What is an Ice Cube Control Relay?

The term “ice cube” is an informal name for a variety of control relays typically found in industrial control and automation applications. Ice cube control relays are like little cubes of ice neatly organized in a control panel. They are typically constructed out of a clear plastic, further reinforcing the analogy. Finally, the unofficial categorization is reserved for relays that are socketable. Representative examples are shown in Figure 1.

Figure 1: A variety of ice cube relays shown with and without their corresponding sockets.

What is the purpose of the ice cube control relay?

The ice cube control relay is a general-purpose device. Here are some of the applications:

Simple control

A variety of simple logic operations may be performed using relay logic. Combinational operations such as AND and OR are easily implemented. More complex sequential operations may be performed using relay-based memory circuits. Here, each relay latching circuit holds one bit of information. Historically, complex circuits were built using relay-based circuits. Today, the Programmable Logic Controller (PLC) and the programmable relay have replaced all but the simplest relay logic.

Dry contact

A dry contact serves as an interface from one piece of equipment to another. The term “dry” is a crude way of saying galvanically isolated. With this circuit, the signal passes through a relay’s contact which has no electrical connection whatsoever to the equipment where the signal originated.


This is arguably the most important purpose of the ice cube relay in modern control systems. The term interposing describes a control relay that is placed between two circuits. For example, an interposing relay may be used to convert the 24 VDC solid state output of a PLC to 120 VAC allowing a large motor starter to be activated. You can learn more about the interposing relay here here and the motor starter here.


Just for fun, and to showcase the ice cube relays, please take a look at Video 1. The DAVIAC-1 computer features hundreds of relays. Do you agree that they look like ice cubes? Do you see the smaller “sugar cube relays” on this or similar relay-based computers?

Tech Tip: Relays are electromechanical devices with moving components. As you watch the DAVIAC-1 computer video, ask yourself how long such a computer will operate before the relays start to fail. The actual time to failure is situation-dependent consists of two part: the physical wear of the armature hinges and the arcing / pitting of the contacts. On a related note, this is why you don’t use a PLC with a relay output to blink a panel lamp. The majority of the relay’s life will be lost over just a few weekends on continuous activity.

Video 1: Video of the DAVIAC-1 relay-based computer in operation. The computer contains hundreds of relays many of which could be considered ice cube relays.

What precaution must be taken when selecting an ice cube control relay?

Coil voltage

Technicians must be careful to select the proper relay. There are many identical looking devices that share common sockets. Perhaps the greatest danger is installing a relay with the wrong coil voltage. This can be especially problematic in systems that feature mixed voltages. How many of us have pulled multiple relays only to put them back in the wrong positions?

Locking mechanism

Some relays contain a mechanical locking mechanism that allows the relay to be locked into the active state. This can be good for troubleshooting. However, it can have unintended consequences if the relay is left in the mechanically locked position. I suspect thousands of man-hours are spent chasing these self-induced errors. Note that two of the relay relays shown in Figure 2 feature this mechanical forcing mechanism.

Surge suppression

It’s important to properly handle the flyback voltage associated with a deenergizing relay coil. This is especially important when the coil is driven by a semiconductor. For example, a PLC with solid state outputs may be damaged if the inductive kick is not handled properly.

There are many ways to handle the flyback voltage:

  • Integral diode where the surge suppression diode is integrated into the relay body itself.
  • Socket integration with the diode integrated into the relay socket such as shown in Figure 1.
  • External diode with the assumption that the installer will use an external diode. A representative example is the 1N4001 for 24 VDC systems.

We must carefully identify and then replace the relay with in-kind diode protection. On a related note, we should also attempt to replace the relay with similar LED indicators.

Figure 2: The two ice cube relays on the right feature a mechanical mechanism to force the relay contacts.

How can I locate an ice cube relay in the DigiKey Offerings?

Searching the term “ice cube” from the DigiKey search bar is likely to point back to this forum post. A search of Power relays, Over 2 Amps will yield better results. At the time of this writing, there are over 27,000 items in this category. We move closer to the “ice cube” category by selecting devices that are socketable or DIN rail mounted. This limits the field to approximately 9,000 items. It’s up to you to determine if each relay fits within the colloquial definition.

Please be very careful when selecting relays, as there are so many closely related options. As previously mentioned, things like coil voltage, locking mechanisms, diode protection, and LED indicators are important and often overlooked.

Parting thoughts

Relays remain an important component in industrial controls and automation. The term “ice cube relay” and “sugar cube relay” are fun historical descriptions that remain relevant for the small relays.
Your feedback is welcomed, especially if you have similar colloquial names for components.

Best Wishes,


Return to the Industrial Control and Automation Index.

About the author

Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer, which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing articles such as this. LinkedIn | Aaron Dahlen - Application Engineer - DigiKey