Can you split the 12v output to two pieces of equipment

Yes.

The device with the trigger will have a current rating on its output. Hook it up individually to each piece with an ammeter in series. Add the two currents together. If they are less than the allowable current draw you are good. If not use a relay.

Craig.

But will it work?

Depends on the current capacity of your 12 volt source and the current draw of the lift and screen.

If your devices exceed the capacity of your source you could have your source trigger a 12v dc relay that would pass on a 12 volt power supply with higher capacity (current capability) to the screen and lift

If you need to trigger a lot of devices and the source of the 12V trigger can't handle all of them, it's not hard to use a higher capacity 12VDC supply and a relay for this. It's a $3 relay and whatever the wall wart costs, plus a little time.

You can definitely burn out the 12v trigger output if the devices you're controlling exceed it's rating.

Recently I was switching two 12v controlled devices with a Marantz AV8802.  The 12v output on the Marantz was rated at 150mA and the sum of the two devices was also 150mA (acording to specs, not measured).

It worked for about 3 months and then burned up the 12v output and I had to send the 8802 out for repair.

After the pre-amp was repaired, I used an additional power source and relay that could easily handle the load.

If you do need a relay. Verify the current draw of it. Many relays from local auto stores and such have too much draw for many 12v triggers. I ordered a bunch of small relays years ago. Think Mousser.

Yes use a solid state relay. Not an automotive relay.

Craig.

Whatever you use, be a pro and learn what's involved.

Automotive relays are built to switch a lot of current and can be made with low resistance coils, so lots of current is needed from the trigger. But they are so simple to use, with slip-on connectors! Maybe someone here could seek out or share with us a good relay for our purposes and recommend it for us to use! The goal would be one with 1K ohm or higher relay resistance.

Too much current draw from a trigger can indeed burn out the trigger circuit, just as putting a 2 ohm load on a power amp can burn out the amp, and there's no kind of protection on a trigger except, perhaps, a resistor to try to limit the current draw.

I wonder, though, if the relays drawing 150 mA might not have ruined the trigger due to another fact: relays output negative voltage when you turn them off! To go back to the power amp metaphor for a moment, this is like connecting a battery to the output of a power amp every time you turn off the relay.

When you turn on a relay, a magnetic field is built up in the coil. That's what pulls the relay contacts closed. When you turn off the relay, the magnetic field collapses very rapidly. And what generates electricity? A rapid change of a magnetic field in a coil.

When a relay turns off, a YUGE* voltage spike (yugeness depending on the coil) is generated by the relay. As it happens, the polarity of this spike is the opposite of the trigger voltage. These voltages, fed back to the trigger circuit, could blow them out.

The cure is to put a diode in parallel with the relay, like so:



The file name of this image uses the term "de-spiking diode." And it is.

The red line shows the turnoff spike being shorted through the diode so that its voltage is not applied to the rest of the circuitry. (Tech note -- this trigger uses an open collector circuit, where one lead is always hot and the other lead is switched. Most triggers we deal with switch the hot and have the ground always connected. The relay works the same with either kind of voltage source.)

If you have not heard of this before, well, it's one of the most basic and important things to know about how to use a relay with a DC coil voltage. Voltage spikes occur with AC relays, too, but they can be either polarity so the protection against them is beefy supplies that don't get damaged by these spikes. The power line is such a beefy supply.



*Thank you, Drumpf**
**Thank you, Oliver

Wikipedia has a discussion.

Bosch has relays with a diode across the coil to prevent spikes from messing with microprocessors- one easy way to tell is by the metal cover with a green stripe on it.

Typically, these require about 150mA-160mA, but this Omron should work- 100mA coil, [email protected]

[Link: mouser.com]

Half you guys do security: use a 4 wire smoke EOL SPST relay module with a 1A wall wart in series. They usually take 20 to 50ma to throw, well under your AVR triggers that top out at 150ma.

You could throw a few screen/motor triggers or Sonance AS/1's no problem.

It's important to note that the coil is 12 volts and the contacts are rated at 24 volts. And this current tells us the coil's resistance is 120 ohms. My guess of a thousand ohms is a bit high as I didn't do the math. Let's say we think we can tolerate 50 mA out of the source. Then the coil resistance would be 240 ohms. I'd go for something in that range or higher resistance.

Every ADI carries these: Altronix Corp. Model #: RBSN

12VDC or 24VDC selectable operation
1,000 ohm coil draws only 15mA
Factory set for 12VDC cut jumper J3 for 24VDC operation
2 amp/120VAC/28VDC DPDT contacts



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For the sake of discussion, if you are doing circuit design, you never run any component or circuit at its maximum capacity. If you are putting in a resistor and calculate that it will dissipate 1 W, you would put in a minimum 2 watt resistor.

So if the circuit is rated at 150 mA, I would probably make sure I am actually drawing 75-100 mA. As Paul noted, you can keep creeping higher, and it will work, but it will not last.