Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Current Voltage And Magnetic Relay Behavior

Status
Not open for further replies.

jimmyboy

New Member
I am still new to electronics and learning the basics of inductors and decided to toy with an assortment of small relays a rheostat and a multimeter to see what kind of current draw each relay has.

The two relays I was interested in was a small 5v and an a larger 9v relay (both normally open) - I noticed that the smaller one drew 120ma at 5v and the larger 24ma at 9v when each closed. If you adjust the pot the relays open and close pretty fast when you raise and drop the voltage/resistance - I decided to take a smaller resistance linear pot and try fine adjusting the resistance to see what would happen.

It seems that the voltage where the relays turn on is fixed but after they have tripped raising the resistance very slowly you can drop the 5v relay down 2v before it falls back and the 9v down to 3v before it returns to its original position - is this related to the fact that inductors resist change?

I always thought that only applied to fast changing voltages like AC at 60hz this is a way slower change - if I am dropping the voltage and the current (I put the meter in series as an ammeter as well with both relays to check) how is the magnetic field staying the same? maybe this has to do with the multimeter? I have tried this with an AC adapter and batteries - same result - any ideas?

Thanks
 
All relays*, well at least all the ones I know about, have a lower drop-out voltage than closing voltage.

This is due to the magnetic circuit within the relay.
When the relay is in the resting state, there is an airgap in the magnetic circuit, and it takes a greater magnetising force to move the armature of the relay.
When the relay is closed, there is no air gap in the magnetic circuit and it takes less magentizing force to hold the armature in the closed position.

JimB


* Afterthought.
While writing this, I am sure I remember some relays which did not completely close the magnetic circuit and so would "drop out" quicker when the voltage was removed.
 
In other words going uphill (closing the contacts) against the spring opening it is much tougher than going downhill - sounds pretty logical - the force to keep the latch closed is much less than swinging across and stretching the spring - not a magnetic effect at all just a result of the design - usually a spring requires more force as you stretch it but there may be a lever at work here as well
 
The spring has some effect, but the predominant effect is the magnetic circuit.

When the relay is open, there is an air gap in the magnetic circuit - effectively a high magnetic resistance, and so needs more magnetising force (current in the winding) to create the magnetic flux required to pull the moving pole (armature) of the relay.

When the relay is closed there is no air gap, so the magnetic resistance (reluctance) of the relay core and armature is much lower, so a lot less current is required to create the flux required to hold the armature against the core.

JimB
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top