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Hi Folks.

The next stage of my little project is to turn on the AC when the fish tank gets too warm. I'm not going to try to use the Arduino as a continuously controlling thermostat, just using it to kick it on. Easiest thing is to just put a switch in between the AC unit and wall power. It's an old Fedders through-wall unit. Landlord is a EE, and he says there's no problem switching just one leg of the 220VAC supply. (I had initially thought of using a DPST NO relay.)

So, off I went, looking at SSRs. I'm wondering whether there's any reason to be skeptical about the $10 unit at MPJA, given that anything else I can find is $30 or more (Mouser, Panasonic). The one at MPJA kinda looks like it has a Kodak logo on it, which makes me nostalgic for Kodachrome, but informs me not one whit about whether it's a good part.

BTW, yes, I will be appropriately careful about using mains voltage. I'll be building something resembling this powerswitch tail.
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Nope, no qualms about MPJA. They're a surplus distributor so sometimes parts are in stock, sometimes they're not, sometimes they just entirely disappear once stock runs out. In exchange for this uncertainty you get a good deal.

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The only minor criticism I have is that it doesn't seem to include a "safe" touch cover.  Many SSR's, even the inexpensive ones, have a clear plastic shield that makes it more of a challenge (not an impossibility) to touch the live terminals.  I'm being picky, but if you can find one similarly priced with a cover, that 's a better solution.

Jim
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Cool. Thanks RC.
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Just a followup. This SSR showed up. It's a Kodak, PN 611490. I have duly searched for a datasheet and come up empty. The input resistance is about 891 ohms, so that's good news. But what I hadn't thought about was whether I need a big heat sink for the load. From the faceplate of my A/C unit, the load will be ~ 8A @230VAC. The MPJA page for it says,
Quote
Opto-Isolated, "0" voltage turn-on, internal snubber
which tells me not much.

So, anyone have some experience or memory to draw on here?

@JimG. It's going to be inside an enclosure, but your point is well taken.
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Just a followup. This SSR showed up. It's a Kodak, PN 611490. I have duly searched for a datasheet and come up empty. The input resistance is about 891 ohms, so that's good news. But what I hadn't thought about was whether I need a big heat sink for the load. From the faceplate of my A/C unit, the load will be ~ 8A @230VAC. The MPJA page for it says,
Quote
Opto-Isolated, "0" voltage turn-on, internal snubber
which tells me not much.

So, anyone have some experience or memory to draw on here?

@JimG. It's going to be inside an enclosure, but your point is well taken.

Most of those industrial SSR are pretty much the same. The input side is straight forward, an internal LED with built in series current limiting resistor. You can control it directly with a arduino digital output pin and a ground connection. The internal snubber part is actually good for you. Most SSR don't handle inductive loads well (fail to turn on, or fail to turn off) when driving inductive loads (like motors). Snubber circuits help deal with this kind of problems, however it kind of depends on the amount of induction and the specific value of the snubber components in your SSR. Worst case you may have to add external snubber components if needed.

As far as heatsink needed, yes you will. The maximum current ratings for SSR only apply if you can also keep the device below it's maximum operating temperature rating. Past experience I've has is that most SSR will 'drop' about 2vac across it's switching device, so at your 8 amp current load, that's 16 watts of heat dissipation that will surely overheat the SSR unless bolted down to a large enough heatsink using proper heatsink paste, etc.
So how big a heatsink? Probably larger then you wish, have room for, or have avalible on hand.  smiley-grin
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I have been using similar units (that I bought used at a hamfest) to control my baseboard heaters for several decades.  The only heatsink is the 4" x 4" aluminum plate that I fabricated to mount them on and to cover the standard outlet box that they are mounted in. 

Don
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As far as heatsink needed, yes you will. The maximum current ratings for SSR only apply if you can also keep the device below it's maximum operating temperature rating. Past experience I've has is that most SSR will 'drop' about 2vac across it's switching device, so at your 8 amp current load, that's 16 watts of heat dissipation that will surely overheat the SSR unless bolted down to a large enough heatsink using proper heatsink paste, etc.
So how big a heatsink? Probably larger then you wish, have room for, or have avalible on hand.  smiley-grin

Heh. Well, my motto is, 'Nothing exceeds like excess'. I'll grab a CPU heatsink at MicroCenter. And then maybe I'll lap the back plate of the SSR. And use heatsink compound.

I have been using similar units (that I bought used at a hamfest) to control my baseboard heaters for several decades.  The only heatsink is the 4" x 4" aluminum plate that I fabricated to mount them on and to cover the standard outlet box that they are mounted in. 

Don

That's an idea too. But, what would Tim Taylor do? smiley
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That's an idea too. But, what would Tim Taylor do?

He would mount a freakin 10HP snow blower pointed right at that SSR.  smiley-wink
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That's an idea too. But, what would Tim Taylor do?
I guess I am culturally deprived.

Don
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That's an idea too. But, what would Tim Taylor do?
I guess I am culturally deprived.

Don

From the show, Home Improvement
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SSRs leak current when "off". The isolation is substantially worse than a mechanical relay.
see http://www.electronicspoint.com/solid-state-relay-leak-t6756.html
This may or may not be a problem for your application.
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Thanks for the tip. I don't know whether that'll be problematic. When I was talking to the landlord about doing this (turning on the A/C when the aquarium goes over-temp) he was unconcerned about my using an SSR. He's a EE, so I figure if it was problematic, he'd've said something. (He's on vacation now, can't bring it up.) But I will keep it in mind.
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  One important thing that has not been mentioned is that you do not want to cycle power to the A/C compressor off and on without giving it a chance to settle. Basically, the "high pressure" side of the A/C needs time to equalize with the "low pressure" side before the unit is restarted. If you do not allow this time the compressor has to start under load and can burn up. Just to throw a time out there, you need to wait about a minute or two between turning the unit off and then restarting it.

 Industrial A/C units have protection from quick restarts but, home units usually do not.

Mark
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That part I do know about, but thanks anyway. I have percolating in my head ideas about how to, for example, switch between manual and automatic control, and how complex I want to get (likely, not very). But the auto mode will function only from noon to 8pm, or something like that, and will just turn it on and latch, and let the unit's thermostat function. And I'll put in some code to use a switch to go between manual / auto, or off / on / auto. Simplest thing is to just hit the off button on the unit at night, and then remember to turn it on in the morning, so the Arduino can then turn it on if need be.
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