3
\$\begingroup\$

Sonoff WiFi controlled power relay. I'm trying to understand how the pictured device works. It's a Sonoff WiFi controlled electrical relay. I was told that it is designed to control mains voltage only, and cannot be used to control a 5v dc current. This doesn't make any sense to me, because I'm under the impression that the relay which ultimately controls the current works by simply completing and breaking a circuit using an electromagnet to move the switch. If so, this would mean that the current may practically be as low as I want, as long as it's below the maximum voltage and amperage rating for the relay.

Am I correct in my belief that a device like this can be used to control a 5v dc current without modification, or do I need something specifically designed for lower voltages? If I need something different, then what do I need, and why?

\$\endgroup\$
4
\$\begingroup\$

Looking at the specs printed on top of your relay it seems like it has a DC rating on it. So the relay alone is probably fine for switching DC.

Note that relays have different ratings for DC because there is no zero crossing in the current to extinguish any arcing. This can cause extra wear on the contacts or even cause them to weld shut if used outside the DC rating.

There's also often a minimum current required to keep the contacts wetted and break through any oxide layer that may form. Switching currents below that can cause high resistance contact unless the relay is rated for "dry" contact operation.

It's possible that for your device there are other reasons why it will only work with AC mains voltage. It may be that they use some of the AC voltage to feed other parts of the circuit (even though you have USB power) or there may be some type of safety function built in that only works with AC. There's no way to tell without knowing the details of the design.

|improve this answer|||||
\$\endgroup\$
  • \$\begingroup\$ If I understand you correctly, the relay itself should work fine for a 5v dc current, so any limitations to the minimum current would be caused by something else on that circuit. From what I can see, it looks like there is nothing else on that circuit, in which case it should be fine. Am I understanding you? I suspect the person who told me about the minimum current had misread the specifications. \$\endgroup\$ – StarCrashr Jul 7 '19 at 22:59
  • \$\begingroup\$ Yes, the relay itself should work for the DC current specified on its case, though if the current (at 5V) is very small you may have dry contact switching issues as I mentioned in the answer. \$\endgroup\$ – John D Jul 7 '19 at 23:14
  • \$\begingroup\$ Based on what you said, I did some more research, and I think I understand now. Low voltage might cause damage to the relay because of resistance in the switch contacts. With that knowledge, I did more searching and discovered that there's a similar device specifically designed for 5 volt currents. Thank you for your help. \$\endgroup\$ – StarCrashr Jul 7 '19 at 23:18
  • 1
    \$\begingroup\$ @StarCrashr: low voltage or low current will not damage the relay, but may not provide enough energy to burn through contamination on the contacts. \$\endgroup\$ – Peter Bennett Jul 8 '19 at 1:01
  • 1
    \$\begingroup\$ @StarCrashr You drew the wrong conclusions based on what I said, but if you found a device designed to switch the voltage and current levels you need you should be fine. \$\endgroup\$ – John D Jul 8 '19 at 14:42
1
\$\begingroup\$

It’s >2A and therefore not gold plated and needs a wetting current of 10% of rated current=10A. The low voltage does not matter. It is the current that matters.

For 5V logic signals all you need is a low ESR e-Cap around >=22uF to discharge the current when the switch closes with any series that is suitable for the signal. With frequent use (1/day) this will satisfy the arc wetting current of 5V/ (ESR+ contact R) to burn off any oxide layer.

The 30Vdc limit is due to cable or load inductance which can create an arc voltage that may burn the contacts enough to create a carbon insulation layer. So too little or too much Current is a reliability issue , yet as I defined should not.

The current rating is only for resistive loads and must be derated for heavy reactive loads like a battery charger surges on closure or heavy motor surge currents on both start and surge voltages on opening contact at least 50% or as specified.

|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.