Other ways to check your defrost thermostat when you don’t have ice around it, is to run the heat pump on heat mode and carefully remove the black wire from the relay on top of the defrost board. This will stop the fan from running and the condenser coil should start frosting, making the temperature of the defrost thermostat 32° ± 3°F and checking the resistance until it closes reading 0.0 ohms indicating that is working properly. If not, it may be stuck open. Replace the thermostat. You can also remove the defrost thermostat from the heat pump and insert it into a cup with ice and water. Make sure the water its 32° ± 3°F, that way you know if it closes, the defrost works. If not, replace.


The other problem could be it’s stuck closed. You always need to know the temperature that surrounds the defrost thermostat. If it is attached to a pipe, you need to know the temperature of that pipe. Otherwise, you will be guessing. In the photo below, we have a temperature above 65° ± 5°F. We know the defrost thermostat should be open and on this defrost sensor it is open.

Harold Nelson

Technical Service & Support Manager

January 17, 2025

How to Measure the Voltage on Triac Output

A triac is an electronic switch, very much like a transistor. Triacs are commonly used in light dimmers, motor speed controls computerized control circuits.

Green circuit board with three red arrows pointing to components on the left side.

What is a Triac?

A TRIAC (TRIode for Alternating Current) is a 3-terminal AC semiconductor switch.


Composed of 2 thyristors facing opposite directions such that it can conduct current in either direction.


MT1 and MT2 are current carrying terminals while the Gate terminal is used for triggering by applying a small voltage signal.


Once triggered, it continues to conduct current until the current falls below a threshold value.

Triac symbol with gate labeled G and terminals MT1 and MT2

Triacs are used on all Carrier and Bryant model thermostats, FE fan coil boards, furnace boards, NIM’s, zone board damper actuator outputs, etc., just to name a few. Plus, many other manufacturers use Triacs.


Triacs are often used because they require less space on a circuit board than a relay. If you don’t see a relay on the board, the output is most likely driven by a Triac.

Pros and Cons

Pros:


  • Better than a transistor as it has much better current surge rating – it can handle more current as it simply turns on more.
  • Inexpensive compared to relays.
  • Eliminates mechanical wear in a relay


Cons:


  • Can’t manually control turn-off with the gate; must turn off by stopping current through the device via the terminals.

How do you measure the voltage?

A triac is essentially an electronic switch. There is a difference, though, in that a triac is not a dry contact and it leaks a very small amount of current when it is “off”.


If you were to check the voltage across a triac with a voltmeter when there is no load connected to the output, you will read a voltage. If a load is connected and the triac is “off”, the voltage reading across the outputs will be 0.


This can be very confusing for someone not familiar with triacs. It is possible to see a voltage when the triac is not actually being signaled to turn on.


A voltmeter has a very high resistance circuit in it. This is so that only a very small current to flow through it so voltage can be measured.


A solenoid, relay, contactor, etc. does not have a high resistance and allows current to flow voltmeter. (remember V=IR)


When a triac is connected to one of these loads the voltage across it then is shown to be zero when the triac is turned “off”.


A load must be connected to the triac output before measuring the voltage. A simple test, if you don’t have a voltmeter, would be to connect a relay to the output. If the relay energizes, you know the triac is “on”, and if not, the triac is “off”.

24V multimeter testing a triac circuit with a disconnected load labeled “Not Energized”

Triac output with no load connected, triac not energized

Multimeter testing a triac circuit labeled 24V, showing energized state and component connections

Triac output with load connected, triac energized

Disclaimer: The technical statements, information and recommendations contained herein are believed to be accurate as of the date hereof, but Mingledorff’s does not make representations or warranties, express or implied, as to its accuracy, its completeness, or the results to be obtained. The information is being provided for informational purposes only and is intended for use by persons having adequate skill and expertise regarding the proper selection, use and application of the products and recommendations and at their own risk and discretion.

< Older Post Newer Post >

More to Explore

The EPA 608 is Changing!

September 16, 2026
Disclaimer: The technical statements, information and recommendations contained herein are believed to be accurate as of the date hereof, but Mingledorff’s does not make representations or warranties, express or implied, as to its accuracy, its completeness, or the results to be obtained. The information is being provided for informational purposes only and is intended for use by persons having adequate skill and expertise regarding the proper selection, use and application of the products and recommendations and at their own risk and discretion. 

DLS A2L Dissipation Error Codes

By Jesse Van Atta • September 16, 2026
Disclaimer: The technical statements, information and recommendations contained herein are believed to be accurate as of the date hereof, but Mingledorff’s does not make representations or warranties, express or implied, as to its accuracy, its completeness, or the results to be obtained. The information is being provided for informational purposes only and is intended for use by persons having adequate skill and expertise regarding the proper selection, use and application of the products and recommendations and at their own risk and discretion.