Time Delay Relay Circuit

by Bryan A. Thompson

Last Updated 11/29/2008

 

Description

This circuit turns a relay on after a preset time.  The time can be adjusted by changing values for R or C.  A potentiometer is included to provide an adjustment of the turn-on time.

By using a SPDT relay, it is possible to choose whether the circuit opens or closes after the preset time, so it is both a programmable on-after-delay and off-after-delay circuit.  By choosing other relay configurations, it is possible to do things like reversing a motor after a specified time.

 

Figure 1 - Schematic

 

How It Works

When the power is applied, the capacitor begins charging through R1+R2+R3 (see figure 1).  When the capacitor voltage reaches the turn-on voltage for the FET (Vgs, threshold), the FET provides a path to ground for the relay coil, and the relay coil turns on (see figure 2).

When the reset switch is pressed, the energy stored in the capacitor discharges to ground through the reset switch and the capacitor voltage is reset to 0V. 

When the reset button is released, the capacitor begins charging through R1+R2+R3 again and the process repeats.

When power to the relay coil is turned off (power is removed, reset switch is pressed, etc), the magnetic field in the relay collapses and the voltage across its terminals spikes.  Voltages higher than Vgs,max will destroy the FET, so diode D1 has been included to protect against this problem.  FETs are susceptible to damage from static electricity (in particular, when the voltage between the gate and source terminals exceeds the max allowed for the FET (Vgs,max), so Zener diode ZR1 has been included to protect against this problem.  Resistor R4 has been included to protect the FET against excessive gate current.

 

Figure 2 - Response of RC Circuit to Step Inputs

 

Figure 3 - Graph of MOSFET Turn-on point

 

Figure 4 - Sample Calculation