24v switched 12v load relay?

Thread Starter

tjonesdfw

Joined Nov 18, 2022
2
Familiar with how to handle a 24v dc load with a 12v dc switch and relay, however I'm trying to do the opposite and struggling to find an appropriate relay and wiring diagram to do so. We need a 24v dc input from an existing switch on heavy equipment to trigger a relay that will control on a 12v load. The constant + to the relay would be fed from a 24 to 12v converter. Could anyone here lend some advice?
 

Irving

Joined Jan 30, 2016
3,190
Why not use a relay with a 24v rated coil? Switching 12v on the contacts is no different to switching 24v.

Its not clear what you are trying to achieve. Are you trying to switch on some 12v equipment, from a 12v source, when the 24v equipment is powered on ?
 

Thread Starter

tjonesdfw

Joined Nov 18, 2022
2
Why not use a relay with a 24v rated coil? Switching 12v on the contacts is no different to switching 24v.

Its not clear what you are trying to achieve. Are you trying to switch on some 12v equipment, from a 12v source, when the 24v equipment is powered on ?
That's what I was trying to ascertain... whether a 24v relay would still switch 12v without issue if the 12v load is small. We're currently using solid state circuit based switch systems for the 12v lighting, but the low draw oftentimes triggers the low voltage shutdown, AND we currently have to switch all 12v equipment separately from the 24v equipment. Someone suggested we get away from the solid state system and use a relay to accomplish both eliminating the low voltage shutdown, and to get 12v equipment to turn on simultaneously with the 24v equipment. So yes, we're trying to switch on 12v equipment, from a 12v source, when some of the 24v equipment is powered on in our machinery. For example, when the 24v running lights are switched on, it would trigger 12v area lights around the equipment.
 

Tonyr1084

Joined Sep 24, 2015
7,199
Relays have various ratings. First is the coil operating voltage. Since you have 24 volts - you want a coil that operates on 24 volts. Anything less and you're asking for a fire. Next, the coil type is important as well. Some coils are designed for AC operation and some are designed for DC operation. Some are designed for intermittent operation while others are designed for long duration operation. You would not use a horn relay to power cooling fans. The horn relay isn't designed for more than a short blast in rare circumstances where the fan relay is designed to run the fans for hours at a time. (examples of automotive relays)

Then there's the contacts. Some are single pole single throw. That means there is only two contacts. Either it's ON or it's OFF. Some are single pole double throw. That means there are three contacts. The common arm makes contact with one contact in its norma resting place, often referred to as "NC" or "Normally Closed". The other contact is marked with "NO", Normally Open. That's the relay's resting position when the coil is not energized. When not energized the "C" and the "NC" are in contact while the NO is open. When the coil is energized the relay changes position. Now the NC is OPEN and the NO is CLOSED.

The types of contacts are rated for different voltages and currents. Those ratings must not be ignored. Switching a high amperage AC load on a relay that is rated for that kind of current - but on DC will likely arc over and the load will not shut down. Also, the relay will suffer a melt-down.

So you can have a coil voltage as low as 5V (or lower) switched by 5V while the contacts can control 120VAC at 20A. That same relay may control 12VDC at 5A. Or whatever the manufacturer has engineered into the design. In short, the switches are independent of the coil voltage. Just be sure to use the correct relay for the job.
 

Irving

Joined Jan 30, 2016
3,190
There are literally 1000s of relays suitable for this task. Here is a typical example spec:

Coil rated at 24v 1.2W continuous operation
Contacts individually rated 300W (30V@10A), minimum load 12v@10mA
2 or 3 changeover contacts rated at 20x10^6 operations
Octal plug-in base with push-on or screw terminal options
Mechanical (and optional, electrical) indication
Test button

Available at Mouser, Digikey, etc.
 

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