0
\$\begingroup\$

I had thought I could just connect the 24vac input to a relay input, and then setup a pin as an input on the arduino and be able to know that the relay had closed. But now I'm reading that relays need a minimum current or some such, and I don't really understand how I could meet that since I'll have no load.

I know just barely enough to even be dangerous. Am I even making sense?

\$\endgroup\$
  • \$\begingroup\$ Why not use diode and R divider with cap to detect 24Vac as DC? Contacts need a wetting current such as a pullup R and 10uF Tant cap. to prevent oxidation unless gold-plated rated <2A \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 22 '19 at 18:22
  • \$\begingroup\$ Yes you are making sense in that a little knowledge can be dangerous but, do you have a proper techy question? \$\endgroup\$ – Andy aka Jan 22 '19 at 18:24
  • \$\begingroup\$ Are you trying to switch the 24V side? Are you detecting the 24V? \$\endgroup\$ – Voltage Spike Jan 22 '19 at 18:24
0
\$\begingroup\$

Your setup is not super clear, but I will try to interpret. It sounds like you have a 24v ac signal connected to the coil on a relay. When that signal turns on, you want the relay to close. You want the arduino to use the relay contacts like a switch and detect whether it's open or closed.

I assume you mean that the contacts need some current? Assuming it's a mechanical relay, that's not really true. Some relays do recommend a bit of current to improve long term reliability, but I wouldn't worry about that right away.

Just hook up the contacts of the relay to your arduino as if it were a switch. Should work fine.

You could even hook up an LED to be turned on by the relay (google Led circuits). Set the current for 20ma, and that will give you a small load and a visual indicator at the same time.

If the relay is a solid state relay, it does need a load. But you haven't indicated that so I won't get into it.

|improve this answer|||||
\$\endgroup\$
  • \$\begingroup\$ Use a signal relay, they have no minimum current. \$\endgroup\$ – Jasen Jan 23 '19 at 8:27
  • \$\begingroup\$ I just browsed for signal relays, and can't find any with 24v AC input. Are there such things, or are they made for DC input only? \$\endgroup\$ – john west Jan 23 '19 at 13:34
  • \$\begingroup\$ Drew, i am worried about reliability. It might sit for months or a couple of years and never get any current, since this is around detecting water leaks, and the 24VAC only flows when a leak occurs (I have no control over any part of that). I have scoured digikey, mouser and other sites, and I see relays that support 24VAC input, but I just don't have experience to know what type and model are best. I'm hoping to learn enough to make that decision :). \$\endgroup\$ – john west Jan 23 '19 at 13:36
  • \$\begingroup\$ I do like the LED idea. And solid state is definitely best. Is "signal relay" synonomous with solid state relay, or are they not necessarily one and the same? \$\endgroup\$ – john west Jan 23 '19 at 13:42
  • \$\begingroup\$ They are not the same. And if you really want to go by the books, just use a resistor as a load. Make sure that if you do that that your power supply can handle it. \$\endgroup\$ – Drew Jan 23 '19 at 17:55
0
\$\begingroup\$

No relay is needed. You could rectify the 24V into DC, smooth it with a cap, then drive the input of an optoisolator with a proper current limit resistor. When AC is applied the opto's open-collector output could pull a uC input with internal pullup resistor low to signal that AC is present. Example part: https://www.onsemi.com/pub/Collateral/HCPL2631M-D.pdf

|improve this answer|||||
\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.