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Sensors, Actuators, and Robotics » Controlling big loads

January 31, 2011
by Twarter369
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I am trying to control 7-8 big loads (120v@3a max) and I am wondering what I should be using? I read a project for a receptacle relay but it wasn't exactly what I was looking for. What are some ideas for reliably switching this with an MCU?

January 31, 2011
by Rick_S
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Like a big motor, or lamp, or heater?? AC or DC??? Usually triacs or mosfets will do the trick depending on what you are switching.

Rick

January 31, 2011
by Twarter369
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I should have mentioned it is AC

January 31, 2011
by Twarter369
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I want to switch a bunch of pond pumps to create a water display fountain I can control from my Laptop. Think the servo squirter project scaled up to a fish pond and 6x350gph pumps plus my outdoor light.

January 31, 2011
by Rick_S
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Triacs would do the job nicely, you can get them that handle 3 amps easily, I have some 12 amp ones at home. They can be switched on and off with a micro-controller pretty easily. Even easier yet, look up solid state relays. You can control them pretty easily and not have to mess around a whole lot with the higher voltage stuff.

Rick

P.S. Be very careful when working with household power. It can be deadly if not wired properly.

January 31, 2011
by Rick_S
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Another option would be to have one large pump that stays on then control the water with solenoid operated water valves. That way the pressure would remain constant and the flow would start when the valves are opened. You could probably get those with 12V or 24V DC coils and keep away from the high voltage altogether.

Rick

January 31, 2011
by mongo
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Give this link a try. Solid state relays are a simple but effective way to control bigger loads.

January 31, 2011
by Twarter369
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Thanks guys. I am thinking about the SSRs. The Large pump idea occurred to me. I have to buy a few Solenoid valves for another project anyway, so I might do that if the SSRs don't work out for some reason.

With the SSR, does it hook up like a MOSFET? With 120v AC source and drain poles and a 5v control pole?

February 01, 2011
by Rick_S
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No, they work like a relay. You have a switched load side, and an input side that would be like the coil of a relay. Depending on what the input side requirements are, you may need to drive it with a mosfet. At minimum, I'd couple it with an optoisolator to keep the micro totally free of any high volatge.

Rick

February 01, 2011
by Twarter369
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I read this: http://wiki.answers.com/Q/How_does_a_solid_state_relay_work and I think I got the idea conceptually before. Also he mentions them being internally optoisolated via an internal LED, should I still add another iso externally?

I am only a little worried about power creeping back to the chip. The relay control pin is going to be physically spaced out from the MCU by about 6 ft of 20awg wire and a 5a fuse. If some 120 did cross it, it would burn the wire and blow the fuse first, right?

February 01, 2011
by Rick_S
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Nope, if it has an internal opto it'll be just fine. As far as the fuse blowing, if you have a 5A fuse on the line from the micro to the SSR, and somehow the 120 did cross over, the 5A fuse would blow Waaay too late. Your micro would be gone. If you are careful, you shouldn't have to worry about that though. And since you are planning on playing with 120AC, please be very careful.

Rick

February 01, 2011
by Twarter369
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Thanks for your very valid concern. I used to do Residential Electric installs, so I have been around main voltages before. I have had my fair share of "oops" moments too, so I always kill the power to the circuit I am working on!

February 01, 2011
by mongo
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There are different types of SSR's. Some require 20 mA, some 3 to 30VDC, some even take AC in. All you need to do is do your homework so it is compatible. I use the Crydom SSR's like in the link a couple of posts back. There are other manufacturers too but they have the biggest selection.

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