out, my application is flawed in places.

Attached with proposed changes/queries.

EDIT: had an hour spare so whipped up a 100nh inductor to replace the resistor. Possibly my imagination, but temperature control is more consistent - that is; less pwm hunting as it endeavours to push the temperature up or down.

Chirping is however, more consistent - resonance in the circuit I guess.

[rcheshire]

Rejigged the circuit.

This arrangement is observably better. Setpoint regulation is smooth, with very little modulation. Setpoint LED is steady ON. I guess temp inputs are less erratic - gradual improvement with each modification.

EDIT: Suggested modifications to PCB. New images

[rcheshire]

This is all in one place now and while there is room for improvement, there is only so much time in the day. I hope to close this now and move on to other ideas. I wish to thank the people who contributed and educated me along the way. I hope this is useful to someone in part, at least.

Incorporated recent changes, except for the PCB etching pdf. The board can be modified easily, pre or post etching with reference to the attached image - no opportunity, otherwise, in due course.

Notes and drawings need updating - time permitting...