I have been working on designing a circuit which can reliable control the on/off cycles of a 1000W (vacuum cleaner) Motor and a 1000W Cartridge heating element. It should be noted that the motor and heating element both have an separate circuit. The circuit is as follows:

current circuit design

It works as follows:

  • The circuit is designed for 230V Mains
  • A micro-controller (ATMega328) will detect the zero crossing of the grid and decide whether to turn the motor/ heating element on or off for the following period.
  • If the triac must be fired, PD4 will be pulled to high, triggering the photo triac, which will trigger the BTA208S-800E Triac.
  • The snubber networks consisting of the resistors/ capacitors are supposed to protect the Triacs from misfiring.

Though, there are a few unanswered questions I would like to ask: First, the motor causes a lot of noise on the grid, does the snubber filter this unwanted EMI or would it need separate filtering (what/how)?

In the datasheet of the BTA208S-800E I found that the thermal resistance from junction to ambient is: 75K/W. How do you calculate the amount of Watts dissipated in the Triac? I plan on using a copper ground plane as heat sink.

Most European wall outlets have 2 pins for the power and a separate for earth. Can I connect the earth wire to the enclosure where both the heating element and motor are placed in?

Last, I would like to detect if the motor is not malfunctioning (if it is, both the heating element and motor should be shut down). I was wondering if detecting this would be possible by monitoring the back EMF caused by the motor?


1 Answer 1


You are asking quite a few questions. I'll answer the one about calculating heat sinks and power dissipation for a triac such as the BT208. First, to determine the power dissipation refer to the datasheet.

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From the RMS current you can find the power dissipation. Since you're not doing phase control, \$\alpha\$ = 180°. So if the motor draws 4A RMS, the power dissipation will be about 5.2W maximum. Note that this says nothing about watts in the load, it's load current that makes the triac heat.

If (and it's an important if) your mounting is the same as the quoted mounting (size of copper pad and so on) and you achieve 75°C/W then you can calculate the temperature rise. Also pay attention to figure 1 that tells you that you should not let the heatsink temperature get much beyond 100°C and the absolute maximum junction temperature of 125°C. Conservative design would keep well below those numbers even on a very hot day. So if your ambient could be 60°C inside the enclosure, and you allowed the heat sink to get to 90°C, then you could allow 30°C rise, or about 2.5W, so it's good for about 3A (with those assumptions).

  • \$\begingroup\$ If the triac heatsink could be placed in the pre-heated airflow from the fan, would it be sensible to monitor the temperature of the triac to determine if the fan motor is working? \$\endgroup\$ Jun 26, 2014 at 20:32
  • \$\begingroup\$ You want to keep the triac as cool as possible. You could suck air over the triac heatsink before it goes to the heater. \$\endgroup\$ Jun 26, 2014 at 20:43

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