Can Relays be Energized Using AC & DC Voltages?

Hello, Watdefak !!
Yes ,some old relays rele were built with four diodes in grec connection and can be energized using both A.C. and D.C. voltages .

A dedicated DC coil has many turns of fine wire which limits the current and yet gives the required flux due to the ampere turns. However AC relays may be run on DC by using a series resistor, initially this resistor is not in circuit so the full DC voltage energises the coil in order to actuate the mechanism and close the flux gap. Once energised the holding current required is much less and since this is DC there is no back-EMF so current limiting is achieved by a series (power) resistor. The series resistor is connected across the normally-closed contacts (often the auxilliary contacts are used) so it is initially shorted out. When the relay / contactor is energised the N/C contacts open and the coil current flows through and is limited by the series resistor. Connect the supply to one side of the resistor and coil and connect the other side of the resistor to the other side of the coil. It's clearer with a diagram but I don't know how to upload one. I hope this helps.

There is an other problem: DC magnetizes the iron core of the relay so the relay can remain "sticked" after the exciter current is turned off, so DC relays have a tiny gap in the magnetic circuit.

Using electronic circuits inside a relay (between its biasing terminals and its coil), the relay can be operated via wide range of ac/dc voltage (say from 24 ac/dc up to 220 ac/dc).

These types of relays are already available in the markets.

Regards... Samak

One could put a bridge rectifier in the AC circuit to convert the AC to DC for a DC relay. It wouldn't matter then if the Dc relay were connected across AC or DC as the rectifier would sort it out.

Regards to all:

My reply: to watdefak
See the following CR4 Post. A lot of information in detail had been there:
http://globalspec.ip09.com/rd/9z1zjh924ietefi92ob5iq8k06g0jj4c874a3h64uqo
How does a DC relay work?

My reply is:

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. Pole is shaded [the cross-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 to quench 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 rivet in Latching-Lip to avoid 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 off. 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 ?-F] on DC output to hold the relay in zero-crossings to avoid CHATTERING.

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

@@@ For attention of "Panta SD" in post #1 and "HUX" in post #2 please.