I appreciated the feedback I got on my switch blog (This Little Switch Went to Market). This time, let's deal with the electromechanical (E/M) device, the relay. I know there are solid-state relays; I used Opto-22 units for a long time. I just want to concentrate on the E/M types. This writing is more for the beginners or those with little experience with relays.
All E/M relays typically have an energizing coil. Some also have an additional holding coil. Others use a latching coil combined with a release coil. These are the most popular types. Other parts of the relay are the armature, which is pulled in by the coil and causes the movable contact (the common ) to make connection with the normally open (NO) contact and break connection with the normally closed (NC) contact.
On many relays there is a spring that returns the armature back to its non-energized location. You can have multiple NO and NC contacts. The simplest is the SPST (single pole, single throw) all the way to 4PDT. There are types with more than four poles, but they are not common. I have seen six-pole relay used for three-phase power, keeping the phase and switching time on the mark.
There are (or used be) multirelay cross-point matrices. The only thing worse than designing a relay matrix is trying to troubleshoot one. I once worked on a cable TV switching matrix. A single DPDT relay was working only on one pole. It was a challenge to figure out. I believe at least four other problems were caused by the looping and chaining of the relay outputs.
Troubleshooting the matrix switcher is tough. But then, so is picking the right relay for the right task. The choices for relays are immense and can be confusing. Let's make things a little easier by having a quick look at some of the types:
- DC power contactors
- Hermetically sealed and intrinsically safe
- Dry reed
- Mercury wetted (not whetted)
- Sequential or steppers
- Sensor relays
- PZA fail-safe (contains additional logic circuitry to prevent unintended operation; or is mechanically configured to prevent unintended operation of the controlled equipment)
- Delay on energization
- Time delay (delayed release)
- Magnetic latching
- Overload protection
- Indicating and manual test
Let's look at the couple of these with which you may not be too familiar. The overload protection relay protects motors from over-current. This works by having a bimetallic strip that heats up and disconnects a set of auxiliary contacts in series with the coil. This current trip works slowly so motor turn-on does not cause a trip. Not an overall precise device, but it works and does the job without additional circuitry.
The Indicating and manual test is one of my favorite relays. It's usually found in 2.3 x 1.37 x 1.39 inch size. The relay often uses an octal or 11-pin socket. It's a plug-in power relay and is typically a DPDT (for the octal) or 3PDT (for the 11-pin). The relay has an LED to indicate coil on/off status. There is a push button for manually operating the armature's action. On some models there is a flag indicator that shows coil action whether via power or manual operation. As you can see this is great for troubleshooting. A contact switching current of 16 or more amps is usually typical for this package.
The hazardous-location, hermetically sealed relays are a breed of their own. The 4PDTs are very common, with contact ratings of three to five amps. Relays meet the UL standards for Class 1, Division 2, Groups A, B, and C hazards. These are intended for use in a variety of potentially hazardous situations. This includes various types of flammable environments or in situations where water is present. Engineers prefer to keep water away from the relay contacts — or to keep electricity away from the water.
This is the start of learning about relays. The follow-up to this article will discuss relay ratings, deratings, inrush, and contact materials. Let me know about your experiences and problems with relays.
- This Little Switch Went to Market
- The Diabolical & the X-Ray
- This Is Really Happening
- Arc-Fault Interrupters: How Can They Tell What’s OK & What Isn’t?
- Everything’s Better After the First Explosion
- ‘That Should Be No Big Deal’ (Yeah, Right!)
- How an electromechanical relay literally wrestled my software to the ground