TEC controller

[richard]

according to this a pid style of control may extend the life of your peltier device, thermal cycling is detrimental

https://thermal.ferrotec.com/technol.../thermalref10/

[NickMu]

Hi Richard,

Thank you for the link. I’ve been reading a lot about TEC controllers lately but did not find this one.
I know my goal might be too ambitious but so far the results are encouraging.
All I did so far is trying to create a poor man’s PID system at its simplest form.
Based on the temperature difference (act_temp – set_temp) I change the output level of my PWM.
It might be a little abrupt in the beginning but after the temperature stabilizes there will be only small steps in the voltage output (about 0.15V per step). That puts me lower than maximum admitted ripple for the TEC (5 – 10%) and it is only an occasional change not continuous like a non-filtered AC to DC voltage.
If I can find the right range (more testing is needed and unfortunately I must travel a lot for the next few weeks) I might not have to turn the TEC OFF at all and only lower the output voltage to maintain the temperature, which in turn keeps the TEC happy.
Since I’m having the fans paralleled with the TEC I must keep this min_pwm adove a certain value. Some brushless fan manufacturers recommend 5V as minimum running voltage and 7V as minimum starting voltage. That is the reason why I’m starting the system with a kick.
Not sure if this is the best approach reason for which I’m posting here.

Thank you for the input. I hope others will be interested in this project and bring new ideas to the table.

Nick

[pedja089]

Why not add another sensor on hot side of TEC to control fans independently from TEC?

[NickMu]

Hi Pedja089,

This might be one of the improvements that I thought about too, in case this first minimal approach will not work.
I’m also planning on adding a temperature sensor / thermostat on the cold side (if my testing finds it necessary) to prevent freezing of the TEC. I have some trusty DS1821 that can be programmed for different trigger temperatures.
I wanted to start simple which is the reason why I’m using the same output for fans and TEC for now.
The nice part is that the cooler is back together, I have access to all wiring and I can do all the work /changes externally.

Thank you for interest and support.

Nick

[NickMu]

This is the latest addition to the TEC controller: One pot on ADC1 sets the temperature.
I’ve done it such way so with a linear pot the target temperature can be set between 40 and 65 degrees F.

'Last release 9_16_16 Implements pot for setting set_temp
@__config_device_pic16f819
@__config_wdt_off
@__config_hs_osc
@__config_pwrt_on
@__config_mclr_off
@__config_lvp_off
@__config_protect_on

DEFINE OSC 8        

DEFINE DEBUG_REG PORTB
DEFINE DEBUGIN_REG PORTB
DEFINE DEBUG_BIT 6
DEFINE DEBUGIN_BIT 7
DEFINE DEBUG_MODE 1                
DEFINE DEBUG_BAUD 38400

DEFINE  ADC_BITS        8
DEFINE  ADC_CLOCK       3
DEFINE  ADC_SAMPLEUS    50
ADCON1=%01001110  

pwm_out         var     PORTB.2

adval           var     word
val_1           var     byte
val_2           var     byte
val_3           var     byte
new             var     word
act_temp        var     byte
set_temp        var     byte
new_temp        var     byte
temp_index      var     byte
i               var     word
pwm_pars        var     word
id              var     byte
pwm_flag        var     bit

start_ch        con     126
end_sym         con     251
samples         con     39 
divider         con     20
hysteresis      con     1
min_pwm         con     30
max_pwm         con     50
full_pwm        con     40

CCP1CON = %00001100     
PR2 = 15            
CCPR1L = 0         
CCP1CON.5 = 0
CCP1CON.4 =0
                                                                                               
        High pwm_out
        Pause 500
        Low pwm_out
        T2CON.2 = 1
'        set_temp=69
        id="1"
        pwm_flag=0
        goto main     

send_out:
        debug start_ch,id,val_1,val_2,val_3,end_sym
        return 

set_pwm_pars:
        CCPR1L = pwm_pars >> 2
        CCP1CON=$0C|((pwm_pars&$3)<<4) 
        return

read_act_temp:
        new = 0
        For temp_index = 1 TO samples
        ADCIN 0,adval
        new = new + adval
        Next
        act_temp=new/divider
        return
read_set_temp:
        new = 0
        For temp_index = 1 TO 9
        ADCIN 1,adval
        new = new + adval
        Next
        set_temp= 40 + new/100  'keeps set_temp between 40 and 65 deg. F
        return

decide:
        If act_temp < (set_temp-hysteresis) then 
        pwm_pars =0
        pwm_flag=1
        endif
        If act_temp > set_temp and pwm_flag =0 then 
        pwm_pars = min_pwm+(act_temp - set_temp)
        goto jump
        endif  
        If act_temp > (set_temp + hysteresis) and pwm_flag =1 then
        pwm_pars = start_pwm
        pwm_flag=0
        endif
jump:
        gosub set_pwm_pars
        val_1=act_temp
        val_2=pwm_pars
        val_3=set_temp
        Return
Main:
        gosub read_act_temp
        gosub read_set_temp
        gosub decide
        gosub send_out
        pause 1500 
        goto Main    
end

I think that I will settle for that now. I will just test its performance for the next week or so and wrap the project up for this particular cooler.
A friend of mine is offering me a huge 125 bottles cooler with refrigeration problems (old Freon compressor type). If this first attempt will prove successful I will spend more time saving the big cooler which is a lot nicer and a lot more expencie than this one.
I manage to get 6 brand new TECs, more powerful than the other ones (7-8 Amps) and I will start experimenting with them and different setups.
It might take a little time but I will keep you posted.

Thank you everyone for the input.

Nick