Frequently Asked Questions about Relays is another in a series of FAQS about electricity and electronics that are oriented toward Appliance Technicians and HVAC Technicians.
Frequently Asked Questions about Relays
Q. What is a relay?
A. A relay is an electro-mechanical device used to switch a high current with a low current.
Q. What is the purpose of a relay?
A. It provides electrical isolation between the driver circuit and the switched circuit, and can effectively amplify the current of the input signal.
Q. What types of relays are there?
A. There are many types, which include those with DC or AC coils, and with single or multiple switching contacts.
Q. What is a DC controlled relay?
A. It is one with a coil intended to operate on DC voltage. It is the most common type found on control boards, with a typical coil voltage of anywhere between 9 and 24 volts DC.
Q. What is an AC controlled relay and how does it differ from a DC controlled relay?
A. An AC controlled relay uses AC voltage to energize the electromagnetic coil. Since a sine-wave spends some period of time at “zero” volts, and reverses polarity, an AC coil has a tendency to “chatter”. Thus it employs a “shader ring”, which acts like a one-turn transformer secondary that generates a magnetic field out of phase with the primary coil’s field – thereby ensuring that the net magnetic is flux greater than zero for any given moment in time, which keeps the contacts closed.
Q. Can a DC coil relay be used in place of an AC coil relay?
A. It is not recommended, as it may “chatter”, and thus not provide a solid connection at the contacts, as described in the previous FAQ.
Q. Can an AC coil be used in place of a DC coil?
A. Technically yes, given that all other specs are the same, but it is best to replace a relay with the same type.
Q. What is the significance of the relay contacts?
A. They determine both how much current can be switched, as well as the logical switching arrangement (SPST, SPDT, etc)
Q. What is an SPST relay?
A. It stands for “single pole, single throw”, which has a single contact that opens and closes.
Q. What is an SPDT relay?
A. It stands for “single pole, double throw” which has a common (C), a normally closed (NC), and a normally open (NO) contact.
Q. Can an SPST relay be used in place of an SPDT relay?
A. No. It does not have the proper logical contact arrangement.
Q. Can an SPDT relay be used in place of an SPST relay?
A. Yes, if all other specifications are equal, and the relay can be physically installed. The extra “throw” is just ignored. Sometimes the extra throw pin needs to be “clipped” so that it can be installed in the SPST relay’s footprint.
Q. What are the primary specifications of a relay?
A. Coil type (AC or DC), coil voltage, coil resistance, contact rating in amps/volts, logical pinout (SPST, SPDT, etc.), and its physical footprint.
Q. What is the significance of the coil’s voltage?
A. It is the voltage the coil is intended to operate at. Using less voltage may not energize it enough to close the contacts. Using more voltage causes more current through the coil than was intended.
Q. What is the significance of the coil’s resistance?
A. It determines the current through the coil and thus through the relay control circuit.
Q. What type of control circuit is used to drive a relay coil?
A. For AC coil relays, it may be a Triac or a smaller relay. For DC coil relays, it is usually a bipolar transistor (and less commonly, an FET) either in discrete form or in an integrated circuit. This transistor is often driven by a microcontroller directly or through a pre-driver stage.
Q. Does the relay coil’s inductance affect the relay control circuit?
A. For AC relays it does. It ultimately determines the impedance of the relay coil when energized, and consequently the relay current. For DC relay’s it doesn’t once the relay has reached steady state (after it is energized and settled in a few milliseconds). The flyback diode is used to clamp the inductive spike at the relay’s “off” transition, in order to protect the control circuit.
Q. What is the reason for the diode across a DC relay’s coil?
A. As mentioned in the previous FAQ, because a coil is an inductor that by nature opposes a change in current, it produces a voltage spike when the relay is de-energized. The diode is used to “clamp” that spike and help protect the relay driver circuitry.
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