Relays are one of the most important components in
electronic circuits. Especially in circuits where high power transfer or mains AC
load switching is involved, relays play the major role in implementing the
an electronic circuit for the required operations.
is used in the form of a switch.
load connected to its contacts in response to a relatively smaller electrical
power applied across an associated coil.
DC is applied to the coil, it energizes and behaves like an electromagnet.
spring loaded contact mechanism placed at a close proximity to the coil
immediately responds and gets attracted toward the energized coil electromagnet
force. In the course the contact connects one of its pair together and disconnects
an complementary pair associated with it.
switched OFF to the coil and the contacts return to its original position,
connecting the previous set of complementary contacts and the cycle may be repeated
as many times as possible.
order to converter it’s low power DC switching output into a high power mains
AC switching output.
be derived from an IC stage or a low current transistor stage may be be pretty
incapable of driving a relay directly. Because, a relay requires relatively higher
DC currents which may be normally not available from an IC source or a low
current transistor stage.
becomes imperative for all electronic circuits which need this service.
stage attached with the relay which needs to be operated. The transistor is
typically and solely employed for operating the relay in response to the
commands received from the preceding control stage.
involves a transistor, a base resistor and the relay with a flyback diode.
before the relay driver stage could be used for the required functions:
source for controlling the relay operations, it needs to be perfectly calculated
for optimal results.
current across the collector/emitter leads of the transistor or in other words,
the relay coil current, which is the collector load of the transistor, becomes
one of the main factors, and directly influences the value of the base resistor
of the transistor.
driver transistor is given by the expression:
- Where R = base resistor of the transistor,
- Us = Source or the trigger voltage to the base resistor,
- Hfe = Forward current gain of the transistor,
out by solving the following Ohm’s law:
supply voltage to the relay.
assumed to be around 150.
equation we get,
connected with the above calculation, cannot be ignored. The diode makes sure that
the reverse EMF generated from the relay coil is not dumped into the
transistor. Without this diode, the transistor would get damaged within seconds
A simple transistor relay driver circuit design was requested by one of the followers of this blog, Mr.Ronald. The suggested circuit by me was tested through a simulation software, the updated video was submitted to me by Ronald.
The entire conversation between me and Ronald can be read here, in the comment section.