Detecting triac failure whether under load or not

  • Thread starter Thread starter THX-1138
  • Start date Start date
  • Tags Tags
    Failure Load
AI Thread Summary
A user is designing a triac-based solid-state switch for a 120VAC receptacle, controlled by a low-voltage DC circuit via an opto-isolator. They seek to detect faults in the diac and triac, specifically when the diac fails to conduct or when the triac conducts incorrectly. Suggestions include moving the triac to the Hot lead and using a large resistance with an analog optocoupler to sense line voltage after the triac. The user is considering using a microcontroller for better fault detection but is currently focused on simpler solutions due to multiple circuits being implemented. They are also exploring the use of a sensing transformer for voltage detection and are still determining the appropriate specifications.
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:

Thread '3-terminal device external modeling'

Very basic question. Consider a 3-terminal device with terminals say A,B,C. Kirchhoff Current Law (KCL) and Kirchhoff Voltage Law (KVL) establish two relationships between the 3 currents entering the terminals and the 3 terminal's voltage pairs respectively. So we have 2 equations in 6 unknowns. To proceed further we need two more (independent) equations in order to solve the circuit the 3-terminal device is connected to (basically one treats such a device as an unbalanced two-port...
View full post »

Thread 'Question about the voltage between two charged capacitors'

suppose you have two capacitors with a 0.1 Farad value and 12 VDC rating. label these as A and B. label the terminals of each as 1 and 2. you also have a voltmeter with a 40 volt linear range for DC. you also have a 9 volt DC power supply fed by mains. you charge each capacitor to 9 volts with terminal 1 being - (negative) and terminal 2 being + (positive). you connect the voltmeter to terminal A2 and to terminal B1. does it read any voltage? can - of one capacitor discharge + of the...
View full post »
Thread 'Weird near-field phenomenon I get in my EM simulation'

Thread 'Weird near-field phenomenon I get in my EM simulation'

I recently made a basic simulation of wire antennas and I am not sure if the near field in my simulation is modeled correctly. One of the things that worry me is the fact that sometimes I see in my simulation "movements" in the near field that seems to be faster than the speed of wave propagation I defined (the speed of light in the simulation). Specifically I see "nodes" of low amplitude in the E field that are quickly "emitted" from the antenna and then slow down as they approach the far...
View full post »

Similar threads

Why Did My DC Motor Fail Under Load?
Replies
8
Views
3K
What Could Cause Odd Failures in a Solid State Relay?
Replies
4
Views
7K
Ten reasons why HIV could not cause AIDS , from my inbox
Replies
14
Views
16K

Hot Threads

Recent Insights

Back
Top