Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Triac problem.

Status
Not open for further replies.

robyelectron

New Member
The annexed circuit is used to drive a fan motor with a nominal current of 3.5 A (rms) with permanently inserted starting condenser.
Casually it happens that, when in modulation, the first quadrant driven halfwaves are OK, while the third quadrant driven halfwaves come entirely to the load. The phenomena is occasional, and goes on some minutes, then all returns OK. It seems the triac doesn't switch off, because of itself or because of the optotriac, but I still haven't found the real reason.
In fact, analyzing some parameters, I can see the following:
1. The opto led is switched off (every halfwave), about 300 microseconds before the voltage zero-crossing (it means about 1.3 ms before the current zero-crossing, because current has a delay of about 1 ms).
2. The dV/dt when triac switches off is about 1 V/us, both for opto and triac (in general, the max ambient temperature is 70 °C, where max dV/dt for opto is 4 V/us; the triac data sheet doesn't report the commutation dV/dt).
3. The commutation dI/dt (when triac switches off) is about 3.4 A/ms (the data sheet gives a maximum of 22 A/ms).
These points don't help to catch the reason of the problem, because the data fall under the max components specifications.
Note: I know that the circuit has some 'errors' (44 ohm resistor is too low, in fact in the opto flows, for a short time during the driving on third quadrant, a current up to 1.4 A, over the 1 A limit; the opto snubber, suggested for inductive loads driving is missing; the opto is a little 'weak' and it should be better to use for example the MOC3052 that has better performances in terms of dV/dt and maximum allowable voltage; eventual other ones). But, I would like to understand the problem on this circuit.
Suggestions?
 

Attachments

  • triaccirc.jpg
    triaccirc.jpg
    26.6 KB · Views: 3,625
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top