How can I calculate a relay's series resistor?

Question

I have been told that always when I add a relay to my design I have to include a series resistor (series with the coil) to limit current in it. So, I want to use a 5 V DC (coil) relay. How do I calculate that resistor? In the relay's datasheet, I can read my coil has a resistance of 178 ohm.

The power of the relay is 140 mW. Actually, if you calculate the power with 5 V DC and 178 ohms the result is 140 mW (I * I * R = 140 mW). So, I guess the series resistor is to avoid that the coil is always at maximum capability. I mean, to have a safety margin for the component, isn’t it?

Is that the purpose?

If it is, I see a problem. The voltage drop in the series resistor will cause that the coil would not be at 5 V DC and maybe the resistance changes too.

If not, could I connect the terminals of the coil directly to V_CC and GND? Or maybe that value of 140 mW is not the coil power(?).

Answer 1

I have been told that always that I add a relay to my design I have to include a series resistor (series with the coil) to limit current in it.

This is wrong advice.

The relay's datasheet will tell you any limitations in the relay's duty cycle (on-time to off-time). In general the coil's voltage rating is continuous.

A series resistor could be used to run a 5 V relay from a higher voltage supply. If series resistance is added when the supply voltage and rated voltage are the same then the pick-up speed will be extended and arcing at the contacts may occur due to slow switching speed.

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From the comments:

What is pick-up speed?

Relays are mechanical. When the coil is powered the armature gets pulled in against the force of the spring and the contacts close - usually with some wiping action to clean the contacts. Using a lower-than-rated voltage for the coil will cause the relay to pull-in / pick-up more slowly than it would at the rated voltage since the magnetic force will be weaker.

So, 140 mW for power coil means I can dissipated that power continuously?

That is what the rating means.

Current for 178 ohm at 5 VDC is 0.028A. So, It can consume 0.028 A continuously.

Correct.

Answer 2

If you have a 5V relay, you give it 5V without series resistors.

I guess what you have been told applies to a case where you have e.g. 12V supply and relay with 5V coil, then you need to have something to drop the 12V to 5V, but in this case it will be better to change to a 12V relay than to put a resistor there to waste power.

Answer 3

The value of 140 mW is not its rated power; it is just its nominal power dissipated at the nominal voltage. It is completely fine to operate the coil directly from 5 V DC, as long as 5 V DC is within the coil's rating.

Actually, if you place a resistor in series, you form a resistor divider that drops your coil voltage too much and the relay may never switch the contacts.

The only reason I would think of for you to want to place a resistor in series is if you want to operate the coil directly from a microcontroller's digital output. I strongly advise not to do that. Digital pins are not meant to drive inductive loads. Use a simple BJT driver rated at the coil current, and don't forget the flyback diode.

Answer 4

There definitely is a reason to limit the holding current of the relay, especially if you care about the power consumption. For that case one may use parallel RC circuit in series with the coil. The capacitor will provide the pulse sufficient to engage the relay; after it is charged, the current will be limited by the series resistor provided it is sufficient to hold the relay active

Answer 5

Very good question... It takes me way back to my 70s school days when I was making all sorts of devices with relays. Then I reasoned like the OP does now:

The relay coil was wound with copper wire, which I imagined had a very low (zero) resistance...ie, it was an "ideal" electromagnet (inductor) whose operation was to create a magnetic field. So, I thought, if we connect it directly to the power supply (voltage source), it would short it; the current will become very high and something would be damaged. Therefore, I concluded, we must limit the current by connecting a resistor in series.

Later I realized that such a limiting "resistor" actually exists and it is the same coil. The relay manufacturers had guessed to make a "2 in 1" by combining the coil and the resistor in the same device (a "resistor coil":-) Apparently they had found that to increase the strength of the magnetic field, they had to increase the number of coils. To be able to fit them in the relay spool, however, the copper wire had to be thin. Because of this, the resistance of the coil increased. Then it occurred to someone that this disadvantage could be turned into an advantage.

So a relay coil is not just a coil but a "resistor coil".