This Little Relay Stayed Home

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)
  • Latching
  • 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.

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20 comments on “This Little Relay Stayed Home

  1. eafpres
    September 17, 2013

    Hi Peter–nice list and some interesting points.  It is interesting that relays are a really old technology, and are used in lots of mundane things.  For instance, the solenoid that controls energizing the starter motor on a car.  I haven't heard that muted clicking sound from trying to crank with a low battery for some time.  Before electronics, doorbells were not quite relays but did have solenoids and armatures, and those nifty musical chimes.  I was trying to think of all the relays in common use.  I think there is one on my furnace blower motor–I think a lot of electric motors in the past had start relays which avoided arcing when first spinning up.  I suspect most of these are electronically controlled now.  

  2. PZman
    September 18, 2013

    Thanks for the feedback, always appreciated.Here is an oddball: good size 110 VAC motors with capacitor start. Engaging the capacitor to even out power factor, one the motor comes up to speed, the cap is disconnected (centrifugally). Oddly enough the cap is also disconnected unless power is applied. Eliminated feeback and arcing on start up and the reconnection of the cap when power is disconnected. Kind of a relay to start and a switch to run. Not only do I love tube circuits, relays & switches through in an old motor and I might take the day off.

  3. WKetel
    September 18, 2013

    EP, in most industrial equipment the motors are still controlled by the good old electromechanical starter relay, which often has larger contacts to handle the heat generated during the starting inrush current. The reason is reliability and safety. Solid state controls often fail in the shorted circuit mode, while contacts sticking closed almost never happens in an adequately sized motor starting contacter. In addition it is sometimes a challenge to get all of the solid state devices to switch at identical times, while contacts do it easily. And those contacts don't need nearly as big heat sinks.

  4. eafpres
    September 18, 2013

    @WK–intersting observation.  A few decades of electronic advancement and integration and the electromechanical relay still has a place!

    By the way, off topic, but if you want to see some serious analog gear, go to a transformer yard and look at the emergency disconnects.  Giant rotating contactors operated by electric motors.  Huge current and voltage transformers.  Insulators you measure in feet.  

  5. Netcrawl
    September 20, 2013

    They may be old technology and aging but they help us powered most of our industrial revolution, there still around, some can be find in cars. @easpres doorbell could be a good example of relay.   

  6. jkvasan
    September 23, 2013


    Relay chattering (switching on and off everytime it changes states) has been one of the main problems we had with Relays. We used to put an electrolytic capacitor – 470 mfd/appropriate volts in parallel with the coil to solve this.

  7. Vishal Prajapati
    September 23, 2013

    Being an young electronic engineer I love the old technology which is mature and has good amount of documentation. It helps in self learning. Old is gold can be applied here as relays and contactors has been the proven and most reliable switching devices uptill now. Even the static swithes can't challenge their reliablility.

  8. RedDerek
    September 25, 2013

    Good list of things to consider, but one mus not forget the other basics: coil voltage, contact voltage and contact current.

  9. Brad_Albing
    September 25, 2013

    @Peter – I too have a real appreciation for relays. Like Blaine said, an old technology. But look what engineers did with them more than 100 years ago – telephone switching circuits, block occupancy signals on railroads – surprisingly sophisticated considering the technology.

  10. Brad_Albing
    September 26, 2013

    @eafpres- like this?



  11. eafpres
    September 26, 2013

    @Brad–yep.  If you notice, right at the start the contactors change positions, which starts the arcing.  Not for the timid!

  12. Brad_Albing
    September 26, 2013

    @eafpres – you're right about “not for the timid.” Kids, we're professionals – don't try this at home!

  13. PZman
    September 27, 2013

    Another thought is when you use a control circuit to drive the coil on a relay, contactor or solenoid, a “snubber” should be across the coil connections. This will protect you against  noise generated by when the magnetic field collapses across the coil portion. A “snubber” is typically a 0.5 uf/ 600 volt cap in series with a 100 ohm, 1/4 watt resistor. This will work on 110 or 220 volt coils.


    As for the chattering have you tried placing a diode in series with the electrolytic?

  14. PZman
    September 27, 2013

    Geat video Brad. I was visiting the GE High Voltage Lab maybe twenty or more years ago. They were testing SHV (super high voltage) transmission lines. I got to see a two million volt testing & failure of a transmission (indoors) I think my ears still ring from the final snap.

  15. PZman
    September 27, 2013

    Glad to here your comments. I believe that relays and the related products are still not final development stage. Reliability can be incredible with some units exceeding 10 MILLION operations. The 'net has been a great accessory to manufacturers catalogs to getting information about relays.

  16. PZman
    September 27, 2013

    RedDerek Take a look at Part 2 of relays & I will include those subject matters. Finishing the article presently.

  17. jkvasan
    September 28, 2013


    I guess you are meaning the same snubber circuit we normally place in parallel with the TRIACs and SCRs. That is a good point. I have not tried it though.

    We add a freewheeling diode in parallel with the Relay coil. This electrolytic cap was in addition to it, again, in parallel.

  18. SunitaT
    September 30, 2013

    Whenever an AC cable is close to the indicator cable, a capacitive link originated between the 2 cables. This crosstalk, allows a lesser current to move from AC cable to signal cable. Whenever the indicator cable is in off state, the small current might move the I/O relay. For example, this might reason electromechanical relay to “latch on”, LED could glow whenever the signal is switched off or in worse case this could switch the relay on while the signal is switched off.

  19. PZman
    October 7, 2013

    The relay I was using comes from Magnecraft 750 series. It has gold flashed contact which I find to be useful in motor circuits. The 750 comes with a mechanical “flag” indicator and an optional 3PDT contacts, with the LED on one of the poles. I can tell when the relay is stuck or welded in position. Trace & induced currents can always cause a relay to continue to stay latched on. I always use sockets with enough pin spacing and run my cables with a twist and at right angles. Thanks.

  20. Davidled
    October 7, 2013

    Everyone loves Gold rather than Silver. Gold plate is more conducting than that of Silver. Gold is costly a bit. Silver plate is still acceptable in some application.

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