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I'm not an electronics student but have a decent grasp on the fundamentals. I am attempting to use an assortment of components to facilitate a driveway gate automation. The parts I have currently are:

  • 2x linear actuators (motors)
  • remote control receiver/relay
  • power supply

The problem I have is that I will be wanting to operate the motors at 24V (as opposed to 12V, which would make them slower; yes, 24V is within acceptable operability of motors), and by my calculation, this would push the max current usage to about 14A total. The wireless receiver/relay maxes out at 6A, and I would like to use this remote system for various reasons.

My thoughts are that I could simply wire in an additional 20A relay to operate the motors, but I'm really not quite sure how that should be done. There are so many relay options, I'm really not sure where to start. Could someone perhaps point me in the right direction? Perhaps even a schematic? Thanks!

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    \$\begingroup\$ Use your little relay to drive the coil of an even bigger relay. Don't parallel them so they share the load since you can't guarantee they switch at the same time. \$\endgroup\$ – DKNguyen Jun 22 '19 at 19:32
  • \$\begingroup\$ That's essentially what I was thinking. But where I get confused is types of relays, be it solid state or electromechanical, the variance in relays with different numbers of pins (some have 2 pins, others have 5 or 6??) so I guess my question comes down to which relay, or how do I go about selecting one? \$\endgroup\$ – jw11432 Jun 22 '19 at 19:53
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Just stick to electromechanical relays for simplicity.

Use your little relay to drive the coil of an even bigger relay. Don't parallel both relays' main contacts so they share the load since you can't guarantee they switch at the same time.

schematic

simulate this circuit – Schematic created using CircuitLab

If a relay has two pins it's probably a reed switch where applying a nearby magnetic field closes the contacts inside. This means that the magnetic field is the control signal and the two pins are for the load. You don't want this.

Relays should have a minimum of 4 pins. Two for the coil (control signal) and two for the primary contacts (main load).

If a relay has 5 contacts it means that two are for the coil just like before, and the three remaining pins are for the load. They are the common, normally open, and normally closed. When the coil is unpowered, the common is electrically connected to the normally closed terminal and disconnected from the the normally open contact. When the coil is powered, the common is electrically connected to the normally open contact and disconnected from the normally closed contact.

If the relay has more than 5 contacts it might be switching multiple electrical paths simultaneously.

There are also latching relays that have springs which hold the primary contact in the current position even if the coil is unpowered.

It is most convenient to pick a relay that has a coil voltage rated for the voltage you intend to drive it with. But the current is what really switches the relay.

The coil has a resistance and a required current and the voltage that can drive that current through the resistance is what ends up being the voltage rating for the coil. If you want to apply a voltage that is higher, then you just need to add more resistance externally.

So if your coil voltage rating matches your applied voltage you can connect it up directly since the coil's inate resistance is able to limit the current being pushed by the applied voltage at a safe, but usable level. But you can always connect a HIGHER voltage than what the coil than what is rated if you add in a properly sized series resistor to limit the current at what the coil operates at so the coil does not burn out.

Note that it's harder on a relay to switch DC than it is to switch AC. When the contacts open an arc is made (especially if it's interrupting current to an inductance like a coil or motor). The AC sinusoid naturally crosses zero which self-extinguihses the spark. DC does not.

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  • \$\begingroup\$ Exactly! I expected at least 4 terminals, one pair for load and one pair for coil, so I was confused by the different pins. Ok, so I suppose the flow would be power supply>receiver>20A relay>motors. Thanks for your input! \$\endgroup\$ – jw11432 Jun 22 '19 at 20:09
  • \$\begingroup\$ Not quite. Lemme draw schematic. \$\endgroup\$ – DKNguyen Jun 22 '19 at 20:11
  • \$\begingroup\$ I must have worded that in a way that didn't come across right lol but that's essentially what I was thinking, thank you for the schematic! I don't suppose you could recommend a relay? I'm sure a 4-pin would suffice for my needs. \$\endgroup\$ – jw11432 Jun 22 '19 at 20:23
  • \$\begingroup\$ The T9A is my goto. Note that it's harder on a relay to switch DC than it is to switch AC. When the contacts open an arc is made (especially if it's interrupting current to an inductance like a coil or motor). The AC sinusoid naturally crosses zero which self-extinguihses the spark. DC does not. \$\endgroup\$ – DKNguyen Jun 22 '19 at 20:23
  • \$\begingroup\$ Since the motors are running on DC, is there any issue there? I really don't have an option to utilize AC switching. \$\endgroup\$ – jw11432 Jun 22 '19 at 20:45
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Use the wireless receiver to drive a relay with contacts rated at 20A or more, then drive the actuators via those contacts.
You don't say what happens when the open signal stops, Do the gates close under some spring action or are they driven shut by reversing the actuators? In either case the contacts on the 20A relay need mimic those on the 6A part.
14A is a pretty large current, you need to ensure that there is an adequately rated wire if it's a long run.

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  • \$\begingroup\$ Correct, reversing polarity (courtesy of remote unit) accomplishes the shutting. As for wire, I'm using 12/2 landscaping wire. It is rated for 600 volts and upwards of 20 or 25A, I forget which. So I should be good there. \$\endgroup\$ – jw11432 Jun 22 '19 at 20:02

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