Relay wiring question with 120v power
Hi all,
I have a standard USA 120v device that i want to control with my parallax basic stamp controller. I need to turn it on/off at certain time intervals. The coding should be straight forward, but I am confused on the wiring.
My initial thought is to purchase a simple relay (https://www.parallax.com/product/27115) and send the control signal to it... but how would i connect the 120v to it so it turns on/off with the control signal?
Can the parallax relay handle 120v? (i see its max is 10amps)
The 3 screws on the top... 120v power in, 120v power out and ground? Is that correct?
Just want to be sure i understand this before buying the items
...and blowing stuff up
Thank you for your help!
I have a standard USA 120v device that i want to control with my parallax basic stamp controller. I need to turn it on/off at certain time intervals. The coding should be straight forward, but I am confused on the wiring.
My initial thought is to purchase a simple relay (https://www.parallax.com/product/27115) and send the control signal to it... but how would i connect the 120v to it so it turns on/off with the control signal?
Can the parallax relay handle 120v? (i see its max is 10amps)
The 3 screws on the top... 120v power in, 120v power out and ground? Is that correct?
Just want to be sure i understand this before buying the items
...and blowing stuff up Thank you for your help!
Comments
John Abshier
https://parallax.com/sites/default/files/downloads/27115-Single-Relay-Board-Guide-v1.0.pdf
I'm not sure about Parallax's relay module, but some of the Chinese relay modules I have are active-low, meaning that the relay is energized when the control pin is low, and off when the pin is high. A bit counterintuitive.
Anyone with doubts or inexperience should NOT wire 120VAC as there are serious shock and fire hazards. This relay will work fine with low voltage DC as well as low voltage AC. Low voltage projects from battery supply are much safer for novices. Consider including a fuse for fire safety against short circuit or overload hazards.
The rating is usually for resistive loads only, an inductive load would be derated to maybe 6 Amps at tops. Use Google for 'derating relays' to get an idea of what industrial installations do. You might also try 'relay snubbers'.
Snubbers on the relay contacts will extend the useful life greatly. A capacitor and resistor that a selected for the the appropriate load work with AC. And different schemes are used with AC and DC. With DC, you might get by with a rectifier diode in the reverse direction of the DC power.
Personally, I have my doubts about relays mounted on printed circuit boards.
The printed circuit board many NOT offer insulation or capacity for 125VAC at 10 amps. I strongly feel you should take a good look at where 125VAC might be exposed to human contact, and limit printed circuit board mounted relays to something like 2-3amps service at best (Yes, de-rate according to the board construction and put the relay in an insulated enclosure to avoid shock and fire hazard.)
And most importantly, wires tend to have 99.9% of their failures at the connections due to flexing and pulling. So just having the terminal block hold the 125VAC wires is not a safe practise. (We tend to not be so concerned if the relay is attached to low power, low voltage uses.) Other means of mechanical strain relief should be included in anything that is going to be a permanent installation.
* There is only one relay on this particular board that required a snubbing RC ... R = 27 Ohms 3 W ... C = 100nF 275V
* As a typical rule of thumb you can run 1 Amp for every 10 mil of board trace ... so 80 mil traces ganged on two layers would be enough to handle 16 Amps
* The green male PCB connectors have a matching female (not shown) connector with screw terminals
* For DC operation you typically derate the AC voltage by 1/8th to find the DC operating voltage at the same current rating ... so about 30VDC
But my whole point is any relay board design is only as strong as its weakest link. And often the circuit board is the weak link both in terms of current capacity and insulation. And fuses are completely absent. Just looking at the relay rating and presuming everything else with regards to safety is included, is foolhardy.
For 120VAC or 240VAC, I still prefer DIN Rail relay systems inside a safe enclosure. Everything is modular, replacable, accessible, and easy to inspect. I admit that it is a bit more bulky, but it won't set the house on fire or electrocute the family dog.
For 12VDC robotics, I like automotive relays that are built to tolerate a hostile environment under the hood.
For beginners, 120VAC or 240VAC does require better construction skills and more diligence about insulation, fusing, and all aspects of safety.
I just want everyone to be safe. That Keyes SR1y and its cousins are everywhere on EBay. Parallax had nothing to do with the quality control or design. With relays on printed circuit board, one must depend on the designer and manufacturer for quality control and safety. Often the safe products are more expensive as they seek UL certification.
Keyes also as 2 relay, 4 relay, and 8 relay boards with similar construction.
True, but this is science, and ...
Extremely helpful, thank you!
They don't have any desire to comply with US or European standards for 120VAC or 240VAC switching. They just happen to make a bunch of relay boards that get shipped all over the world under the guise of Arduino expansion boards. They get to do so because they are considered 'components' and not finished product.
The relay systems that are certfied safe in the USA are much more expensive than the Keyes products, and well worth the extra cost for industrial installations.
The bottom line is that if you are wiring 120VAC or 240VAC for anything, it is up to you to do everything that assures a safety outcome. Skipping an insultated housing, a fuse, strain relief for wirings, and so forth is risky.
Trying to get an overloaded relay to work because the right part is too expensive is never going to work out well. So I take a very conservative approach and de-rate the capacity.
If you want to do something durable and useful, take the time and effort to do it right... even if you are just rewiring an old table lamp or temporary lighting for a birthday party in the back yard.
And find out how to do it right from your local authorities. The internet is full of opinions that will not apply. In California, the government is likely to say you need a licensed electrican for anything on AC mains. In Oregon, a homeowner can get a permit to wire his own residence and the inspectors with mentor you. With 50 states and other regions making different rules, we can't keep up.
But we all tend to do a bit of DIY that doesn't have an electrical permit on a don't ask, don't tell basis. That doesn't excuse poor quality.
If you just want to fool around and learn for fun, stick with low power circuits and stay away from AC wiring, as the mistakes are less hazardous.
I am not a licensed electrician, but I was a licensed general contractor and pretty much picked up all that needed to be done from electricans I hired and inspectors telling me what wasn't right.