Solenoids and relays, Part 2

Part 1 looked at the solenoid, an electromechanical component which translates applied current into linear motion. The design of the electromechanical relay uses a coil and current drive (or a voltage source) just as with the solenoid. However, the function of the relay is quite different. Despite the availability of alternatives for some applications such as the optical solid-state relay (SSR) and MEMS-based relays, the electromechanical relay is still a vital and versatile component for switching both AC /DC signals and power, and at low and high levels.

Q: What does a relay do?

A: It’s conceptually simple: it allows one signal to control the switching of another circuit, with complete electrical isolation and without any electrical contact between the two circuits, Figure 1.

The operating principle of a basic relay uses the energized coil of the solenoid. However, instead of moving a plunger in the core, it instead “pulls in” an armature on which are one of more electrical contacts. As the movable armature pulls in, which then makes (or breaks) connection with a fixed contact, completing (or opening) a circuit path through the armature and contact. When the coil is de-energized, a spring pulls the armature back to the power-off position. Thus, the relay is an electrically controllable on/off switch.

Q: What are some important aspects of the electromechanical relay?

A: There are many reasons why a relay is a unique and still viable component, even with the availability and low cost of SSRs and MEMS relays:

• The coil circuit and the contact circuit are electrically isolated from each other completely, and can have very different voltage and current levels;
• The relay contact forms a basic switch closure, and current through it can be AC or DC, independent of the coil drive. Neither side of the closure is grounded or connected to circuit common, so the closure can be placed anywhere in a circuit;
• While the relay can close a contact on activation (called normally open, NO), it can also open a contact (normally closed, NC)—or both, using multiple contacts;
• The relay can control more than one NO or NC contacts; you can get three, four, or even more independent NO and NC contacts (Figure 2). These multiple contacts do not have to be carrying the same type and rating of loads, which is another benefit; some contacts can be for low-level signals while others can be for power;

• The contact circuit does not have to be “live” when the relay is activated, a feature which is actually a necessity in some designs; it can be switched while the load circuit is off (called a “dry contact” closure);
• The relay is electrically and mechanically rugged and robust, and simple to troubleshoot; it can withstand transients that would damage a solid-state equivalent;
• Relays can be designed for coil currents as low as 10 or 20 mA or as high as tens of amps, with contacts handling milliamps and a few volts to several orders of magnitude greater for both parameters;
• Once the relay is energized and the armature has moved, it only needs a weaker field to hold it in place; thus, the relay “holding current” is far less than the “actuation current”—typically about half. This is the same as with the solenoid, and the same or very similar circuit can be used as a solenoid driver or a relay driver;
• The relay load does not have to be fully known or defined, as long as it is within the design limits; this is useful in cases where the load may have uncertain or hard-to-control characteristics;
• A properly designed relay can use a very low-level voltage/current to switch a much-higher voltage/current;
• The relay is very easy to troubleshoot; all that is needed is an ohmmeter to measure the coil continuity and DC resistance, and to measure the contact resistance when the relay is open and closed;
• Relays can also be used to switch RF signals–but these require very different internal construction.

Q: What are some of the key parameters to consider when selecting a relay?

A: The choice begins with coil ratings, then contact type, number, and ratings:

• Coil rating: designed for what amount of current (or voltage)? Optimized for AC or DC drive, or both? Vendors also specify the coil resistance, so the engineer can be sure the drive circuit is compatible;
• Contact configuration: simple NO, NC, or multicontact version?;
• Contact rating: how many amps do the contacts carry when closed (physical contact size), and what is the voltage across the contacts when open (contact spacing)?;
• AC or DC load: a high current can an arc as the contact opens, which damages the contact plating (AC arcs self-extinguish, as the voltage reverses); the contact shape and material must be designed for this situation.

Q: What are some of the limitations of relays?

A: Relays are well suited for some situations, and not for others. Among their weaknesses:

• They are relatively slow, with switching speeds on the order of tens of milliseconds. This is not acceptable for those switching applications which need microsecond-range or faster speeds;
• They will wear out; although a well-designed quality relay used within its design limits can last over a million cycles, that may not be enough;
• Not only will the moving mechanical elements wear out, but the electrical contact surface plating will abrade from the repeated make/break action and thus eventually make poor or intermittent contact;
• Unless they are sealed, the contacts will accumulate dirt and may even corrode, which affects the contact-side performance;
• They are larger than their SSR or MEMS counterparts, and generally more costly;
• They require current drive at relatively high levels, and can consume (dissipate) significant power, especially if they are in long-term energized mode.

Conclusion

Despite their age and “old-fashioned” electromechanical nature, both the solenoid and relay still have important roles in applications where all- or mostly electronic counterparts lack the versatility, ruggedness, ease of use, and flexibility of these venerable components. Don’t assume they are outmoded or obsolete solutions, as they may well be the best options for linear motion or circuit switching for a given set of objectives and priorities.