Design Considerations Using Relay and Relay Datasheet Explanation

There are several design considerations using relay. A detailed relay datasheet explanation is also the key on obtaining a good relay circuit. Like other electronic devices a relay has its own operation. But before jumping to these, we will take a tour on the basics; what is a relay and how a relay works for full understanding.

A relay is an electromechanical device or switch. It consists of a coil and contact. The coil is an inductive part which needs to be energized to change the state of the contact. A contact is may be a normally open or a normally close one. A relay coil has already a significant internal resistance and a of course an inductance. When the coil is driven with a transistor or a MOSFET a protection against reverse voltage or kickback voltage should be added. To know the complete analysis of relay kickback voltage and how to clamp it, read this. You can also read the relay driver circuit comprehensive design here.

Figure 1 below is a schematic of a 4 pin relay. Pin 1 and 2 are connected to the coil while pin 3 and 4 are connected to the contact leads. When the coil energized, the contact will close and pin 3 and 4 are shorted.

Design Considerations Using Relay
Figure 1: Schematic of a four terminal relay

There are also relay which has 5 terminals such in Figure 2. This relay type has two close contact states. The relay wiper or the common terminal may connect to NC or NO when the coil is energized. Unlike with Figure 1 wherein the contact has only one close state and the wiper is simply float otherwise.

Design Considerations Using Relay
Figure 2: Schematic of a five terminal relay

Figure 3 is some of the relay used in the industry nowadays. The blue one is a general purpose relay while the rests are used for automotive.

Figure 3
Figure 3: Some relay used in the industry nowadays

Sample Circuits Where Relay is Used

Figure 4
Figure 4: Relay circuit using a BJT switch

On above circuit the relay is being driven by a BJT. The contact is driving a load when the coil energized. The load is can be a motor, lamp or whatever. Another variation of this circuit is using a MOSFET as below. Some design engineers prefer MOSFET by the fact that it is a voltage controlled device and easier to set it into saturation than BJT which is a current controlled. In using either device, a protection for the reverse voltage must be added such as diode D1 in the schematic. Another way of protecting the circuit is using a TVS across the drain to source of the MOSFET or collector-emitter of the BJT.

Figure 5: Relay circuit using a NMOS switch

Design Considerations Using Relay

Relay Grade/Class

Top of the list for the design considerations using relay is the grade or class. For general electronics a commercial or industrial grade is enough. Using a higher grade will correspond to a higher cost. For automotive applications, you should use an automotive grade relay. There are specific qualifications and tests that are conducted by vendors per application. If your application is for industrial then you use a military grade relay, it is advantageous to your design since the reliability of the device is much higher than industrial. However, the price is much higher as well.

On the other hand, if your application is space then you use an automotive grade only, your design will be in trouble.

Common relay class/grades are Commercial/industrial, Medical, Automotive, Military and Space.

Part Numbering System

Part numbering system is off topic; this is not one of the design considerations using relay. However, this is part of a relay datasheet so it’s worth mentioning it.

Relay vendors has its different part numbering systems. Below is the example part number of a particular relay manufacturer. This is provided in the datasheet and does not need technical knowledge. You need to familiarize this when you want to order a relay.

Figure 6
Figure 6: Sample part numbering system of a relay

Nominal Coil Rating

One of the most important design considerations using relay is the coil rating. Coil is being rated at nominal condition. Nominal rating is a reading taken from a nominal ambient temperature of 20’C for relays. Nominal means average or the recommended point where the operation should be.

The usual coil specification is like in Figure 7. Take note that all these parameters are taken at 20’C. This means that these values are not anymore the same at operation other than 20’C.

Figure 7
Figure 7: Nominal coil rating of a particular relay

Nominal Voltage

This is the typical voltage that should be applied across the coil for ideal operation. Vendors are calling this nominal because vast of the tests and qualifications were based on this.

Coil Resistance

This is the internal resistance of the relay coil. This value is taken at 20’C ambient temperature and subject to change on different ambient temperatures. You need also to consider the tolerance it has for critical applications aside from temperature variations.

Nominal Current

This is simply the resulting current when you divide the nominal voltage by the coil resistance. How about if the actual coil current exceeds this, will the relay get damaged? It can be, depending how big that current is. However, a current higher than the nominal current is only possible when the applied voltage is higher than the specified nominal voltage. By design if the applied voltage is set always at the nominal the coil current will be at the nominal always.

Must Operate Voltage

This is the same to pull-in voltage or pickup voltage. Figure 8 below is another relay specification and it is using the term pickup voltage. A pickup voltage is the level of voltage needed to change the state of the relay contact coming from an OFF state (especially cold start). At voltage below this level, the relay coil will not be energized and there is no change in the contact state.

Figure 8
Figure 8: Coil rating of other relay

Must Release Voltage

This is similar to the term drop-out voltage used in Figure 8. Some manufacturers are using the term holding voltage also. Once the relay is energized and changes state, it will only be de-energized completely when the voltage across the coil is less than this level.

Nominal Operating Power

This is the resulting power considering the nominal voltage and typical internal resistance. This power rating will not be exceeded as long as the applied voltage is set at the nominal rated value.

Usable Voltage

This is the range the applied voltage may play with. Supposing in Figure 8, the usable voltage is from 10V to 16V. 10V is a guaranteed minimum voltage that can be applied on the coil to be energized fully. In case of Figure 8, the pickup voltage is only 7.2V but the minimum usable voltage specified is 10V in order to consider tolerances and other unexpected factors. On the other hand the maximum limit of the usable voltage is 16V. Meaning you can operate up to this level even though the specified nominal voltage is only 12V. However, this rating must only be reserved to abnormal conditions and not for steady state. For example, at startup the coil voltage increases to 16V and go to nominal value after few milliseconds, this is okay. But operating the coil voltage with 16V continuously is not a good idea.

Figure 9 below is a coil data specification of another relay vendor. As you can see it is using the same terminologies with the two above.

Figure 9
Figure 9

Contact Rating

The critical parameters in a relay contact are the switching voltage and switching current. In Figure 10 below for instance, the maximum switching voltage is 16Vdc, so do not operate above this. For the switching current the continuous rating at 16Vdc is 25A maximum. You can exceed this level for short period of time as specified in the table below. In practice relay loads are very much lower than the maximum current capability so the stress on the contact due to current is not common to happen. Keep on mind not to exceed the current stress to above 80% because it may shorten the life of the relay.

Another thing I will highlight is the contact resistance. If efficiency is a must on your application you should consider a lower contact resistance. A lower contact resistance will give a lower power loss and the higher the efficiency will be.

Figure 10
Figure 10: Relay contact specification

Operating Temperature Range

This is one of the most important parameters to take note. Exceeding the ambient temperature range, the device will not function normally and eventually get damaged. For the relay in Figure 11, you cannot use this for your application wherein the maximum ambient is exceeding 85’C. Ambient temperature is the temperature measured near the relay case but not touching it. This temperature may be higher than the surrounding ambient temperature if the relay is surrounded by heat generating devices such as power MOSFETs and BJT, power diodes and magnetics.

Figure 11
Figure 11: Operating temperature range

Coil Resistance at Operating Temperatures Other than Nominal

Most of the time, the specified resistance of a relay is the nominal value which is taken at 20’C. When operating above or below this temperature the resistance will change. Relay coil is made up of metal alloys and mostly with copper content. Copper has a positive temperature coefficient of resistance. Meaning the resistance will increase with higher temperature and decrease with lower temperature. If the relationship of temperature and coil resistance is not given in the relay datasheet (which is most of the time not), we can use this equation

Equation 1


  • R1 – the target new resistance
  • R0 – the resistance of the coil at nominal temperature
  • α – the temperature coefficient of the coil material (0.00393/°C for copper)
  • T1 – is the new temperature of operation
  • T0 – is the nominal temperature (most relays are rated at 20°C nominal temperature)

Example 1:

The rated coil resistance of the relay is measured at 20°C and the reading is 320Ω. What will be its resistance at -40°C and 85°C ambient temperatures?


Equation 2

Above answers don’t include tolerance yet. It should be included for more accurate result.

Pickup Voltage at Operating Temperatures Other than Nominal

The same with the coil resistance the pickup voltage will also change with respect to ambient temperature. Some relay datasheets has a graph between ambient temperature and the corresponding pickup voltage. It is easy to determine the needed pickup voltage from there when the ambient temperature had been changed.

If ever there is no such graph provided, you can compute the pickup voltage the same way the coil resistance computed. You just need to replace the resistance R0 and R1 with V0 and V1 where the former is the nominal pickup voltage while the latter is the new pickup voltage at a particular temperature.

Example 2:

The rated pickup voltage of a relay is measured at 20°C and the reading is 8.4V. What will be its pickup voltage requirement at -40°C and 85°C ambient temperatures?


Equation 3

Coil Temperature Rise

When the applied coil voltage is higher than the specified nominal value, the tendency is that the coil temperature will rise. Below figure is a graph between coil applied voltage and the temperature rise of a particular relay. This particular relay has a nominal voltage rating of 12V. As you notice, when the applied voltage is increased, the temperature rises. So for a 12V relay, the applied voltage must be contained within 12V under continuous operation to avoid over heating the coil that will lead to relay failure. As I explained above, it is okay to operate at the maximum applied voltage (say 16V for a 12V rated relay) during transient condition only.

Figure 12: Temperature rise of relay with respect to the applied voltage
Figure 13: Temperature rise of relay coil with respect to coil power

Transition between Pickup and Dropout Voltage

There is some applications that strategically decreased the voltage across the coil. The best example of this application is shown in Figure 14. Vss is set equal to the nominal coil voltage requirement. Initially Vin is off and the MOSFET is at cutoff and capacitor C1 will be fully charge and with a voltage equal to Vss. When the MOSFET is on, momentarily the voltage across the coil will be equal to Vss and then go to the steady state level which is equal to the voltage divider of R2 and the coil resistance. This approach is mostly used in power supply. Without R2 the power dissipation on the relay is high and also the MOSFET which are liability in terms of efficiency.

The challenge in this strategy is to ensure that the voltage of C1 will stay at the level of Vss until the relay coil is fully energized. This timing diagram is not popularly included in the relay parameters in datasheets.

Figure 14

Figure 15 is an example timing diagram for this strategy. I based this timing diagram based on the vendor’s explanation when we used this approach. The level of V1 should stay higher than the minimum pickup voltage requirement until the delay is satisfied before it can go to the level of V2. Otherwise the relay may not be fully energized. The level of V2 should be set also higher than the dropout voltage.

Figure 15

Expected Life

Below is a graph of switching current versus the number of operations of a particular relay. The higher the current the lower the number of operation will be. Figure 17 is a more detailed relay life specification from another vendor. One operation has a 1 second on time and 9 seconds off time for a resistive load.  A motor load one operation has 0.5 seconds on and 9.5 seconds off time.

Figure 16

Actually life expectance is not a big concern in relay. 100,000 operations is already several years and some other circuit device may fail prior to this. Supposing a relay is used to drive a car headlamp. How many times the car headlamp transitions from on to off? Surely a few times; let us consider that the driver is doing this 10 times average per day, for a month the total operation is 300 (10×30) and for a year the corresponding operation is 3600 (300×12). The 100,000 operations is equivalent to 27 years and the usual product life is only ranging from 5-10 years.

Figure 17

Considerations Using Relay: Mechanical Dimensions and PCB Layout Considerations

These are important for the proper spacing and fitting on the PCB. These are self-explanatory though.

Figure 18
Figure 19

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