Detecting triac failure whether under load or not

  • Thread starter Thread starter THX-1138
  • Start date Start date
  • Tags Tags
    Failure Load
Click For Summary

Discussion Overview

The discussion revolves around the use of a triac as a solid-state SPST switch for controlling a 120VAC receptacle, with a focus on detecting faults in the triac and diac components. Participants explore methods for fault detection and circuit design considerations, including the integration of low-voltage DC control signals.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant describes their intention to use a triac controlled by an opto-isolator and seeks advice on detecting faults in the diac and triac.
  • Another participant suggests that the triac should be connected in series with the Hot lead rather than the Neutral lead and proposes sensing the high voltage after the triac to determine if the circuit is functioning correctly.
  • There is a discussion about using a large value resistor and an analog optocoupler to sense line voltage after the triac, though clarity on this method is lacking.
  • One participant expresses uncertainty about how to visualize the proposed sensing method and inquires about the use of a microcontroller for fault detection.
  • Another participant questions the utility of fault information without a microcontroller, suggesting that a small PIC with an internal ADC could be beneficial.
  • The original poster indicates that the fault signal will simply activate an indicator light and mentions the challenge of designing multiple circuits for three receptacles.
  • There is a mention of needing to determine the rating and part number for a sensing transformer, with the original poster admitting to confusion about this type of transformer.
  • The original poster later notes an error in their circuit diagram regarding the orientation of the transformer.

Areas of Agreement / Disagreement

Participants express differing views on the necessity and method of integrating a microcontroller for fault detection, and there is no consensus on the best approach for sensing the line voltage or the specifics of the transformer required.

Contextual Notes

Participants have not fully resolved the details of the sensing method or the specifications for the transformer, and there are uncertainties regarding the circuit design and fault detection strategy.

THX-1138
Messages
4
Reaction score
0
[First post here, please make allowances..]

I want to use a triac as a solid-state SPST switch for a normal 120VAC receptacle. The on/off state will be controlled by a low-voltage DC circuit through an opto-isolation MOC3020.

http://ken.coar.org/images/opto-triac.png

So far so good. However, I want to be able to detect faults in either the diac or the triac -- faults being defined as:

  • the diac conducting when TP1 is low, or failing to conduct when TP1 is high; and/or
  • the triac conducting when the diac isn't (VTP2 = 0), or failing to conduct when the diac is.

I'm basically a low-voltage DC kinda guy, so I'm unclear on how to detect the lack of signal at TP2 and TP3 in general, or at TP3 when nothing's plugged into J1 in particular.

Any/all pointers appreciated..

Thanks!
 
Last edited by a moderator:
Engineering news on Phys.org
THX-1138 said:
[First post here, please make allowances..]

I want to use a triac as a solid-state SPST switch for a normal 120VAC receptacle. The on/off state will be controlled by a low-voltage DC circuit through an opto-isolation MOC3020.

http://ken.coar.org/images/opto-triac.png

So far so good. However, I want to be able to detect faults in either the diac or the triac -- faults being defined as:

  • the diac conducting when TP1 is low, or failing to conduct when TP1 is high; and/or
  • the triac conducting when the diac isn't (VTP2 = 0), or failing to conduct when the diac is.

I'm basically a low-voltage DC kinda guy, so I'm unclear on how to detect the lack of signal at TP2 and TP3 in general, or at TP3 when nothing's plugged into J1 in particular.

Any/all pointers appreciated..

Thanks!

Welcome to the PF.

First, your switch triac D1 needs to be in series with the Hot lead, not the Neutral lead as you have it shown now. What does the symbol in the Hot leg represent? Is it a motor or something?

To tell if you have a fault, I think the best/easiest thing would be to see if you have the high voltage after the switch D1 (after you move it over to the Hot side). If you can sense that voltage and process it, you will know if your circuit is working correctly.

To sense that line voltage after the triac, you can use a large value resistance and an analog optocoupler or similar arrangement. Do you have a microcontroller (uC) that you are going to be using to control the circuit? Does it have an ADC on-board?
 
Last edited by a moderator:
berkeman said:
Welcome to the PF.

First, your switch triac D1 needs to be in series with the Hot lead, not the Neutral lead as you have it shown now.

Circuit diagram updated. It was just meant to be illustrative, anyway, not exact. :)

What does the symbol in the Hot leg represent? Is it a motor or something?

As I mentioned, it's a receptacle. Whatever is having its power switched by the remote DC circuit.

To tell if you have a fault, I think the best/easiest thing would be to see if you have the high voltage after the switch D1 (after you move it over to the Hot side). If you can sense that voltage and process it, you will know if your circuit is working correctly.

Indeed; it's the 'how to sense it' part that's my problem.

To sense that line voltage after the triac, you can use a large value resistance and an analog optocoupler or similar arrangement.

How do you mean? I'm not visualising this.

Do you have a microcontroller (uC) that you are going to be using to control the circuit? Does it have an ADC on-board?

Nope, just a simple momentary SPST switch, a 555 monostable, and TTL signals.
 
Hmm, if you don't have a uC, what are you going to do with the fault information? A small PIC with an internal ADC costs less than $1...
 
Simply enough, the fault signal is just going to red-light an indicator.

Yeah, a µC is cheap enough, but this entire circuit is going to be triplicated; I have three receptacles to be controlled by three separate circuits. I may end up using one eventually, but for now I'm just trying to sort out the sensing details.

This is where I've gotten so far. Aside from putting a couple of Zeners across the input to V02 for overvoltage protection, I'm now trying to figure out the rating/part number for the sensing transformer.

http://ken.coar.org/images/opto-triac3.png

And I have no clew about the transformer. I'm used to step-down and step-up power transformers, not this sort of thing. So I'm still feeling my way.
 
Last edited by a moderator:
Meh. I just noticed I put the transformer in backwards.

http://ken.coar.org/images/opto-triac4.png
 
Last edited by a moderator:

Similar threads

Why Did My DC Motor Fail Under Load?
  • · Replies 8 ·
Replies
8
Views
3K
What Could Cause Odd Failures in a Solid State Relay?
  • · Replies 4 ·
Replies
4
Views
7K
Ten reasons why HIV could not cause AIDS , from my inbox
  • · Replies 14 ·
Replies
14
Views
17K