PWM Frequency and the Solid State Relay

How Thermocouples Work

The solid state relay is built using a TRIAC (or something similar, like two thyristors) because it needs to handle large alternating currents. Remember that we are trying to put 120V AC at 60 Hz (these numbers are North American) through this relay.Once you trigger a thyristor, it starts to conduct, but it will not stop conducting until the voltage reaches zero.This also means if you tried to power the heating element with a DC power source, once the relay has been triggered, it will stay on forever. So do not attempt to use a DC power source. (Also, if you did use DC, you would probably fry the USB charger).The fact that thyristors won't stop conducting will make high frequency PWM ineffective as the PWM duty cycle won't correlate to the power delivered to the heating element. When a pulse from the PWM output ends, it doesn't mean the AC current is turned off.If a low PWM frequency is used, then you have more control over how much power is being delivered. If the PWM frequency is 1 Hz, PWM duty cycle at 50%, and the AC power is 60 Hz, then you are basically saying "I want to let about 30 out of 60 oscillations through per second". The relay will try to turn it self off 60 times per seconds at regular intervals, unless you keep the input signal high.The gallery includes a few diagrams that illustrates what I'm saying in terms of voltages and waveforms.The Crydom solid state relay is actually more complicated because it has special zero-cross triggering. This means the relay will conduct immediantly after the voltage is zero (instead of at any time) and turn off immediantly when the voltage is zero again if the trigger is removed. What I am saying still applies. The output waveform will always start at 0V and end at 0V, which is good for equipment that doesn't like being turned on too fast.There are special solid state relays that can control the output waveform according to a linear input (and thus, filtered high frequency PWM can be used). But these cost more, and the heating element doesn't react fast enough for this to be useful anyways.Wikipedia about thermocouples http://en.wikipedia.org/wiki/Thermocouple Maxim application note 4026 http://www.maxim-ic.com/app-notes/index.mvp/id/4026 I used a K type thermocouple, with the AD595AQ thermocouple amplifier chip. The K type thermocouple is made of a junction between chromel and alumel, where a small voltage is generated proportional to the temperature difference at the junction. The AD595AQ is designed to amplify the small voltage between the chromel and alumel to a voltage that is 10 mV per degrees Celcius. The AD595AQ features "cold junction compensation", which basically it means it can use room temperature as the reference voltage.Remember how I said you must use the same metal for the wire that connects the thermocouple to the terminal block? For example, using copper, the voltage beween the alumel and copper would be cancelled out by the voltage between the chromel and copper, while the left over voltage would be proportional to the temperature. As another example, if you used copper connected to the chromel and steel to connect the alumel end, then your final voltage reading would be the voltage between the alumel and chromel plus the voltage beween the copper and steel, which is not accurate.