Reflowino

use an ESP8266, an ADS1115, two NTCs, an SSR and a cheap electrical toaster grill to build an SMD PCB reflow oven

Status

Work in progress.

• Built into oven. Works fine. Currently no ADS1115 but internal A0 and only one NTC.
• Temperature can be set via webpage and is maintained by pid loop.
• PID parameters need optimization. 20% overshoot.
• A pwm style fixed duty cycle of the SSR can be controlled via webpage
• OTA is working to avoid touching high voltage stuff
• Syslog works. Needed to give A0 to WIFI ~10ms within 40ms
• Theory for temperature measuring is done (see below). Maybe needs a bit more calibration.

Todo

• PID parameters via webpage
• Define temperature profile via web page
• eans to store/retrieve profiles (could be spiffs, EEPROM, MQTT persistent topics, ...)
• Provide status via Neopixel colors, mqtt, webpage

NTC Temperature Measurement

Formula for getting temperature in K from measuring Rntc

T = 1 / (1/Tn + ln(Rt/Rn)/B)  (1)

• Tn = Temperature where Rntc = Rn. Usually 25°C -> 298.15K, but check datasheet to be sure.

• Rn = NTC Resistance at Tn. Usually 10k .. 100k. Better measure yourself, can differ quite a bit from datasheet.

• B = B-constant from datasheet. Often 3950 K.

• Rt = measured resistance

  Rt,min = Rn * e^(B/Tmin - B/Tn) with Tmin = 0°C -> Rt,min ~ 340k Rt,max = Rn * e^(B/Tmax - B/Tn) with Tmax = 260°C -> Rt,max ~ 290

Using an ADS1115

Using an ADS1115 you can compare Vcc to Vntc like the following sketch. Since using Va3 as Varef === 0 it eliminates errors at Vax due to Vcc fluctuations. Comparing Ax to A3 with A3 === 0 gives Ax ~ 0 for very high Rntc and Ax ~ -Amax for very low Rtc.

Vcc ---+--- Rv ---+--- Rntc --- Gnd 
       |          |
       A3         Ax (Vcc=0...-32667=Gnd)

Voltage divider and some algebra with ohms law gives

Rntc = -Rv * (1 + Amax/Ax) (2)

Sanity check: If Rv chosen to be same as Rn, then Vntc at Ax should be Vcc/2 at 25°C. Vcc/2 should result in Ax = -Amax/2. Put this Ax in (2) yields Rntc = Rv --> as expected. Put this Rntc as Rt in (1) gives T = 1 / (1/298.15 + 1/3950*ln(1)) = 298.15K = 25°C --> as expected.

Choosing Rv: 100k is good for low to medium range up to 150°C but 10k gives ~10x better resolution at the high temperatures around 250°C that we need. An Rv of 1k gives bad resolution at room temperature, so my Rv will be 10k.

Using ESP8266 ADC via NodeMCU A0

This method is less accurate, but maybe good enough for the usecase:

                          ADC
                           |
              +--- 220k ---+--- 100k --- Gnd
              |
              A0 (Vcc=1023...0=Gnd)
              |
Vcc --- Rv ---+--- Rntc --- Gnd

Calculation

Vcc/(Rv+Rntc) = Vntc/Rntc                | * (Rv+Rntc) * Rntc
-> Vcc * Rntc = Vntc * Rv + Vntc * Rntc  | - (Vntc * Rntc)        
-> Rntc * (Vcc-Vntc) = Rv * Vntc         | / (Vcc-Vntc)
-> Rntc = Rv * Vntc/(Vcc-Vntc)           | * (1/Vcc)/(1/Vcc)
-> Rntc = Rv * Vntc/Vcc/(1 - Vntc/Vcc)   | Vntc/Vcc = A0/Amax (Amax = 1023)
-> Rntc = Rv * A0/1023 / (1-A0/1023)     | * (1023/1023)
-> Rntc = Rv * A0 / (1023 - A0)

Solve for A0 to do some test calculations: Rntc = Rv * A0 / (1023 - A0) | * (1023 - A0) -> Rntc * 1023 - Rntc * A0 = Rv * A0 | + (Rntc * A0) -> Rntc * 1023 = (Rv + Rntc) * A0 | / (Rv + Rntc) -> Rntc * 1023 / (Rv + Rntc) = A0

PID Loop or Bang Bang?

I can only switch power on or off -> classic usecase for bang bang algorithm -> switch on if below (Tset - lower tolerance) and switch off if above (Tset + upper tolerance). Choose tolerances such that T is close enough to Tset (-> low tolerances), switching is rare enough (-> high tolerances) and mean of T over an on/off cycle is as close as possible to Tset (-> since cooling is slower than heating, lower tolerance will be less than high tolerance).

But since I can switch on/off easily and relatively fast with SSR I can also do a kind of slow PWM and use that for PID control. Should result in T following Tset with less oscillation. Maybe that means even less stress on material, but could also mean more EMV due to more high power switching.

Used Hardware

• Steinborg Mini Backofen 13 Liter | Timer | herhausnehmbares Krümelblech | 1200 Watt
• Haobase Temperatur Controller AC 24V-380V Output Solid State Relais 25 A
• LEORX NTC 3950 100k Thermistoren mit Teflon 5 PCS

Pretty good oven for that purpose. Could keep all controls (Max tmperature, heating from bottom and/or top, 60min timer). ESP8266 is only on while timer is on.