Project #1 – PID Temperature controlled oven, start of project

I’m building a PID temperature controlled oven.  I’m going to use a triac (BT138), thermistor and microcontroller to control the oven.  I speculate that the oven does not have lots of electronics between the power plug and the element, therefore I’m going to try to control the element’s heat via the power supply directly.

(Note: I’m not an expert in power electronics.  I studied mostly small-signal electronics and software development.)

This is how I designed the temperature sensor circuit that connects to the microcontroller:

A0 connects to the analogue input for the Atmel chip (ATMega328P), and I use the op-amp to buffer the input as the datasheet states that the ADC input should not have more than 10k impedance.  I use the Arduino circuit board.

The switching circuit I designed as follows:

The forward voltage of Vce has worst case maximum of 0.4V at 100mA, and the gate voltage trigger of the BT138 is approximately 2V with a minimum of 0.7V.  Therefore the current through the resistor will be approximately (5 – (0.4 + 2)) / 10 = 240mA as a minimum worst case, and at most (5 – (0.3 + 0.7)) / 10 = 400mA as the maximum worst case.  This gives an (0.4)^2 * 10 = 0.2W power dissipation for the 10 ohm resistor.  A 1/2 Watt resistor should suffice.

There should be better opto-isolators for triac circuits, but this one I already have on hand.  D0 connects to a digital output on the ATMega chip.  I use the BT138 600E as the voltage in South Africa is 230V and the oven is 1500W (the oven part of it).

The oven should not draw much more than (1500/230)= 6.52A, but I’m using an oversized triac (should be able to handle 12A) to make sure that it doesn’t overheat or otherwise break with an overcurrent fault.

Care should be taken to isolate the high-voltage parts from any people or animals that might not enjoy frying.  To do that, I’m going to put the high-voltage parts (the half of the circuit with the optocoupler, triac and power supply) in a separate enclosure.

I’m going to source the parts I don’t already have and post again to indicate progress soon.

Keep well!

-Peter Peiser

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