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I want to use SPST relay to control resistive load as well as inductive load i.e. ceiling Fan (Load<150W).

I used the 5VDC relay (HF46F-G) which is capable of 7A current, 250VAC.

Datasheet: http://www.hongfa.com/pro/pdf/HF46F-G_en.pdf

As we know, P=VI so that we can control the appliances which power rating is 1000W with this relay (P=230V*4.5A = 1035W; current is less than 7A)

  • then why datasheet says that its maximum switching power is 300W?
  • What does it exactly mean?
  • What would happen if I switch the appliance which is 1000W rated?
  • Is ceiling fan (Inductive load <150W) control safe (arching, relay health perspective) with this relay?

My understanding

We can safely control the appliances rated below 300W which means safe rating is 1.3A current if voltage=230V and 7A means that relay can handle the sudden peak up to 7A without any damage if it happen.

And if load is greater than 300W then arching chance increase more and relay life become shorter.

Datasheet

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Your relay can switch 7A of a resistive load at 277V AC It can handle 10A (but not whilst switching) so that gives a theoretical maximum power of 10A × 277V AC = 2770 VA

The figures on the other side of the solidus (or slash) refer to DC, 10A × 30V = 300W.

If you switch an AC appliance that takes 1kW at 230V that presents a resistive load, then it will work fine.

For an inductive load you need to de-rate (reduce) the permitted switching current, your datasheet should provide the information, but typically relays are de-rated between 2x and 8x for inductive loads. If you take the extreme end of that spectrum (8x) you get

7A/8 × 230VAC = 161W

So your ceiling fan should be fine. The life of the relay will be shortened, but should still be acceptable.

It usual to switch on inductive loads at zero crossings to avoid this problem, solid state relays exist which will do this for you automatically.

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  • \$\begingroup\$ Oh! I misunderstood that 300W is for AC. Max switching power of relay for AC (resistive load) is 1.6kW (230VAC*7A) Right? \$\endgroup\$ – Mohit Agnihotri Jan 28 at 9:29
  • \$\begingroup\$ One more relevant question under relay control. If I control the 2 ton Air-conditioner (3 star power rating) then 16A relay is okay? Is Air-Conditioner resistive or inductive load? \$\endgroup\$ – Mohit Agnihotri Jan 28 at 9:34
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    \$\begingroup\$ If it's an inverter one with PFC it'll be a resistive load, if it's not then it's a very large inductive load. \$\endgroup\$ – james Jan 28 at 12:45

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