TEC controller

[NickMu]

Hi All,

After gathering all the stock ingredients I ventured to prototype something to get me started (see attached schematic). Most of the stuff was already made from older projects so I just needed some re-wiring.
To my surprise, at the first try, I was able to drive a TEC with an 80KHz output from an older PIC 12F683 board. I will have to look back to find the code that runs it but it basically outputs 80KHz of about 40% duty cycle PWM for about 3 seconds and about 75% for 3 seconds.
The output voltage to the TEC goes from 5.5V to 8.5V with a current draw of 1A and respectively 3A.
Checking with an old scope the TEC voltage is pretty clean.
I separately built a refrigeration box out of ¾” Styrofoam and I was able to maintain a bottle of wine at about 45⁰F for two days running the TEC under 5.5 V from a separate PSU. I will do some more testing like this to figure out the minimum voltage that is needed to maintain a decent temperature in the cooler without having to turn the TEC OFF. I will do so after replacing the first wine bottle which became “a casualty of the project” and I opened it to celebrate the first successful test. This is becoming a fun project. LOL
Next step will be to fine tune the switching frequency for efficiency and the needed PWM range to get my lowest and highest output.

I will keep you posted and I might come back with some clarifications and maybe questions. I will also post the code that I’m running after I get it optimized.

Nick

[NickMu]

I trying to control PWM output on a PIC12F683 and I wrote the following code for a quick test:

@ device  pic12F683, intrc_osc_noclkout, wdt_on, mclr_off, protect_off

OSCCON=%01110111     
DEFINE OSC 8        

ADCON0=%00001101
ANSEL   = %00111000       
TRISIO=%00011000        
CMCON0=%00000111        
            
i           var         byte
pwm_out     var         GPIO.2

CCP1CON = %00001100     
PR2 = 15            
CCPR1L = 0         
CCP1CON.5 = 0
CCP1CON.4 =0
    

    Pause 200
    T2CON.2 = 1             
    Goto main
Main:
     for i=8 to 14
     CCPR1L = i
     CCP1CON.5=0
     CCP1CON.4=0
     pause 250
     CCPR1L = i
     CCP1CON.5=0
     CCP1CON.4=1
     pause 250
     CCPR1L = i
     CCP1CON.5=1
     CCP1CON.4=0
     pause 250
     CCPR1L = i
     CCP1CON.5=1
     CCP1CON.4=1
     pause 250
     next
     for i=14 to 8 step -1
     CCPR1L = i
     CCP1CON.5=1
     CCP1CON.4=1
     pause 250
     CCPR1L = i
     CCP1CON.5=1
     CCP1CON.4=0
     pause 250
     CCPR1L = i
     CCP1CON.5=0
     CCP1CON.4=1
     pause 250
     CCPR1L = i
     CCP1CON.5=0
     CCP1CON.4=0
     pause 250
     next
     CCPR1L=0
     Pause 5000
Goto Main

What I’m trying to do is use the finest possible steps in controlling the PWM.
Please ignore the low case spelling on some lines of the code. I did not realized I have the auto capitalize feature enabled and I became lazy. The code works since the Micro Code Studio fixes them for me. I know is a bad and risky habit and I will try to fix it.
The code rumps up and down from 5.5V output to 9.5V in 28 steps and after 5 seconds stop repeats the cycle which should take 14 seconds without the pause. It is just to easily explore all possibilities.

The question is: Can I somehow combine the CCPR1L and CCP1CON<5:4> in one single variable so it is easier to handle in the FOR / NEXT loop and make the code more elegant?

Any input will be greatly appreciated.

Nick

[Dave]

This is how I do it for a word sized variable that gives you 10 bits of resolution:

CCPF = CCPF MIN MAXPWM
CCPR1L = CCPF >> 2 'UPDATE PWM1 UPPER 8 BITS
CCP1CON = $0C | ((CCPF & $3) << 4) 'UPDATE PWM1 LOWER 2 BITS

[pedja089]

For CCP1CON bits this is much faster
CCP1CON.5=CCPF.1
CCP1CON.4=CCPF.0

[NickMu]

Thank you Dave and Pedja089 for your input.
I will try implementing your suggestions and report back the results. Unfortunately I just started work so it has to wait for a few good hours.
I was thinking to try something like this subroutine but it was too late last night:

Set_pwm_pars:
	CCP1CON.4 = Pwm_pars.0
	CCP1CON.5 = Pwm_pars.1
	CCPR1L = Pwm_pars >> 2
	RETURN		
Main:
	FOR i = min_pwm to max_pwm
	Pwm_pars = i 
	GOSUB Set_pwm_pars
	PAUSE 250
	NEXT
	PAUSE 5000
	GOTO Main

My goal for now is to get enough steps with about 0.25V between them so the TEC will not complain.
BTW does anyone know if slow, occasional (every few minutes) 0.25V steps combined with heavy capacitor filtering will affect the TEC?

Thanks again for your input.

Nick

[NickMu]

Just a quick update: I’ve tried all three suggested methods and they all work as expected.
I will do some more hardware changes, now that I have a stable test code. I will experiment especially with frequency changes and inductors values. I’m trying to get as much efficiency as possible.
What is encouraging though is that running the prototype for long time does not generate heat at all. The main power components are cold and the current draw (from the 12V source) swings between 1A and 2.5A.
I will post the latest working code as soon as I get it cleaned up.

Nick

[NickMu]

This is the working code. It steps up through 21 steps of PWM (from 30 to 50) and then steps down (from 50 to 20). The result is an output voltage between 5.5V and 8.5V (the numbers are rounded up). This is design for testing the flexibility of the system only. Final code will implement a temperature controlling algorithm. BTW I switched to PIC16F819 since this is the PIC I'm going to use for this task.

Code based on Dave's input (183):

@__config_device_pic16f819
@__config_wdt_off
@__config_hs_osc
@__config_pwrt_on
@__config_mclr_off
@__config_lvp_off
@__config_protect_on
@__config_CCP1_RB2

OSCCON=%01110111     
DEFINE OSC 8        
            
i           var         word
pwm_out     var         PORTB.2
pwm_pars    var         word
min_pwm     con         30
max_pwm     con         50
CCP1CON = %00001100     
PR2 = 15            
CCPR1L = 0         
CCP1CON.5 = 0
CCP1CON.4 =0
                                                                                               
    High pwm_out
    Pause 500
    Low pwm_out
    T2CON.2 = 1 
    goto Main
set_pwm_pars:
     CCPR1L = pwm_pars >> 2
     CCP1CON=$0C|((pwm_pars&$3)<<4) 
     RETURN
Main:
     for i=min_pwm to max_pwm
     pwm_pars = i
     gosub set_pwm_pars
     pause 250
     next
     for i=max_pwm to min_pwm step -1
     pwm_pars = i
     gosub set_pwm_pars
     pause 250
     next
     Pause 5000
Goto Main

With the same main loop I changed the subroutine set_pwm_pars based on Pedja089 input (163):

set_pwm_pars:
     CCP1CON.5 = pwm_pars.1                
     CCP1CON.4 = pwm_pars.0
     CCPR1L = pwm_pars >> 2
     RETURN

Finally my simplified version for the subroutine (187):

set_pwm_pars:
     CCP1CON.5 = pwm_pars.1                
     CCP1CON.4 = pwm_pars.0
     CCPR1L = pwm_pars / 4
     RETURN

The output from all is the same (as expected). Cannot comment on execution speed and for my purposes it doesn’t really matter. I will only set a step, based on the temperature control algorithm, and sit on it for a while. I’m sure that the wine will not mind a few microseconds shifted response.
The one difference between the three versions is the code space required. The numbers in parentheses are showing that.

So far I’m happy with my results.
I will work on the hardware for the next few days and start working on the temperature control algorithm after that.
Before I start this new task I’m asking all the people in this forum that might have some experience in this field: Am I on the right track? Is there a better way to do it?
I love its simplicity and the fact that I fully understand how it works but in the same time I hate to find it out that is wrong after I already spent a lot of time on it.

As always looking forward for input.

Nick