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106  Using Arduino / Sensors / Re: Peltier temp control PWM method - sensor analog reading value on: November 16, 2011, 09:31:42 pm
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Simple PWM control of a TEC:  Use a logic level N-channel MOSFET as a low-side switch (ie, between the TEC and GND).  A FET with low Rds(on) will minimize losses and reduce/eliminate the need for a heatsink

Thank you, that's mighty helpful. More reading comes up with these, which seem to be popular;

IRL520 Logic Level N-Channel MOSFET, 100v, 9.2A drain. Rds(on) 0.27ohm.
http://www.hobbyengineering.com/specs/IR-irl520.pdf)

IRFZ44 MOSFET; Drain Source Voltage, Vds:60V; Continuous Drain Current, Id:55A; low Rds(on) Test Voltage, Vgs:10V (0.028 ohm);

Not sure of my facts here;

The IRL520 has around 50 watts power dissipation - the TEC is 68.5 watts, but I presume that there will be losses to GND.

The IRFZ44 looks like a good all rounder if I have my facts straight. Power is not mentioned, but at 55A I don't think its a problem.

Pin wiring: Arduino to S - TEC GND to D - there is a diode between S and D, so at a guess I think that's correct.
107  Using Arduino / Sensors / Re: Peltier temp control PWM method - sensor analog reading value on: November 16, 2011, 05:44:25 pm
Ah yes. I see that - amplify or on/off.

12V 8A maximum rating for this application.

There must be a package to suit, or this could be adapted for my purposes - http://freecircuitdiagram.com/2008/08/27/variable-adjustable-current-limiter-circuit/
108  Using Arduino / Sensors / Re: Peltier temp control PWM method - sensor analog reading value on: November 16, 2011, 05:35:24 pm
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The reading you get from the analogRead() call is (1024 * input_voltage/reference_voltage). The default reference voltage is the 5v supply, but you can change it by connecting the AREF pin to a different reference (e.g. the 3.3v output) and calling analogReference() to say you have changed it.

Changing the analog reference from 5v to 3.3v will give you slighty better resolution, i.e. around 0.49 deg c resolution at 5v vs. 0.32 deg C resolution at 3.3v.

Thank you, that makes sense now. I intend using aRef 3.3v for better resolution.

With a bit more reading, I see that a current regulator is not the right terminology for increasing and decreasing current. I guess it needs to be a variable current limiter.
109  Using Arduino / Sensors / Re: Peltier temp control PWM method - sensor analog reading value on: November 16, 2011, 04:22:10 pm
Thank you both.

I have been reading about PWMing the TEC and it seems complex, except for hard banging which I understand is not the best treatment for a Peltier. I need to regulate the current to the TEC. In simple terms, I don't know where to start.

My current set up (to which the TEC control will be added) is supplied by a 12v 1.5 amp power supply, which also supplies the Arduino.

I need to add temperature sensing and current control for the TEC (12v, 68.5 watts, 8 amp, 68C DeltaT), which needs its own power supply - 12 V 8.5amp or 12 - 15v 12.5amp, and a breakout board for current control...

I don't know of the top of my head how to build a current regulator. Conceptually, resistance needs to change in response to temperature read by the Arduino... there needs to be a means of controlling resistance. The system is closed loop with temperature sensing as the variable.

How do I know how much current to supply to the Peltier for the desired effect? I imagine, it would work something like this. Supply maximum amps until temperature is reached and then regulate current about the set point temperature, read by Arduino, feeding back to current regulator.
110  Using Arduino / Sensors / Re: Peltier temp control PWM method - sensor analog reading value on: November 15, 2011, 03:41:49 pm
Thanks. I'll follow the discussion smiley

While I think of it. Can jitter be managed with a delay? The cooling system has a reasonable degree of inertia. Temp wont change significantly over .5 - 1 second.
111  Using Arduino / Sensors / Peltier temp control PWM method - sensor analog reading value on: November 14, 2011, 05:27:08 pm
Hi. I have 2 questions concerning Peltier temperature control.

There are two methods available for my project, to maintain a temp of -30C, +/- 0.1 (realistically, I will be happy with +/-0.5C

Sensor - TMP36 TO-92 located near cold site (-40 - 150) to avoid negative voltage - easier I guess (ladyada)

1. Which is the better method? (opinion)

- PWM the heatsink cooling fan, which is easy, but how effective.

- PWM the Peltier voltage - I have previously been advised against this, but I see others using it, providing the voltage is not cycled on/off.

2. What is being read by the analog pin connected to the temperature sensor?

- voltage

- a number 0 - 1023. I'm a little confused about this one. With digital pins it's HIGH and LOW. In this case I want to read a set point voltage of 0.2v (-30C) and maintain that by one of the PWM methods above.

Many thanks.
112  Using Arduino / Project Guidance / Re: Peltier cooling temp control- photographic application on: October 03, 2011, 06:21:43 am
Providing condensation is not an issue, fridge should be ok. Sealed plastic bag and desicant
113  Using Arduino / Project Guidance / Re: Peltier cooling temp control- photographic application on: October 02, 2011, 05:29:28 am
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Q: why do you want to cool the camera so much ? special effects? sensitivity? ...

That's correct a.d. Cooling the camera / sensor reduces thermal (dark) noise. A dedicated cooled scientific ccd camera is best, but good ones run into many thousands of dollars. I want to cool a Canon XS/1000D DSLR.

http://ghonis2.ho8.com/rebelmod450d16c.html - The hardware requirements are fairly straight forward.

So far, Arduino is controlling the imaging session almost completely, except for guiding. That will be an interesting project smiley-cool

Right now I need to get the camera temp down to -30, ideally. It seems that most people use a variable power supply to the Peltier module to control temperature.

114  Using Arduino / Project Guidance / Re: Peltier cooling temp control- photographic application on: October 01, 2011, 09:33:39 pm
Thanks. Much simpler to control than I imagined.
115  Using Arduino / Project Guidance / Peltier cooling temp control- photographic application on: September 30, 2011, 11:50:02 pm
The project I'm working on - several posts on this site - drives a stepper motor, camera shutter and illuminated lens cap (led array). Thanks to the help I've received on this forum all is working OK. To complete the project I want to drive a peltier cooler, to cool the camera sensor, quite simply, as cold as it should reasonably go, preferably -30C - 40C below ambient. The cooler will be on throughout the photographic session with temperature control.

There are two options - use a heat sink to the back of the sensor (about a 1.5mm gap between the sensor back and circuit board (use hot glue to seal the electronics from condensation), or put the camera in a cold box with the lens sticking out the side with a desiccant. Not as efficient.

I have seen it done both ways, but I want to regulate the temperature and keep the power demands low. So I think the heat sink is the way to go.

I will need a new 12v high amp power supply to handle the peltier current and an H-bridge circuit, I think. The idea is to have the peltier operate close to maximum cooling but control the temperature using a thermistor, which will be the most complex part. I read that peltiers can be controlled much like DC motors - PWM, but I'll have to work out how to do that.
116  Using Arduino / LEDs and Multiplexing / Re: LED power switching power transistor - code hangs? on: September 22, 2011, 03:46:52 pm
I used the data sheet this time. This makes more sense.

Solution 1: 2 x 4 array uses 8 LEDs exactly

+12V

R = 220 ohms

R = 220 ohms

R = 220 ohms

R = 220 ohms

The wizard says: In solution 1: each 220 ohm resistor dissipates 198 mW the wizard says the color code for 220 is red red brown the wizard thinks 1/2W resistors are needed for your application together, all resistors dissipate 792 mW together, the diodes dissipate 768 mW total power dissipated by the array is 1560 mW the array draws current of 120 mA from the source.
117  Using Arduino / LEDs and Multiplexing / Re: LED power switching power transistor - code hangs? on: September 22, 2011, 04:02:58 am
Thanks Cross Roads and Grumpy Mike.

I had the same thought about the resistance and ran he wizard again - I get the same values. I'll use cross roads calculation.

I'll use 12v for the LEDs as suggested. I run a stepper and camera off the same shield at 5v. I can tap 12v off the Vin pin.
118  Using Arduino / LEDs and Multiplexing / Re: LED power switching power transistor - code hangs? on: September 20, 2011, 06:11:10 am
OK. I relocated the LED switching out of the main loop and set up a separate void loop for that part of the code - works fine now.

I have a 12v supply that is currently powering the Arduino board and a separate 5v power supply for the LEDs.

The LEDs should be wired in parallel, powered directly from the Vin 12v supply instead of a separate 5v supply for maximum brightness?

Referring to this site http://led.linear1.org/led.wiz for circuit calculation - 12v 3v forward voltage 10 mA forward current 8 LEDs. I've guessed the forward current. As suggested, a pair of 4 LED - 2 x 1 ohm resistors.

119  Using Arduino / LEDs and Multiplexing / Re: LED power switching power transistor - code hangs? on: September 19, 2011, 06:48:46 am
Thanks. I will get back soon - have some ideas for the code. It can be simpler. Meantime, will try the circuit suggestions. I want max brightness - Cree 30000mcd LEDs.
120  Using Arduino / LEDs and Multiplexing / Re: LED power switching power transistor - code hangs? on: September 18, 2011, 06:45:05 am
Thanks.

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Regardless: do the switches change state after this:

Code:

// camera shoot - when finished

digitalWrite(pin12, LOW); // leds should turn off here - but they don't

so that the next if () isn't just skipped and goes right to the top of loop to turn the LEDs back on?

The pin remains HIGH.

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The solution might be as simple as turning on the internal pullups to make sure switches see a good high:

pinmode(pin2, INPUT); digitalWrite (pin2, HIGH); // enable internal pullup pinmode(pin3, INPUT); digitalWrite (pin3, HIGH); // enable internal pullup

I am doing this too.

While the camera code works fine testing with a single led as described. There is no action whatsoever once the LEDs light up. The hardware switch to pause turns LESs off as mentioned.
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