For the use in electrical engineereing:
Generally speaking, using a relay may involve anything from a very robust approach to an extremely delicate approach. Consider some aspects of a relay. A relay has contacts, arcing and sparking because of back e.m.f from load circuits may damage these contacts and reduce the life span of the device. Some inductive applications might require a low value capacitor across the relay contacts. Just remember that the peak voltage (not rms or AC) of the mains running through the contacts is not more than the rated capacitor voltage value. Do not use polarized capacitors where AC is used. Often a 10nF might make a difference already depending on your back e.m.f of the load obviously. It will also reduce some EMI (electromagnetic interference)
Even if your DC control signal may only see resistance of the coil at the time the coil is on or off. A coil that has inductance and store magnetic energy, the same as any coil will also produce back e.m.f when potential is removed. The collapse of the magnetic field cause a high potential difference, possibly thousands of volts at high frequency. The frequency may in fact be so high that regular diodes or transistors might not stop it. One would recommend a series resistor and use the collector of a NPN transistor with it's emitter to the ground to switch the relay on or off via the base, be sure to have a base resistor to protect the source of the control signal as well as the B-E junction of the transistor, calculation as follow:
Rs = collector series resistor
Rb = is base series resister
beta = current gain for the transistor
Vsource = the control signal voltage minimum that will turn relay on or off
By applying ohm's law we get:
Rc = [Rrelay x (VCC-Vrelay -Vce)]/(Vrelay )
Rb = (Vsource-Vbe)/[(Vrelay/(Relay x Beta)) x 10]
{use factor 10 to compensate for error in beta values}
Don't ever be to comfortable with the fact that there are no electrical connection between the contacts and the coil of a relay. Do not think relays are safe to use with computers or related micro controller devices because they are electrically isolated. It will be a bad mistake!! There are back e.m.f. from the coil aswell as e.m.i from contacts what is induced back into the coil when it has high impedance across it.
It's then recommended to have a fast recovery diode in reverse bias condition across the coil to prevent back e.m.f. of finding its way back home.
This is recommended to protect the driving signal source. Often in small relays using HC or TTL family logic to drive it via a transistor to a transorb or fast recovery diode across the coil may be good enough. Especially if you use it for your self and not to design a heart-lung machine for a hospital.
But if you have micro controllers, computer ports. The damage of a strong back e.m.f may be more than what you bargain for and will probably happen at the worst time in the wrong place. The best is to use a opto-coupler.
The control source will via a series resistor, let say about (330R for a 5V signal) switch an internal LED inside the opto-coupler device on. The light will drive a photo-transistor into forward bias condition. That can be used to drive the transistor that will energize the relay. Since there is no electrical connection between the output pin of the microcontroller that produce the control signal and the relay coil. There is only a one way communication via a light beam. This is by far safer. It creates a typical, 2500V isolation between the robust electrical circuit and it's e.m.f 's, and the sensitive and delicate micro controller or computer circuit environment. A dip in Vcc or pin for less than a millionth of a second dipping with 2 volt or spiking can cause freezing, erratic response or failure of the micro controller. For good electronic design opto-couplers are a very important part of a reliable control system that use micro controllers to control relay output with.
1. Relay Drive by Means of a Transistor 2. Relay Drive by Means of SCR 3. Relay Drive from External Contacts 4. LED Series and Parallel Connections 5. Electronic Circuit Drive by Means of a Relay 6. Power Source Circuit 7. PC Board Design Considerations
A circuit breaker when energized will latch and stay in that position until the load is reduced and manually reset. a relay will change state continuously if the source continuously changes
This circuit can switch on and off light a fan or a radio etc;by the sound of a clap This circuit is constructed using basic electronic component like resistor transistor relay transformer capacitor .This circuit turns on light for the first clap . the light turns on till the next clap .for the next clap turns off this circuit works with 12v voltage . therefore a step-cown transformer 12v/300ma is employed. The working of the circuit is based on amplifying nature of the transistor . switching the nature of transistor and relay as an electronic switch.
The most energy used in a circuit is at the load.
Switch it with a known good one from a non critical circuit to see if it solves the problem.
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There is no relay used on the speedometer circuit.
A relay cannot be used to power a motor. It can be used to control the motor by wiring the motor circuit in series with the relay contacts.
A relay is an electrical switch. It allows a low current "switch" circuit to control the electricity flow of a high-current circuit load
A local breaker backup relay is used to check the operation of distribution circuit breakers and to trip the feeder circuit breaker if the distribution circuit breaker fails to trip on an overload.
basic circuit of electrical protective relay
A relay race could be a good model of a series circuit. In any circuit, current flow is the daisy-chaining of electrons from atom to atom.
Yes. Older refrigerators used a hot wire relay in the starting circuit of the compressor.
There are numerous applications for a normally-closed relay contact. It is used whenever you want to disconnect a circuit when the relay coil is energised.
Remote-Control Circuit. Any electrical circuit that controlsany other circuit through a relay or an equivalent device.
They can be used as a kind of safety switch to allow a circuit with a small current through to switch on a circuit that will hve a larger current flowing through it.
Sounds like a relay to me.