Hey guys, I'm newish to electronics and the Arduino, but an old hand at programming. I'm trying to build a modified light-chase for a friend who's doing an art project. 40 incandescent lights, in groups of 4, need to light up to the top and come back down. I was looking at doing this with a decade counter and flip-flops, but I decided to go to an Arduino instead as it's more flexible.
In any case, these incandescent groups will be about 5A/100VAC. Obviously, the arduino can't supply this so I've been speccing out relays. Per a tutorial's recommendation, I bought several transistors (p.n. 2n222a), so I'll use the Arduino to the leg of the transistor, and use it to switch ~12v for use in the relay.
Please tell me if any of the above looks wrong, but that's not my real question. I'm a little confused by all the specifications for choosing a relay. I think I want my "coil voltage" to be 12ish v DC and my "contact voltage/current" to be >110v/>5A
That relay will work... you mention your load is 5 amps at 100vac? That relay is rated at a max 5 amps AC, at max voltage of 240vac. So, Yes, the relay will work - but is being pushed to the max. It seems you will be breaking the contacts under full load? That puts even more stress on the contacts.
Interesting - I just checked the datasheet linked on the Newark website and it shows a max of 15amps switching with 15amps at 120 vac, but as I get to the graphs at end of 4 pages, it seems to show a max of 10amps - those specs would say the relay is just right.
Relay should be ok, but I am very much open to other ideas.
Incandescent bulbs have a lower resistance when they're cold, so they can draw quite a bit more power (called "inrush current") when they're first turned on. So your 5A nominal load is probably pushing those relay contacts too hard.
Relays are mechanical devices: if you're going to be running the chase lights fast and/or continuously, you have to worry about how long the relays will last with all that switching.
If I were doing this, I'd definitely consider these solid-state relays. But note that they derate rather dramatically above 30 degrees C (a serious concern here in AZ), and they have a zero-crossing trigger inside. Which is better for bulb life and inrush current concerns, but will have a small effect on the timing of your chase lights. This probably won't be a problem, but might be.
As you guessed correctly, these will be switching rather fast and frequently. So it sounds like solid-state is the way to go.
This is far from a professional installation. If it's up long enough for stuff to fail (doubtful), I can fix it. IOW, downtime is pretty much unimportant.
The timing is also pretty much unimportant, as long as it's faster than like a half-second. In fact, timing is irrelevant, as long as it's consistent.
These won't be used in a super-hot environment, but that 30c is good to know - that's quite a bit lower than I would've thought.
Ran - I like the look of that relay. At that price, I can buy extra. What do you mean by a zero-crossing trigger?
A "zero-crossing trigger" means that the voltage isn't turned on the instant your circuit says "go": what actually happens is that it arms a circuit that enables power the next time the AC voltage swings from positive to negative, or vice-versa. That's nice for bulbs, because it means they warm up a little more gradually, but it also means that the actual turn-on could be delayed up to about 8mS (12mS on 50Hz lines).
This probably won't matter, but I haven't actually tried this with incandescents (the only "chase lights" I've ever done were relatively slow, and with LEDs), so I brought it up because I have the perverse sort of mind that instinctively seeks out and flags possible design flaws
OK, so a zero-cross trigger also sounds like it'll reduce noise on the Arduino/coil side of that relay, which is good. 8ms (max, of course) is plenty fast enough. The timing on the first light is approximately 2s, and the last light is approximately 100ms. 8ms is well within the margin of even the light coming up.
I really don't have anything else to worry about - thanks a bunch! The fact that those relays can be powered from the Arduino itself greatly simplifies the design as well.