Relays?

Yes initially it is but as the magnetising current is established it looks like a high resistance, note that when a relay or solenoid is switched off the stored energy is released causing a back EMF that often has to be diverted by the use of a diode across the winding. If a semiconductor device such as a transitor is used to switch the relay it can be destroyed by this large reverse voltage.

Just like Brainwave says, they look like a high resistance once the current is flowing. For the same reason a DC motor isnt just a dead short.

Note however, the number of turns on the coil is to some extent matched to the expected voltage. e.g. You can get 9v coils 24v coils etc. The relay spec ususlly includes a DC resistance which is actually fairly high, say 10-100 ohms. They will often work on a much lower voltage...say a 9v relay will probably work on 5v.

Note also once they have pulled in, they will hold in on a much lower voltage so a 9v relay will pull in at 9v but once in the voltage could be dropped right down to maybe 3v before it opens again...this is due to the magnetic circuit being made. e.g even a very weak magnetic force will hold 2 magnets together once they are actually touching.

Whoops #2, that was me....forgot to log in!

You're forgiven Del, just don't let it happen again.

John

Roy.

Brainwave (who knows all has screwed it up again)

AC and DC relays are not directly interchangeable. What operates the relay is the magnetism in the core and is determined by the number of ampere turns.

So, With a given number of turns, and a fixed voltage, what controls the amperes.?

1. With an AC relay it is the combination of AC resistance (impedance) and the DC resistance.

With the DC relay the amperes in controlled by the impressed voltage and the DC resistance. Ohms Law remember.

i.e. DC relays require a smaller wire with a higher resistance per turn so to limit the amount of current flowing.

AC relays have much fewer turns (with a low DC resistance) because the AC impedance is high and becomes the major factor in limiting the current.

Another note.. A diode across the coil of an AC relay will be a direct short on 1/2 the cycle. Guaranteed smoke will follow.

You could operate a DC relay from an AC source by putting a diode in SERIES with it however, is will probably chatter. But If you also put a resistor and capacitor in the circuit, you can make a time delay relay from it. You can delay the pick up or the drop out time by the circuit that you use.

Keep in mind that AC relays normally have laminated cores with shading poles. These features have no practical value on DC.

Nice answer, my reply was confined to low voltage DC relays really...so that has filled out my knowledge of AC relays. Thanks!

And for anyone called O'Reilley lets not get into the O'Reilley? A relay? Oh really?

puns.

I'm glad to see someone caught this very large mistake. First yr EE kids know this stuff.

Try this link for more details : www.howstuffworks.com

Any coil is not pure inductance. It has some resistance. When voltage is first applied, it looks like a high resistance (actually high impedance). The current increases with a constant voltage applied until it approaches V/R where R is the internal resistance.

Since AC is always changing, AC sees a higher impedance which depends on the frequency and the Inductance of the coil. For a given coil, more current will flow DC than AC with rms voltage equal to the DC voltage. Another factor is that the polarity of the current is reversing so the magnetic field actually becomes zero twice each cycle.

A diode, or better still, a full wave bridge is a good idea.

Yep, that information is correct

And a full wave bridge circuit will be less apt to cause relay chatter.

Now add a series inrush current limiting resistor and a small filter capacitor in parallel with the coil and you have a DC relay that will operate on AC.

But expect there to be a finite time delay in pick up and drop out.

Now the other way around, there are some more considerations. !

I would only add that relay manufacturers will spec. both the
inrush and holding currents of their coils.

An AC and DC relay are the same thing, but with one minor exception. One day while my friends and I were in the electronics shop at work, we tore apart a AC 24v relay. Inside, on top of the coil was a metal washer. We wondered what the washer had to do with it because an identical DC relay DID NOT HAVE IT. When we removed the washer and hooked the relay up, it buzzed like an angry bee hive. We put it in the DC relay, applied AC, and it didn't make a sound. The AC relay also worked fine when powered by DC. So the only difference we found between the two was that simple metal washer.

You don't know much about relays do you.?

You did not leave it connected to a good voltage source very long did you?

If you did, you didn't feel how hot it was getting, did you ?

You did not read comment #5 did you ?

You do not know what a shading coil is, or understand what it does, or how it works, do you.

Come on guy, this is supposed to be a technical engineering forum, Get a little professional, These are not parts you put on a car because they look alike.

I'm sorry I got so angry, but please do study a little more before commenting.

If you want to play with the electrical engineers, they will welcome you, but do not expect them to respect you when you make these comments. Don't feel bad, there are many that walk among us.

Q. How does a DC relay work?

A. A voltage is applied which energises it magnetically to operate & hold ass long as supply is

supplied. Moving & static parts make a magnetic circuit as all are made-up of generally speaking mild-steel. A winding is around the Pole-piece & contacts are operated by a Latching-lip. Moving contacts are springy so return when power is removed.

Q: DC sees a coil as a short so how can this be applied to the coil in a relay?

A: No it is not a short-circuit but have a sufficient resistance of winding wire. Winding is of appropriate wire & number of turns.

Q: Can AC and DC coils be interchanged by using a diode across the coil in a DC relay using AC?

NO, for DC relay to be used on AC

Partly YES for use of AC relat on DC.

See below:

The physical construction of AC & DC relays differ quite a lot & they cannot be replaced interchangeably even by modifications in electrical circuit:

1.) All AC whether relays or contactors have special design consideration:

a. Core is laminated to decrease eddy-current losses

b. Poie is shaded [the crss-section is cut into 2 sections 1:2 ratio approx to accept a copper ring across about 1/3 portion] to give 2 phase effect [as in shaded-pole motors]

This action ensures that relay is not de-energised in zero-crossings [Chattering of relay].

c. No gap is necessary between the pole & Latching-Lip as in AC there is no possibility of residual-magnetism which may hold the relay after the power is removed.

d. An RC Network is connected across the relay toquench Back-EMF produced on switching off.

2.) A DC relay looks as an AC Relay but:

a. Solid core is used as no eddy-current losses are there on DC.

b, No Shading of pole as there is no zero-crossing thing in DC

c. A gap is generated between magnetic-pieces eg pole & Latching-Lip by putting a copper rivot in Latching-Lip to aviod holding of relay after the power is removed [Instant de-energising of relay]

d. A Diode is always connected across a DC relay to quench Back-EMF generated on cutting the power. It is not necessary that the relay is being operated by Semiconductor or a conventional switch. There is alway a danger of arc-over the switch or the semiconductor by Back-EMF.

Note: An AC relay can be used on DC provided:

a. an air-gap is produced by sticking a piece of paper or copper-shim on pole-face or pole-facing surface of Latching-Lip.

b. a bridge-rectifier be used with a small capacitor [2-10 μ-F].

c. if a single diode is used in series then a larger capacitor will be required [50-100 μF]

HAAJEE Good Engineering report and a

Excellent comment.

Snakers..

Back-EMF generated on removal of input pwr should be explain to those not knowing what it is and or the value generated depending on the input voltage, which in itself is higher then said input voltage during initial pwr removal, which then answers the diode value type needed for the task and or the semiconductor which might have a diode built within, its value.

who remembers the older chopper circuits which at one time used relays.

Thanks for Snakers & Hap's remarks.

I have to add a few pionts to my Posting at #14

1. Coils [all wound parts] store energy = LxI sq [as Capacitor stores =C x Vsq] & this energy is to be dissipated across the Supression-Network [A diode in DC & an RC NW in AC across it]

2. Relay [& contactors] initially start at higher voltage [like all electrical devices eg Tube light & other Lamps excluding Tungston-lamps, Contactors, Motors etc etc] but are held [keep on operating] at much lower than the Starting Voltage.

This is reffered as HOLDING-Value. A 12V relay normall operate at 9-10V but held at as low as 7-8 V. This is due to the DECREASE OF RELUCTANCE OF MAGNETIC CIRCUIT [as resistance in electrical Cct] due to decrease in air-gap which has the major part of RELUCTANCE.

In this context where you want a quick-make & Quick-break of relay or contactor [In industrial equipment] hey are operated on higher voltage, voltage decreased to HOLDING voltage by an aux-contact.

Thats correct. In the power industry where large power grid protection is required and we need Very High Speed mechanical relays, we us a 12 volt DC relay on 120 volts DC to obtain a very high speed pick up and then use a holding contact and resistor to hold it at 10 volts.

Using this method, We are able to initiate and open a bank of 3 phase 345,000 volt circuit breakers at 20,000 amps in 1/2 cycle. (8.333) milliseconds.

New solid state devices can do it even faster but it still takes at least 1/4 cycle to recognize a fault condition.

Of course, these breakers are either Oil or Sulfer Hexafloride quenched.

VERY NICE COMMENT.

Now should we also cover contact bounce and settling out time?

.

Not unless you want to include terms such as contact wipe, make before break, transfer, silver, paladium, gold, long lever, short lever, Heel end slugged coils, armature slugs. long sluged lo-capacitance, co-ax, shielded, RF type, antenna, heel clearance, armature travel, spring tension, residual screw settings, copper shims and a few other terms that come to mind. I have adjusted them all but..

Lets not go there. OK

O-yes, then we can move on to Dashpot circuit breakers and arc shoots...LOL

or cleaning a Slip-ring/commutator and seating new brushes on a running generator while underway in high seas.

Hello Snaker & Guest

#18 & #19

Thanks for your comments