realys

Relays

Choosing a relay |
Protection diodes | Reed relays | Advantages & disadvantages

Also see:
Switches |
Diodes


Circuit symbol for a relay

A

relay is an electrically operated switch. Current flowing through the

coil of the relay creates a magnetic field which attracts a lever and changes

the switch contacts. The coil current can be on or off so relays have two switch

positions and they are double throw (changeover) switches.

Relays allow one circuit to switch a second circuit which can
be completely separate from the first. For example a low voltage battery circuit
can use a relay to switch a 230V AC mains circuit. There is no electrical
connection inside the relay between the two
circuits, the link is magnetic and mechanical.

The coil of a relay passes a relatively large current, typically 30mA for a
12V relay, but it can be as much as 100mA for relays designed to operate from
lower voltages. Most ICs (chips) cannot provide this current and a
transistor
is usually used to amplify the small IC current to the larger value required for
the relay coil. The maximum output current for the popular 555 timer IC is 200mA
so these devices can supply relay coils directly without amplification.

Relays are usuallly SPDT or DPDT but they can have many more sets of switch
contacts, for example relays with 4 sets of changeover contacts are readily
available. For further information about switch contacts and the terms used to
describe them please see the page on switches.

Most relays are designed for PCB mounting but you can solder wires directly
to the pins providing you take care to avoid melting the plastic case of the
relay.

The supplier's catalogue should show you the relay's connections. The coil
will be obvious and it may be connected either way round. Relay coils produce
brief high voltage 'spikes' when they are switched off and this can destroy
transistors and ICs in the circuit. To prevent damage you must connect a
protection diode across the relay coil.

The animated picture shows a working relay with its coil and switch contacts.
You can see a lever on the left being attracted by magnetism when the coil is
switched on. This lever moves the switch contacts. There is one set of contacts
(SPDT) in the foreground and another behind them, making the relay DPDT.



The relay's switch connections are usually labelled COM, NC and NO:

   
• COM = Common, always connect to this, it is the moving part of
      the switch.
   
• NC = Normally Closed, COM is connected to this when the relay
      coil is off.
   
• NO = Normally Open, COM is connected to this when the relay coil
      is on.
     
  
     
   
• Connect to COM and NO if you want the switched circuit to be
      on when the relay coil is on.
   
• Connect to COM and NC if you want the switched circuit to be
      on when the relay coil is off.
   

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Choosing a relay

You need to consider several features when choosing a relay:

   
1. Physical size and pin arrangement
   
       If you are choosing a relay for an existing PCB you will need to ensure that
       its dimensions and pin arrangement are suitable. You should find this
       information in the supplier's catalogue.
   
2. Coil voltage
   
       The relay's coil voltage rating and resistance must suit the circuit
       powering the relay coil. Many relays have a coil rated for a 12V supply but
       5V and 24V relays are also readily available. Some relays operate perfectly
       well with a supply voltage which is a little lower than their rated value.
      
   
3. Coil resistance
   
       The circuit must be able to supply the current required by the relay coil.
       You can use
      
       Ohm's law to calculate the current:
      
   
   Relay coil current   =  supply voltage
   
                                   
       coil resistance
   
   
       For example: A 12V supply relay with a coil resistance of 400
       passes a current of 30mA. This is OK for a 555 timer IC (maximum output
       current 200mA), but it is too much for most ICs and they will require a
      
       transistor to amplify the current.    
   
4. Switch ratings (voltage and current)
   
       The relay's switch contacts must be suitable for the circuit they are to
       control. You will need to check the voltage and current ratings. Note that
       the voltage rating is usually higher for AC, for example: "5A at 24V DC or
       125V AC".
   
5. Switch contact arrangement (SPDT, DPDT etc)
   
       Most relays are SPDT or DPDT which are often described as "single pole
       changeover" (SPCO) or "double pole changeover" (DPCO). For further
       information please see the page on switches.
      

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Protection diodes for relays

Transistors and ICs must be protected from the brief high voltage produced when
a relay coil is switched off. The diagram shows how a signal
diode (eg 1N4148)
is connected 'backwards' across the relay coil to provide this protection.
Current flowing through a relay coil creates a magnetic field which
collapses suddenly when the current is switched off. The sudden collapse of the
magnetic field induces a brief high voltage across the relay coil which is very
likely to damage transistors and ICs. The protection diode allows the induced
voltage to drive a brief current through the coil (and diode) so the magnetic
field dies away quickly rather than instantly. This prevents the induced voltage
becoming high enough to cause damage to transistors and ICs.

---

Reed relays

Reed relays consist of a coil surrounding a reed switch. Reed switches are
normally operated with a magnet, but in a reed relay current flows through the
coil to create a magnetic field and close the reed switch.
Reed relays generally have higher coil resistances than standard relays (1000
for example) and a wide range of supply voltages (9-20V for example). They are
capable of switching much more rapidly than standard relays, up to several
hundred times per second; but they can only switch low currents (500mA maximum
for example).

The reed relay shown in the photograph will plug into a standard 14-pin DIL socket
('IC holder').

For further information about reed switches please see the page on switches.

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Relays and transistors compared

Like relays,
transistors can be used as an electrically operated switch. For switching
small DC currents (< 1A) at low voltage they are usually a better choice than a
relay. However transistors cannot switch AC or high voltages (such as mains
electricity) and they are not usually a good choice for switching large currents
(> 5A). In these cases a relay will be needed, but note that a low power
transistor may still be needed to switch the current for the relay's coil! The
main advantages and disadvantages of relays are listed below:
Advantages of relays:

   
• Relays can switch AC and DC, transistors can only switch
      DC.
   
• Relays can switch high voltages, transistors cannot.
     
   
• Relays are a better choice for switching large currents
      (> 5A).
   
• Relays can switch many contacts at once.

Disadvantages of relays:

   
• Relays are bulkier than transistors for switching small
      currents.
   
• Relays cannot switch rapidly (except reed relays),
      transistors can switch many times per second.
   
• Relays use more power due to the current flowing through
      their coil.
   
• Relays require more current than many ICs can provide, so
      a low power transistor may be needed to switch the current for the relay's
      coil.