Kat Joycee January 22, 2021
How Much Voltage Does a 12V Relay Need to Work?
I know that this seems like a trick question, but it’s not. I have come across many questions like “will 12V relay work 6V?”, “can I use a 12V relay in a 24V system?” or “will a 12V relay work with 9V?”. So how much voltage does a 12V relay need to work?
Let’s start by looking at a datasheet for an AD-PR40-1C-12D 12V relay, which is an open style power relay. A datasheet is a document with detailed specifications of a component.
What is a pull-in Voltage?
As you can see from the snippet of the datasheet, there is a term Pull-in voltage. Another name for this is Pick-up voltage. Pull-in voltage is the voltage required for the relay coil to energize and build enough strength the overcome the spring tension on the contact and pull it in. In simple terms, activate the relay.
All relays and contactors have some mechanical resistance, and the electromagnetic coil needs to pull against that tension. The image here shows this spring.
In the datasheet, the pull-in voltage is listed as 80% of the nominal voltage. The nominal voltage of a 12V relay is 12V, so 80% of this is 12 x 0.8 = 9.6V. So in theory, this relay should activate at 9.6V. But what about in reality? In the video below, I’m going to conduct this test using our Electromag pracbox that has this relay.
The experiment
I have tabled my test results below. The exact voltage when this relay energizes at is 7.7V. So there is a difference between the theoretical value and the practical.
| Voltage (DC) | Relay activated? | % of 12V |
|---|---|---|
| 7.0 | No | 58.33 |
| 7.2 | No | 60 |
| 7.4 | No | 61.67 |
| 7.6 | No | 63.33 |
| 7.8 | Yes | 65 |
| 8.0 | Yes | 66.67 |
Does this happen in all relays?
Absolutely, but the pull-in voltage may be different. Let’s look at another example, SRD-05VDC-SL-C, which is a 5V relay that is very popular in devices like Arduino and Raspberry Pi. These devices are used quite extensively in Science Technology Engineering Mathematics (STEM) and Internet of Things (IoT) projects. Here is a datasheet snippet for this relay
As you can see, the pick-up voltage, what we know as the pull-in voltage is 75% of the maximum. That means the minimum voltage required to activate this relay is 5 x 0.75 = 3.75V.
Why is knowing the pull-in voltage significant?
In the previous example of the SRD-05VDC-SL-C 5V relay, I mentioned that they are quite popular with STEM and IoT devices like Arduino and Raspberry Pi. The standard output of Arduino is 5V, so this relay works well with it; however, the standard output of a Raspberry Pi is 3.3V, so you will need some additional circuit to operate this 5V relay. Even the Arduino output may require an additional circuit component called the flyback diode if used with the 5V relay, but that’s the topic for some other time.
Another reason why knowing pull-in voltage is essential is because of the voltage dips and rises. In all electrical systems, the voltage seldom remains the same, sometimes it dips or decreases and sometimes it rises or increases.
For your 12V relay or any relay to work, you will need to ensure the voltage is always within a range that will activate the relay when you require. Otherwise, you will have an unstable system.
Finding pull-in voltages for other relays and a contactor
To give you more accurate data, I conducted experiments with another 12V relay, a 24V relay and a 24V contactor, which are all in the Electromag pracbox. Here is the video of my experiment.
| Component | Rated voltage (DC) | Pull-in voltage (DC) | % of the rated |
|---|---|---|---|
| Interface relay | 12V | 5.9 | 49.17 |
| DPDT relay | 24V | 13 | 54.17 |
| DC Contactor | 24V | 14.4 | 60 |
As you can see from my experiments and observations, all relays and contactors require less voltage than their rated voltage to activate. A broadly accepted rule of thumb is 60% to 80% of the rated voltage but this depends on the relay’s sensitivity, as evident from my tests.
So why supply more voltage than it needs?
I’m sure the question that begs to be answered in this situation is that why supply 12V to a relay that requires only 7.7V to activate? And the answer is Stability. As I mentioned earlier about all electrical systems having some degree of voltage instability, you don’t want the circuit to behave erratically. Supplying 12V instead of 7.7V will ensure that the relay will activate when it’s supposed to and stay activated as long as it’s required.
Conclusion
I hope this article answered the questions like “How much voltage does a 12V relay need to work?”, “will 12V relay work 6V?” or “will a 12V relay work with 9V?”. Roughly 60% to 80% of the rated voltage.
The next question is when the relay will deactivate? 0V or before it reaches 0V? We will discuss this in another article.
Thanks for dropping by. If you liked the article, I would appreciate it if you left a comment. And feel free to drop a comment about your experiences with the pull-in or pick-up voltages in relays and contactors.
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About the Author
Husnen Rupani
I help electrical training organisations increase learner engagement by designing innovative training equipment. I have a saying "Electricity - you cannot see, you cannot hear it, but by the time you feel it, it may be too late." My main aim is to turn this black magic that we call electricity into something that people can understand.
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Kat Joycee January 22, 2021
10 thoughts on “How Much Voltage Does a 12V Relay Need to Work?”
thank you for the information you shared.very nice information
Appreciate the kind words Sabri, thanks for dropping by
If there is no trouble, can you give information about amps?
No trouble at all. Because the relays and contactors I showed are DC, the operating current can be found using Ohms law. So if we measure the resistance of the coil and we know what voltage we are using, the current will be V/R.
Just a word of caution if you’re planning to use a relay with a sensitive device, like a transistor or microcontroller, always install a flyback diode in parallel with the coil.
selamun aleykum
I want to run a sim module with an arduino with a relay.first relay coil 12v, second relay coil 5v.I want the system to start the 12-volt battery automatically when power goes off.
-How can I operate these two relays with an adapter?
-How can I charge the battery externally without touching the cables of the system when the battery is discharged?
I don’t want the system to charge the battery automatically.
(I write english from translation.excuse me)
Peace be upon you too Sabri.
I don’t completely understand your requirement, may I request you to explain it more. If it’s easier to share the information via email, please feel free to send it to me on info@infinispark.com.au. I’ll try my best to help you.
Hi guys I teach the Army/ Air-force Electrician Apprentices in Bandiana/ Wodonga, I/we did an experiment with an Energy Space Skills Practice, we used a NHP Finder 24VDC 2 pole Relay, the Pick up Voltage was 14 Volts, requiring 27mA to energize, the drop out voltage was 5.3V and drop out current was 9.6mA.
Thanks for sharing Gary. This is such an important concept for anyone learning about relays and contactors.
I want to use a 3.7volt – 12 volt relay (the exact model is a UD2-4 5NU made by NEC) which has a pick up minimum voltage of 1.13V. In my project I want this relay to be battery powered so that I can remotely operate a small strip of 5V LED lights which are also powered by the same battery(s). Taking into account that battery voltage decreases with time what batteries would you recommend to use to get the maximum amount of time with the relay working before the batteries have to be replaced. Either 4 AA or 1 or 2 9V batteries in series or parallel.
Theoretically although not at all practical for my situation would a 24V battery be the best it would take a while to get down to 1.13V.
Hi Cam, the part that you quoted, is that UD2-4.5NU? because that has the coil voltage of 4.5V and shouldn’t be used with 12 or 24V. Please correct me if I’m wrong, you may have looked at the UD2 series datasheet where they have options of 3V, 4.5V, 5V and 12V. This means that you can purchase any of these but you won’t get one relay that will work on all of these voltages. You can get multi-voltage relays if necessary but not from this series.
It is possible to use a 5V relay if you choose to use the same battery pack for LED and relay, 4xAA in series will give you 6V. I’m guessing you’ll trigger the relay with some smart device?
As for how long the battery will last, depends on how much current the LED strip will draw. Relays don’t need too much, in fact, the datasheet mentions less than 140mW power.
24V is possible if you series 2 x 12V batteries and then you can use a voltage regulator to limit the voltage to the required value, but this might be an overkill. We can’t tell for sure till we know the current for the LED strip.
Please feel free to share more details and I’ll see if I can guide you to achieve your project.
All the best!