Hot Plate temperature controller code for a raspberry pi

This contains code to control a hot plate and display the temperature. The raspberry pi is connected to a thermocouple, a solid-state-relay, and an OLED display.

Running Instructions

The following steps are required after power-on of the Raspberry Pi. Python is run twice because of a startup bug in the code.

• sudo pigpiod
• python3
  • import control
  • exit()
• python3
  • import control
  • control.runScript()
  • ... as desired after it completes
  • control.runScript()

Pinouts

HOTBIT = 18 -- the pin that controls the relay

FANBIT = 22 -- the pin that controls the fan

graphicslib

graphicslib contains the graphics routines

pigpio

pighelp is a simple helper for the PI-GPIO gpio code for the raspberry pi.

see: https://github.com/joan2937/pigpio for the source

Power requirement

At 100 percent duty cycle it uses 1000W

Measurements

Percentage	Stable Temperature F
20	293
25	340
30	375
35	425

Reflow Profile

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see: http://micro.apitech.com/pdf/Surface_Mount_Reflow_Soldering.pdf

• Initial Pre-Heat Stage. PWBs should be preheated prior to solder reflow. During the pre-heat stage, the solder paste begins to dry as volatile ingredients are allowed to evaporate. The initial pre-heat stage takes place during the first 90 seconds of the reflow profile as the temperature is slowly increased from room ambient conditions to approximately +155°C.

• Flux Activation and Pre-Heat Soak Stage. Following the initial pre-heat stage, the temperature is gradually increased to +183°C over a period of approximately 90 seconds so the flux in the solder paste can clean the bonding surfaces properly. During the pre-heat soak (also know as the flux activation stage), the solder paste and soldering surfaces should be roughly the same temperature.

• Ramp Up. The PWB now enters the solder reflow stage. Over a period of 30 seconds, the temperature should be increased to the appropriate peak reflow temperature shown below. Depending on package dimensions, exposure time at peak temperature should be minimized (re. J-STD-020A).

• Peak Reflow Temperatures: • +220°C to +225°C for IR and FC Reflow Systems. • +215°C to +220°C for most VPR Systems.

• Ramp Down. The reflow stage is completed as the temperature is reduced to +183°C (the original melting point of the solder paste) over a period of approximately 30 seconds. Caution - Over baking the solder paste and/or exceeding the glass transition temperature of the FR-4 printed wiring board material should be avoided.

• The profile is completed as the temperature is gradually reduced from +183°C to < +40°C over a period of approximately 3 minutes. The gradual cooling of the printed wiring board after reflow is important. It is during this period that the molten solder solidifies to form a strong joint fillet.

Summary

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• Pre heat: <2.5C per second. Get to Flux activation temp of 160C.
• Soak: 60-90 seconds (120 seconds max). Rate of 0.5-0.6C/sec up to Liquid temp (183C).
• Reflow: Go from 183C to 210-220 at 1.3-1.6C/sec then return to liquid (183). 30-50 seconds recommended. 70 seconds max.
• Cooling: 180 secs minimum.

Fan Testing

I've got a high powered 12V fan and an aluminum foil shroud. This is all Fahrenheit and values per second.

Fan	Heating Rate	Cooling Rate
None	1.2	-.35
6V	1	-1.2
12V	1.1	-1.6

So, the heating rate isn't quite linear. It starts at maybe 1.4 (when cold) and decreases to 1.0 when hot.

With the 500W heating cartridges

type	0-200F	200-300F	300-400F
250W	1.2	1.3	1
500W	2.4	2.6	1.6