What is a "TRIAC"??

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new guy
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What is a "TRIAC"??

Post by new guy » Tue Jul 25, 2006 4:38 pm

I read a tutorial on "TRIAC'S" . In order to better understand what it is and how it works, it is easier if I know what old mechanical component it replaced and what that component was used for. I work on elevators and the manufacturer's technicians are always replacing triac's every time there is a problem with the elevators operation. They are small (fit on the tip of my index finger) square and black and have thee pins sticking out one side. Is there a way to bench test these components.

Robert Reed
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Post by Robert Reed » Tue Jul 25, 2006 5:03 pm

new guy
Are you familiar with SCR's (Silicon contolled rectifiers) used mainly in DC circuits. The Triac is very simalar to SCR's in inverse parrallel. Basically, these replace mechanical relays as do SCRs. Where as the scr is used in dc circuits, the triac is its counterpart for AC circuits ( conducts in both directions). The three leads on the package are MT1 (generally connected to hi side of circuit) ; MT2 Connected to low side of circuit) and GATE The input that triggers MT1 & Mt2 into conduction. The gate will usually require 20 - 50 ma to trigger the device except for sensitive GATE versions which operate anywhere from 100 - 500 microamps. Triacs current handling run any where from 0.5 amps tp 40 amps in standard solid state packaging. You can test these by connecting a suitable load to it for the ACV you are feeding it with and then connect a 1 to 10 K resistor from Gate to MT1 of suitable wattage. It should turn on at this point and by increasing resistance, you can also determine GATE sensitivity. There is a little more to this as Quadrants come into play here that will alter these results, but these are thec basics.

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Bob Scott
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Post by Bob Scott » Tue Jul 25, 2006 6:18 pm

Hi Guy,

A triac is a power switching device that comes from a family of devices called thyristors, and includes the SCR (Silicon Controlled Rectifier), Triac and Diac. In my opinion and in my experience, this whole family of devices has a high rate of failure. This explains why your factory techs are always replacing them, why the "solid state relays" made with Triacs in stage lighting controls are always failing. Has anybody else noticed that room light dimmers are unreliable? They use Triacs too.

The word "thyristor" is a combination of the words "thyratron" and "transistor". (A thyratron is an old type of vacuum tube used as a switch where the grid input is used as a trigger to turn the thing on.)

A Triac is like two SCRs wired back to back so it can switch AC power. An SCR operates like the above mentioned Thyratron and is used for DC power. The gate input of the SCR is used as a trigger to turn on the SCR like a switch. Once triggered on, the SCR will not stop conducting current, no matter what gate voltage or current is applied, until the current is interrupted by some external means. Hence, Triacs are handy for switching AC on and off because AC current goes through zero (turns current off) every 1/120 of a second at 60 Hz.

The mechanical component it replaces is a RELAY. That's a plain old solenoid controlled mechanical power switch.

Regards,
Bob :cool:

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Chris Smith
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Post by Chris Smith » Tue Jul 25, 2006 7:57 pm

Failure with these devices usually occurs from poor design.

Triacs, [Diacs] and SCRs are actually very reliable if you don’t over heat them like most, spike their gate like most, and instead of over rate them, under rate them so they can last.

Power outages and power spikes however can blow them as will the soft start up designs.

They are nothing more than a reliable relay, either for AC or DC designed from the common Diode and the common Zener, added with a junction in the middle to set them off “whenâ€

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MrAl
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Post by MrAl » Tue Jul 25, 2006 10:51 pm

Hi there,

There are ways to test triacs but there are quite a few tests that
have to be performed. To name a couple:

gate turn on current (bi directional, with bi directional main current)
holding current (bi directional main current)

The above 'two' items comprise a total of 6 individual tests, where
the levels of the currents must be measured for both the gate or
the main terminals. The measurements are often different depending
on the direction of the gate and main currents:

gate turn on current positive with positive main current,
gate turn on current positive with negative main current,
gate turn on current negative with positive main current,
gate turn on current negative with negative main current,
holding current with positive main current,
holding current with negative main current.

Also you could test voltage drop in both directions at the max
current rating.


Even so, when reliability is in question, after a high voltage shock
the triac might still appear to work normally, but could blow out
more easily...i dont remember what this is called, but it leads to
the practice of replacing devices like this when there is the possibilty
that they received a high voltage over their rating for a short period
of time, not long enough to blow it out. In any case, there is no
direct non distructive test for this. This could be why the manufacturer
recommends replacement when any problem comes up even though
the device is not blown out.

BTW the triac is often used in place of a relay so there are no contacts
to wear out. The triac, when conducting, drops a small voltage unlike
the relay however.

To perform a static test you could set up a jig to push current through
the device when the gate is energized, turning up the gate current
little by little until the triac turns on (gate turn on current). Then, reduce
main current until the triac turns off (holding current).
LEDs vs Bulbs, LEDs are winning.

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Post by cato » Thu Jul 27, 2006 3:24 pm

You don't seem to have gotten the simple answer you were hoping for, so I'll give it a try:


Tricas are a solid state means for providing a High Voltage (eg. 120, 220) contact closure output for digital electronic based systems.

They replace relay contacts. As you have learned, they are somewhat more delicate than relay contacts.

You may be familiar with Opto 22 modules. The AC modules have triacs inside.

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Post by Robert Reed » Thu Jul 27, 2006 4:56 pm

Newguy
Since this post is giving you more and more detail, I will add a few more that may be of interest to you. As stated, triacs replace mechanical relays, but they also replace rheostats and small variacs ( both mechanically operated devices) That were formerly used in light dimming and similar functions.This type of triac operation is known as PHASE CONTROL in which each cycle of ACV is "chopped" to increase or decrease the available power in that cycle and consequently the available power to the load (i.e.-dimmer, brighter as a lamp ; Faster-slower as a motor). As to my prior mention of Quadrants, there are 4 modes (or quadrants) of triggering the triac and these are 4 scenarios of GATE voltage phase in relation to either MT1 voltage phase or MT2 voltage phase. Each one will show different trigger sensitivities and have to a lesser effect their own pros and cons. One main headache with SCR, Triac design is a phenomena known as transient turn on. When the circuit sees transients from other sources applied across the triac or scr, and if these have a fast enough risetime and large enough amplitude( called delt V/ delta T), they will literally turn on the device without a gate signal present ( Well in actuality, there is an internal gate signal generated by the internal capacitances reacting to that very same transient).This is an annoying and sometimes dangerous situation, and as wonderful as sensitive gate triacs sound, they should be avoided whenever possible in favor of standard gate triacs which are MUCH less sucseptable to this. Cures run the gamut of a simple Shunt diode for SCR loads to complicated specific modeling circuits some inductive loads. Thyristor circuits have proved to be quite reliable if original design parameters are met. One short coming I see in a lot of cheaper commercial products is no provision to keep from overdriving the GATE, either voltage or wattage, and inadequate heat sinking (every bit as important here as with any other solid state device). I hope this is within the range of your electronic knowledge, If not forgive the overflow of technical information. For some reason, I just felt like rambling on tonight :smile:

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Bob Scott
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Post by Bob Scott » Thu Jul 27, 2006 7:38 pm

Robert ,

Thanks for expressing what I've been thinking. This supports my contention that the design Crydom solid state relays used in studio lighting control and all of its immitators are unreliable because their design is basically drek, merde if you excuse my Dutch and French.

Regards,
Bob :cool:

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Post by Robert Reed » Thu Jul 27, 2006 9:08 pm

Bob
One area of poor design I have seen in using solid state relays, be it Crydom or others was in Tower light controls. The manufacturer of these panels incorporated a lot of SSR's in their product. The relays are usually potted with an aluminum backing-maybe 2"x4"x 1/8" thick.This is merely meant to be a bonding device to a proper heat sink for it's application, much like the metal tab on TO-220 cases. Apparently the panel makers paid no heed to this and assumed the backing provided adequate heat sinking. I don't know if this was through ignorance or from misreading the SSR spec sheets properly. At any rate they were installed as is with no proper bond to a lager heat sink. These relays handled very large bulbs and handled large amounts of current. Time and time again I saw them blow out at different sites because they could not scrub off enough heat. Of many situations where I have seen these installed, they all had improper heat sinking, and this was so common, that I almost started beleiving that was the way to use them. That is until I read The SSR's spec sheet and did some heat transfer calculations myself to find out they were grossly undersinked which probably led to their failure. But I still have to admit, other than contact pitting ,I still always have a comfortable feeling with good old mechanical relays.

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