Can You Use A 24 Volt Relay On A 12 Volt System?

Electrical relays are essentially switches that are controlled electronically rather than mechanically. They consist of an electromagnet coil and a set of contacts. When power is applied to the coil, it creates a magnetic field that actuates the switch contacts either open or closed based on the relay design.

Relays play an important role in electrical systems and circuits. They provide electrical isolation between control circuits and the high power circuits they drive. Relays also provide overload protection and one control signal can drive multiple relay contacts to control different loads. Common applications include motor starters, safety interlocks, automotive circuits, and industrial control systems.

Relays are designed to operate at a specified voltage referred to as the “coil voltage.” This is the voltage applied to the coil that creates the magnetic field to actuate the contacts. The question arises of whether a relay designed for a higher voltage can operate on a lower voltage system.

Voltage Ratings for Relays

Relays have a rated coil voltage that they are designed for. This voltage rating is usually printed on the relay itself and listed in the relay’s specifications. The coil voltage rating indicates the nominal voltage that should be applied to the relay’s coil in order to properly magnetize the electromagnet inside and change the switch state. Most standard relays have coil voltage ratings of 5V, 12V, or 24V, but other ratings are available.

According to an article on DigiKey’s forum, “The coil voltage rating on a relay, sometimes referred to as the nominal coil voltage, is the voltage required for the relay coil to operate per its specifications and achieve maximum life expectancy.” (Source)

Exceeding the rated coil voltage can result in the relay coil overheating. Under the rated voltage may result in intermittent operation or failure to energize properly. Therefore, it’s important to operate relays within their specified coil voltage ratings.

Using a Higher Voltage Relay

Using a 24V relay in a 12V system can potentially cause issues due to the difference in coil voltage ratings. The 24V relay is designed to operate at 24V, while a 12V system provides half of that voltage. This discrepancy can lead to the following problems:

Reduced magnetic field strength – The magnetic field produced by the 12V system will be weaker than what the 24V relay expects. This may result in the armature not fully engaging or disengaging from the contacts (Source 1).

Intermittent operation – With a lower voltage, the relay may chatter or not reliably switch between open and closed states. This could cause erratic operation of the circuit (Source 2).

Overheating risk – Since the coil resistance remains the same, the lower voltage can cause much higher current flow through the coil. This may lead to overheating and damage of the coil over time (Source 3).

Shortened relay life – The relay contacts are rated for a certain load at 24V. Using at half the rated voltage may lead to arcing and accelerated wear of the contacts. This shortens the overall service life of the relay.

Coil Resistance

The coil resistance of a relay plays an important role when operating it at a lower voltage than its rating. Relay coils are designed to function optimally at a specified voltage. This voltage creates a magnetic field in the coil strong enough to reliably close the relay’s contacts.

When powered by a lower voltage, the coil produces a weaker magnetic field. The coil resistance also determines how much current will flow through the coil at the lower voltage. A relay coil with a lower resistance will allow more current to flow, compensating somewhat for the lower voltage. However, there are limits to how low the resistance can go before the coil overheats.

For example, if a 24V relay coil has a resistance of 400 ohms, at 12V it will only have 30mA of current flowing through it. This may not be enough to strongly activate the relay. If the coil resistance was 200 ohms instead, the current flow would be 60mA at 12V. While this compensates for the lower voltage, it could potentially overheat the coil.

Therefore, when operating a relay at a lower voltage, the coil resistance needs to be low enough to allow sufficient current flow, but not so low that the coil overheats. Relay datasheets will provide the coil resistance specification to determine compatibility with lower voltages. Generally, operating a relay more than 20% below its rating is not recommended. For critical applications, relays designed for the lower voltage should be used.

According to https://amin.ug/kampala/207375/electronics-components/relays-and-contactors/relay-coil.html, “The DC resistance of the coil must be low enough so that the rated coil voltage will produce the magnetic field necessary to operate the contacts reliably.” This emphasizes the importance of verifying coil resistance when operating relays below their rated voltage.

Reduced Magnetic Field Strength

When powering a relay coil with a lower voltage than its rating, it will produce a weaker magnetic field. As the Quora article explains, “the strength of the magnetic field is directly proportional to the current passing through the electromagnet coil. Since current is equal to voltage divided by resistance, a lower voltage will result in a lower current for a given resistance. Therefore, a 12V supply will generate a weaker magnetic field in a 24V rated relay coil than a 24V supply would.”

The magnetic field strength needs to reach a certain level to reliably activate the relay’s internal contacts and switch states. If the field is too weak from the lower voltage, it may not fully pull in the armature or contacts, leading to intermittent operation as the LibreTexts source describes.

Overall, the lower voltage supply can compromise the magnetic field strength needed for proper activation. This is a key downside when operating a relay outside its voltage rating.

Intermittent Operation

A potential issue that can arise from powering a relay coil with a lower voltage than its rating is intermittent operation. At the lower voltage, the magnetic field produced in the coil may not be strong enough to reliably close the relay contacts every time. This can lead to the relay sometimes failing to activate when it should.

As this Mike Holt forum post explains, operating a relay coil below its rated voltage can result in intermittent failures to activate. The lower coil voltage means the magnetic field strength is reduced. This can cause the armature to sometimes not pull in fully or drop out early. So the relay contacts will make and break inconsistently.

Intermittent relay operation is frustrating, as the problem comes and goes. Each time the coil is energized, whether the relay activates is basically random. Running relays outside their voltage ratings risks this unreliable performance.

Overheating Risk

One major risk of using a 24V relay in a 12V system is overheating. Relays contain an electromagnet coil which generates a magnetic field to switch the contacts when energized. This coil has a rated voltage, usually 12V or 24V, which matches the field strength it can handle without overheating. If you apply a higher voltage, it will generate a stronger magnetic field and draw more current, causing the coil to get hotter.

As explained in this article, operating a relay coil above its rated voltage can make it abnormally hot. The increased temperature causes faster aging of the coil insulation and connections. It also stresses the internal components of the relay, shortening its lifespan.

If a 24V relay on a 12V system gets too hot, it could sustain damage or even fail entirely. The contacts may become pitted or welded together in the closed position. The plastic case around the coil could also melt from excessive heat. Therefore, running a relay at higher than its intended coil voltage risks overheating and premature failure.

Shortened Relay Life

Running a relay at a lower voltage than its rated coil voltage can significantly reduce the lifespan of the relay. This is because operating the relay coil at a lower voltage produces a weaker magnetic field in the coil. According to research by TE Connectivity, this weaker magnetic field can result in the armature not fully opening or fully closing during operation [1].

As the armature repetitively moves from open to closed, these partial strokes put more mechanical wear and tear on the relay over time. This leads to contact oxidation and debris buildup which can cause the relay to fail prematurely. TE Connectivity’s white paper showed relay life decreasing by as much as 68% when operated at a lower voltage [2]. Therefore, running a 24V relay at 12V could potentially cut its lifecycle almost in half compared to operating it at the full 24V.

Solutions

There are a few ways to safely use a 24V relay in a 12V system:

Use a resistor in series with the coil to drop the voltage. Select a resistor value that will drop the voltage to 12V at the coil’s rated current draw. This reduces the magnetic field strength to a safe level for continuous operation. Make sure to use a resistor with an adequate power rating.

Use a 12V relay driver circuit. This uses transistors or other components to switch the higher voltage relay coil safely at 12V. Many 12V relay driver modules and circuits are readily available.

Use a DC-DC buck converter to step down the 24V supply to 12V. Make sure the converter can supply adequate current for the relay coil. The converter provides a safe 12V power source to operate the 24V relay.

Replace the 24V relay with a 12V version rated for the application. For example, considering voltage, current, and contact ratings. This is often the simplest solution without any additional circuits or components.

In summary, use a resistor, relay driver, buck converter, or 12V relay rather than directly connecting a 24V relay to a 12V system. This avoids issues with reduced coil magnetic field strength, intermittent operation, overheating, and shortened relay life.

Summary

In summary, using a 24V relay on a 12V system is possible but comes with some drawbacks. The relay may operate intermittently or not at all since 12V may not generate a strong enough magnetic field to close the contacts. This could lead to unpredictable operation, overheating, and shortened relay life. To use a 24V relay on 12V, increase the coil resistance through additional resistors so less current is required. Use a relay rated for both 12V and 24V operation. Or use a DC-DC voltage converter to step up the 12V to 24V. While possible, it’s best to use a relay rated for the system voltage to avoid issues. Carefully consider the risks before substituting a higher voltage relay.