24Vdc relay

[Trivo]

I picked up a 24v DC relay and would like to know how it works because my limited understanding of DC Volts is that they can not create a magnetic field to make the relay activate and you need AC current. when i conected it to 12v battery it activated? and i got a shock when holding the wires on the terminal any ideas

Trivo

[tecker]

  What you have there is commonly called called three pole contactor .It is usually used for 30 amp loads and has  a 24 volt control voltage . The reason it shocked you is the back emf spike around 300 volts is generated when you release the 24 volts ( If you were using 12 volts it was less). The control voltage was probably 24 volt transformeraround 100 va .

[drdongle]

12 volts probably won't activate it, though if it did you would know it as it will make a noise as the armature slams in to the top of the coils core.

[finnsawyer]

Your understanding is wrong.  A dc current (as created by a dc voltage) does create a magnetic field around any conductor it flows through.  In fact, for a long straight wire the magnetic field forms concentric circles around the wire and drops off as 1/distance (from the wire).  Bending the wire into a coil will concentrate the field.  

Any time a charge moves it creates a magnet field, so it doesn't matter whether the cause of the motion is ac or dc.

[geoffd]

A relay normally works on DC, not AC.  When DC voltage is applied it creates a steady magnetic field that attracts an armature which in turn closes / opens the switch contacts. The switches remain operated until the voltage through the coil is switched off.

There are two ratings involved.  That of the coil, in this case 24 volt which requires a certain current to activate it, and that of the switches which says how many volts and amps they can safely switch.  Looking at the construction I would say this is a pretty hefty relay.

If you were to feed AC through it the relay would attempt to switch on and off at 60 times a second. Probably not what you want at all.

Cheers

Geoff

[finnsawyer]

Actually there are ac relays.  I worked one summer at Allen Bradley's switch test facility, where we ran ac relays to death.  These work because the iron armature will still want to get to the center of the coil (has to do with minimum field energy}.

[ghurd]

And I have used DC coil relays on AC just fine.

Nothing for long term use, just toys, but it worked fine.

G-

[richhagen]

Probably not a perfect basic analagy and description, but:

If you wrap a few turns of wire around a nail, and run current through it, either AC or DC, you will induce a magnetic field in the nail.  In a relay, the magnetic field generated by a coil is used to throw a switch.  The Magnetic force is usually counterbalanced by a spring which throws the switch back to its original position when the magnetic force is removed.  The coil is an inductor, which stores energy.  When you attempt to disconnect the leads, that stored energy has to go somewhere, hence the EMF spike and arc when disconnecting the leads.  Rich

[Ungrounded Lightning Rod]

Some AC relays have a shaded pole, like a motor, to eliminate chatter.

The end of the core, where it comes out of the coil and attracts the armature, is split , forming two smaller poles each with a D-shaped cross-section.  A D-shaped piece of heavy copper (sort of like a copper washer) is wrapped around one of the two poles, forming a one-turn shorted secondary coil.

When you energize the relay with AC, the pole that isn't "shaded" by the copper D acts normally, producing a magnetic attraction of the armature that varys with the current in the main coil.  But the changing magnetic field induces a current in the D, and that lags the current in the main coil by a quarter-cycle.  The current in the D magnetizes the pole that passes through the D, and THAT pole's field, like the D's current, also lags by a quarter cycle.

So the field from the part of the core that goes through the D is strongest when the field from the part that bypasses it is reversing, and vice versa.  The force on the armature is the sum of the magnitudes of the forces from both poles, and that sum is strong over the whole cycle and doesn't even vary a lot.  With the armature pulled in strongly throughout each cycle the relay doesn't chatter or even hum much.

[finnsawyer]

Is that how "Square D" got it's name?

[healerenergy]

The place I work at goes through about a hundred contactors a month all the way up to size 4. I strip as many as I can for good coils and copper. We also go through a number of ac to dc 180 or 90 volts the main thing that goes is the rectifier diodes. I saw Three of them in the trash last night nice power transisters in them some other stuff to.