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4QD-TEC Electronics Circuits Reference Archive
Modular Relay System

In case you have not read it, Part 1 of Modular Relay System

Circuit diagram


Circuit description

The relay driver uses two transistors: this gives this gives lots of gain. The two benefits are

  1. The circuit will drive relays up to 100mA coil current (or more if suitable values are chosen)
  2. The circuit has a high input impedance, 470K with the values shown.
  3. Overall positive feedback can be added to increase switching speed: this gives fast, positive response and avoids the driver transistor lingering in a high dissipation state - which could blow it.

In normal circumstances, R6 is absent and ZD1 linked out: there is a scratch-through link which must be broken if this is to be fitted.

There are two inputs to the circuit, Pins 3 1nd 4 of the connector on the circuit. Both feed through to the 'personality socket'. Pin 3 also feeds through to the pinstrip and (without a personality module fitted) SW1 is left in the position shown so that Pin 3 feeds straight through to R1. It is R1 which defines the high input impedance.

R1 together with R2 define the operating point. A transistor requires about 0.6v on its base to make it conduct. If there is 0.6v across 47K, then there must be 6c across 470K (R1) so the input operating point is about 6.5 volts. This can be altered downwards by decreasing R1 or increasing R2.

It is also possible to fit R6 and ZD1: this puts a voltage on the emitter of Tr1, so raising the operating point (on Tr1's base). This is rather more accurate that amplifying a transistor Vbe. It is useful where tight control of the operating point is required, or to stabilise against thermal drift: at around 25°C, transistor Vbe decreases by about 2mv for every 1° change. This is amplified, so the input operating point varies by around 14mV/°C. Fitting the zener can remove most of this variability but is not required for most applications.

As soon as Tr1 conducts, it passes current through R3. This turns Tr2 on, so the voltage across the relay starts to rise. But R5 applies positive feedback so that any rise in voltage across the relay coil will aid the turn-on of Tr1. The circuit therefore snaps from one state to the other and cannot remain in an in-between state where the relay is half on and Tr2, in consequence, cooking itself to death.

A fairly conventional circuit which needs only a little explanation. There are two 6 way connectors indicated: Sk1 is the input connector and Sk2 is the expansion connector into which plugs the 'personality module'

Notice also that, on Sk1, pins 1 & 2 are connected, as are pins 5 & 6. This is to simplify wiring a machine: the 12v feeds to a collection of relays may be 'daisy-chained'. On the circuit board layout, split 6.3mm push-on tags are used. These will accept two 2.8mm push-on terminals, also allowing daisy-chaining.

Coil catching diode

The relay coil (RL1) has a normal catching (or flyback) diode across it but there is a resistor in series with the diode. A straight diode allows the energy (stored in the relay coil's inductance) to be dissipated slowly when the relay is de-energised. It does this by conducting when the drive transistor is turned off. If it were not present, the rapid switching off of the coil would result in a high voltage spike - high enough to cause the drive transistor to break down. Although such breakdown is not necessarily harmful to the transistor, it can have unwanted side effects.

A necessary effect of this flyback diode is that it extends the relay's release time because the coil current is kept circulating. The added resistor in this circuit can, if chosen correctly, makes the release time the same as the operate time - which seems the most useful compromise.

The circuit may be simple but, as will become apparent, a lot of thought and experience has gone into designing a simple and reliable system which is very versatile and also very economic to build.

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