Relays and solenoids and their use are pretty straightforward, but there are some basic requirements to be aware of when choosing the type of relay or solenoid that will best suite your needs.

When choosing a relay, such as a standard Automotive Horn Relay or Headlight Relay, the current rating marked on them should be their maximum constant current rating, in other words, this will be the maximum current that the given relay can handle for an unlimited time.

The current ratings for Solenoids is not so straightforward. Many solenoids are made for short time periods of operation, such as your starter motor solenoid. The contacts on these types of solenoids are designed to take very large currents but only for a short time.

Solenoids usually have a PULSED or MOMENTARY current rating, but if these types of solenoids can be used in constant current applications, then they will have a CONSTANT current rating marked on them as well and this will often be half, or even less than half the PULSED current rating and the confusion doesn't end there.

Many PULSED or MOMENTARY type solenoids can NOT be used in constant current applications such as Dual Battery Systems, because their coils are not designed to be powered for long periods of time and doing so will cause the coil to over heat. So choose your solenoids carefully.

As stated above, standard Automotive Horn Relays or Headlight Relays are current rated for the maximum continuos load. But if you are using non Automotive type relays you must check that the contact load for these relays are designed to be used for DC applications.

Even though the relay might have a 12 volt coil, many non Automotive type 12 volt relays are designed to be used for AC control applications only and are marked with only an AC current rating.

Unless the relay is specifically marked with a DC current rating, or both an AC current rating and a DC current rating, the relay should not be used in DC applications, particularly high current applications as the contacts are not designed to handle the arcing that occurs when switching large DC currents and the contacts may not last long meaning the relay could need to be replaced after just a short period of use.

Where a relay is marked with both an AC current rating and a DC current rating, it is not uncommon for the DC current rating to be much lower than the AC current rating, so make sure you check the DC rating when using the relay in automotive set-ups.

Solenoids are commonly available in two forms, Normally Open and Changeover, while Automotive Relays are available in a large number of different configurations but only data relating to the most common Automotive type relays is displayed bellow.