checking a TRIAC

I have been trying to repair my dishwasher for quite some time. Trouble is, it mostly works.
It sometimes does not fill with water.

When the dishwasher is started, it begins to fill with water. A float will determine whether or not to stop the water from overfilling. The float is attached to a switch, which powers a solenoid, which then allows water to enter via a fill valve. I have checked the switch, the fill valve, the float, everything is working.

I even removed the fill valve, connected the water supply lines, and connected 110 VAC directly. It works.
(on the housing, it shows 110VAC input)

I then checked the voltage going to the fill valve solenoid, and it is only 79 - 80 VAC. I am thinking this might be the problem since I have checked everything else. The voltage should be 110-120 VAC.

I have the schematic from the fill circuit, I was wondering if anyone knows what could be wrong?

How can i test the triac to determine whether or not it is faulty?

How can i test the triac to determine whether or not it is faulty?

Click to expand...

I would suggest
(1) remove it from the circuit,
(2) connect a suitably-rated mechanical switch in its place; if that ensures solenoid operation then the fault lies with the triac or the triac driver.
(3) build another circuit (e.g. lamp controller) using the triac and see it it works.

Hello,


Testing a triac isnt too hard at all really. You need a battery or two or power supply and a couple resistors and a DC voltmeter.

What you do first is look up the data sheet on the part number to determine what quadrants the triac can turn on in. You should test each quadrant. You should also look up the minimum gate current in each quadrant and the min holding current.

The setup is as follows with say a 12v battery...with the negative lead grounded...

Ground MT1, connect MT2 to the positive of the 12v battery through a 10 ohm resistor (assuming the triac is rated for 1 amp or more).
Measure the voltage at MT2, and it should be 12v, then connect another resistor of about 470 ohms from the positive of the 12v battery to the gate, the triac should turn on, which means you'll see the voltage at MT2 go down quite a bit to 2v or less. Then disconnect the 470 ohm resistor and the voltage at MT2 should stay low.

Now disconnect the 10 ohm resistor from the 12v battery, then connect it back again after a second.

Next you would reverse the 12v battery connection and do the test again.

With a second small battery or another 12v battery, you can do the test two more times with the smaller battery used to drive the gate through the 470 ohm resistor (assuming another 12v battery). The first test is with the first 12v battery with it's original connections and using the smaller battery to energize the gate with a negative drive signal, then reverse the first 12v battery and try to drive the gate with the smaller battery and resistor with a positive gate signal.

If all 4 tests work as the first one does, then the triac can trigger in all four quadrants. If only one fails, it may be the quadrant that the triac is not designed to fire in or the triac may be bad, but the date sheet will tell you this.

We can go through this one step at a time if you like. The idea is to test the triac with both polarities for MT1 and MT2 and for both polarities of the gate and MT1. This creates four distinct tests with MT1 common: one test with MT2 positive and gate positive, one test with MT2 positive and gate negative, one test with MT2 negative and gate positive, and finally one test with MT2 negative and gate negative.

johnsmith123 said:

A float will determine whether or not to stop the water from overfilling.

Click to expand...

Just to clarify
It's the CPU/ timer deck that determines how long the water valve is activated.

The float switch stops the water flow only if it overfills, For instance: the triac shorts, cpu fails to stop it, water valve stuck open etc. It's a safety not a control.

Since you have wired it directly and it works and only 80v through its own wiring, then the triac is gone bad or you have a high resistance either with a bad plug at the valve/ board or cold solder on the triac.

Also be aware that some units will act weird if the hot and neutral are reversed. Some never appliances will not even turn on if they are wired incorrectly.

cheers