Relay Questions

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Assuming these are all related to the coil...
  • To make certain you circuit can deliver enough power to change the contact state when invoked
  • Goes hand in hand with pull-in voltage; coil requires a minimum current voltage [drop] to change to the energized state
  • relay will not change from energized to deenergized state until voltage [drop] goes below this level
As long as relay receives enough power to energize it and discrete control [on/off] is used, the latter two are not of any concern typically. When you use analog control [variable power] all three become a very significant design issue.
 
150224-1603 EST

kody916:

As an electrical engineer I would think your questions could be self answered.

Why do we need to look at relay coil resistance?
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There are several questions to ask here. How is resistance related to wire diameter, and length? In a tightly wound coil how much current can a given size coil tolerate? Indirectly how much magnetic field can be produced by a given coil at some current or voltage? How do the number of turns and current relate to the magnetic field? How does resistance relate to the number of turns in a given cubic volume? How does temperature rating of wire insulation influence wire size and magnetic field?

what is switching current for a relay?
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You need to clarify the question. If "switching current" means the load current switched by the relay contact, then obviously this is determined by the circuit being switched. If it is the maximum load current capability of the relay contact, then it is dependent upon your criteria for the life of the relay contact. I have run a single cycle test with a mercury relay rated at 10 A at 120 V DC where the current was in the range of 5000 to 10,000 A without destruction of the relay.

why is dropout voltage important?
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You want the relay to open. Think about this? A typical problem today is that a controlling circuit has a solid state output device and associated with this is snubber circuit across the switching element. This snubber produces a leakage impedance and thus potential current to a load. If the load is a relay and that leakage current is sufficient to keep the relay or other load energized when the solid state switching element is off, then your circuit has failed to perform as expected.

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This is all good info.
I would like to answer 2 of them from a practical prospective.
Comparing coil resistance between identical relays helps us to evaluate the condition of the relay; does the coil have shorted turns? is the coil open circuit; both indicating
a defective relay.
The dropout and pickup voltage on an undervoltage (phase sensing) relay is very important in how the system operates in a power failure condition.
 
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