Safety and switching 1500w of AC

Other Hardware Motors, Mechanics, Power and CNC
1

I'm really not sure whether this should be here for the power considerations, project guidance, or general electronics, but this seems the most likely.

I will be using a dedicated board to monitor temperature and switch a heater on and off in my hothouse. It's not about warm, but about keeping it warm enough that the tomatoes don't die (40F should suffice).

I have optically isolated 5v transformers to do this, and the programming and circuit are ready to go.

The issue is really how to mount this safely.

My current thinking is to use a blue plastic wall box to mount a regular AC socket, chop the socket end from an AC cord, wire ground to the socket with a screw and bolt, neutral normally, and hot (black) to one side of the relay, and a leftover bit of romex or the donor cord to connect from the other side to the hot side of the socket.

There is plenty of room inside the box for the relay board with about an inch of clearance (maybe fix it to the back of the box with a zip tie?).

The whole thing plugs into an outdoor GFCI socket (otherwise, I'd be inclined to use one in the box).

Any thoughts on safety that I'm missing?

For general info, the hothouse is 16'x16', treated lumber frame, with 6 mil plastic sheeting tacked on for the winter.

Irrigation is direct to plants and citrus trees with emitters--but 1/4" hose, "spider" manifolds from the underground PVC manifold, and the hose Ts are all known to fracture and spray at times . . .

Right now, the MCU is mounted on a breadboard with a DHT-22 temperature sensor and an AT-09 or similar BLE transceiver, running off an old iPhone 10W 5v adapter, which will power both the MCU at 3.3 by onboard regulation and the relay board (5v relay).

hawk

2

Good morning!

(new user to arduino and first time poster. I am a power system engineer in my day life and have an electronics background).

I am designing something very similar for my grow tent with lettuce and herbs in my basement in a chilly part of Canada.

You have covered things well for grounding, however there is one thing I want to point out and some ideas to fix the situation.

1500W will likely kill the relay you are using if you are using the standard 10A/120V Songle relay module as many use with the Arduino (you will be drawing about 13.5A through the relay on 110VAC and 12.5A on 120VAC operating voltage, which depends on where you live in the US). Your breaker on your house panel will not see this as an issue, however your relay will fail or worse, catch fire. First, put the heater on the medium setting, and look at the nameplate. Do not exceed 1000W for your setting. Second, install a 10A dual element fuse just upstream of the relay module to protect it from overloaded current. You will likely need more space for a fuse and holder (you can get a small fuse and holders from places like Digi-Key or hobby electronics stores) and you can use the tiewrap to mount it in the box.

With the needed space for the fuse, I would recommend using a 2-gang device box and putting the relay module and fuse in space #1 and your socket in space #2. I had thought about using my relay module below my outlet in my project too but the bend radius on the wire was just too tight and it will inevitably cause the wire on the socket or the terminal block to walk off the connector in time, causing a poor connection and possibly a teasing arc fault. You can get device cover plates that have a blank on one side and the outlet on the other, so you will still have ingress protection for the relay and fuse.

Finally, make sure you have the right entry fitting as well to reduce moisture and dust getting into the relay module. Relays are sensitive to moisture as they are electro-mechanical devices with some sensitive moving parts.

Have fun and I hope that helps :).

3

.Those mounting notions are helpful, and the fuse is something I hadn't even considered the fuse, but a great idea.

I've been toying with the idea of an outdoor GFCI box, perhaps with a hat over it (but why? any direct rain would be the time of year the drive isn't used . . .)

A pigtail fuse holder could also be the connector from fuse to socket . . .

My Chinese relay claims to be rated for 15A at 125VAC--and 10A at 240. Shouldn't that be enough?

Ultimately, I expect to use multiple relays on the high/low settings of the heater, and/or using distributed 250W rather than a central heater.

The first night, I used the heater's own thermostat, and beat a fnear-reeze that would have killed tomatoes. By luck, it seems--two weeks later, I came out and my tomatoes were dead and the heater was off. Diddling with the control shows that it was turning off ten or fifteen degrees lower than where I set it! Apparently, its own heat affected. Short version: Walmart and its infamous "quality" killed my tomatoes . . .

4

dochawk:
.Those mounting notions are helpful, and the fuse is something I hadn't even considered the fuse, but a g

My Chinese relay claims to be rated for 15A at 125VAC--and 10A at 240. Shouldn't that be enough?

Those claims may or may not be true. But look at the printed circuit board the relay is mounted on. Those traces and connections will not handle more than 1 amp. And the distance between traces and connections may not stand up to 125 VAC. Remember that is 125 volts RMS, so the peak is around 140 volts.

Paul

5

Grr.

There are indeed traces connecting relay to screw pins; I had though that they were integral!

I've also noticed that there do not seem to be opto-isolators on these single relay board, unlike my 16 relay board for my sprinklers. There are three surface mount resistors, an SOT23 device, and a little inductor, along with two LEDs. It also has the single power source, unlike the large board. Looking closer, it seems that the isolates are only on boards with two or more relays, notwithstanding the caption. It's one of these: 5v 1 2 4 8 Channel Relay Module With Optocoupler. Relay Output 1 2 4 8 Way Relay Module For Arduino In Stock - Integrated Circuits - AliExpress

My traces are .1" wide.

Should I solder 18 gauge directly to the pins of relay and screwdown?

Googling has found calculators for current and width, but it seems that I need to know the thickness of the trace, too (OK, that's obvious in hindsight . . .)

6

Wow, good call on that Paul, thanks. I am using a near identical relay module (Elegoo 4-relay module). To date, I have just been using LEDs on the relay outputs until I have my code tightened up; I haven't wired into my 120Vac outlets yet, but was going to do that this weekend.

I am wondering if a solid state relay would be a better way to go that is fully integrated in a single unit? I have seen the ones with screw terminals on both the low voltage control side and the Form C contact side. It is more money, but I think I would rather that than have my board smoked inside my device box!

Tim

7

Won't a solid state relay dissipate more heat in the unit itself?

These singles only cost about four bits, anyway. They are disposable--but my poor tomatoes!

Perhaps I should move directly to disassembling the heater to see if I can replace the power switch and the level selectors.

In a sane design, I would expect those all to be low current, and to use internal relays. However . . .

While I'm at it, I assume that I would either short out or open the potentiometer on the heater thermostat to make it "always on" if the power switch is on.

(and to add insult to injury on y dead tomatoes, I found my other old heater in the garage while looking for a power cord to cannibalize for this project. They would still be alive . . .)

8

dochawk:
Grr.

There are indeed traces connecting relay to screw pins; I had though that they were integral!

I've also noticed that there do not seem to be opto-isolators on these single relay board, unlike my 16 relay board for my sprinklers. There are three surface mount resistors, an SOT23 device, and a little inductor, along with two LEDs. It also has the single power source, unlike the large board. Looking closer, it seems that the isolates are only on boards with two or more relays, notwithstanding the caption. It's one of these: https://www.aliexpress.com/item/4-channel-relay-module-4-channel-relay-control-board-with-optocoupler-Relay-Output-4-way-relay/1899443885.html

My traces are .1" wide.

Should I solder 18 gauge directly to the pins of relay and screwdown?

Googling has found calculators for current and width, but it seems that I need to know the thickness of the trace, too (OK, that's obvious in hindsight . . .)

You can parallel the traces as you suggest. BUT first cut both traces at each end so the traces cannot participate in the AC circuit. The SOT 23 is the transistor that your Arduino drives, which switches the relay coil on/off. The "inductor" is a diode that protects the SOT23 for sudden death from the collapsing magnetic field in the relay coil.

Please use insulated wire for your 18 gauge.

Paul

9

Paul_KD7HB:
You can parallel the traces as you suggest. BUT first cut both traces at each end so the traces cannot participate in the AC circuit.

That would have been the next question :slight_smile:

The SOT 23 is the transistor that your Arduino drives, which switches the relay coil on/off.

I'm
That would have been my first guess, but . . .

The "inductor" is a diode that protects the SOT23 for sudden death from the collapsing magnetic field in the relay coil.

I'm going to have to take your word for that--even with my reading glasses, magnifying lamp, and the booster bubble in the magnifier, I just can't tell. ::slight_smile: That coppery color (which I thought was a coil) doesn't look ridged, though

Please use insulated wire for your 18 gauge.

Yikes! Heavens yes!

I have plenty of the cord available, and about 20 feet of Romex left.

10

tricard:
Wow, good call on that Paul, thanks. I am using a near identical relay module (Elegoo 4-relay module). To date, I have just been using LEDs on the relay outputs until I have my code tightened up; I haven't wired into my 120Vac outlets yet, but was going to do that this weekend.

I am wondering if a solid state relay would be a better way to go that is fully integrated in a single unit? I have seen the ones with screw terminals on both the low voltage control side and the Form C contact side. It is more money, but I think I would rather that than have my board smoked inside my device box!

Tim

Those relays are wonderful for switching LEDs! I have examined a couple that had welded contacts and one that refused to close a contact. The contacts are actually TWO tiny balls of metal that touch when the relay is powered. They are balls because the alignment is pretty iffy and being balls, they will always find some contact. Real relays will have slightly convex contact surfaces, so as they erode, they will still make a contact.

MY one relay that refused contact had a spot on insulating material on one of the balls. The relay worked for a while, so I don't know where the material came from, unless it was trash left from the manufacturing process.

I love SSRs! So easy to work with and you can find them for any current needed. Depending on the application, they will need a heat sink and it may need to be attached to a metal surface. I have some with built-in heat sink/mounting plates and others with plastic cases that are air cooled.

Paul

11

Hmmm. I may need to do a bit more research. I am not driving my heating loads with these relays, just fans (about 100W total), LED lights (at most 2 per relay (190W per light fixture), or roughly 400W per relay) and maybe a heating pad for under my seedling tray (50W). So, highest load is 400W or 3.3A. I may risk this setup as an open face prototype for a bit to see how hot it gets and if it performs as expected. If needed I will go to SSRs in rev 1 :).

Do you have a preferred vendor or package for SSRs? I am familiar with P&B and Omron in my day job, but those are for industrial applications and may be more rugged than what I would need for this non UL/CSA/commercial home-brew product :smiley:

12

My preferred vendor is Ebay. Even though my general manager purchases thousands of dollars of components a week, I prefer Ebay for hobby type stuff.

Paul

13

dochawk:
I have plenty of the cord available, and about 20 feet of Romex left.

I noticed a post in the tips thread suggesting metal wick solder remover for bolstering current capacity of traces.

It kind of sees like it would make sense, and it would be physically attached where there was already a spare trace, but I'm really skeptical about the notion . . .

14

Copper is expensive. Solderwick is good copper. It is very easy to get it to take solder. :slight_smile:

Another source of good, cheap braided wire is the shield from coax TV cable. Very good as a grounding strap where real copper ground strap is difficult to get.

15

Gee, and I threw out a whole bunch of old tv coax recently, too . . . probably still tons left, but there's only 2" of trace to reinforce per relay, anyway . . .

16

You could also go with one of these - https://www.digikey.com/product-detail/en/omron-automation-and-safety/LY1-DC12/Z785-ND/126853 and eliminate traces altogether. Solder directly to the relay or mount it on a socket.

relay.PNG

relay.PNG

17

Wouldn't that mean generating 12vdc, building my own driver circuit, and so forth?

I'm not seeing what that would get me over just soldering to the relay leads on my little board . . .

18

dochawk:
Wouldn't that mean generating 12vdc, building my own driver circuit, and so forth?

There are different voltage options for driving the relay but, yes, if you're not using the system 5VDC you'll need something to generate that voltage.

dochawk:
I'm not seeing what that would get me over just soldering to the relay leads on my little board . . .

Adding solder-wick to increase the current capacity just seems kludgy to me. A relay, with optional socket, rated to carry the current with some healthy headroom would be my preference.

YMMV