How to use spdt relay as spst

Written by refugio jones
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How to use spdt relay as spst
An SPST relay is identical to a single throw switch. (switch image by Clark Duffy from

An SPDT relay refers to a Single Pull Double Throw electric or electronic switching device. SPDT relays allow electrical signals, digital or analogue, to switch from a single input to one of two outputs. On the other hand, an SPST relay is a Single Pull Single Throw device that connects or disconnects (turns on or off) a single signal path. Replacing an SPDT relay for an SPST device requires knowledge of electronics and some experimentation to make it work.

Skill level:

Things you need

  • Multiple output voltage source
  • Oscilloscope with fine probes
  • Multimeter with probes (instead of oscilloscope)
  • Solder-less evaluation test board (breadboard)
  • 22 AWG copper wire in red, black, white colours
  • 100 mega ohm or larger resistor

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  1. 1

    Learn how much power (voltage and current) will flow through the SPDT device. Read the manufacturer's datasheets to select the most appropriate devices. Relays conduct micro to mega volts and will have internal resistance values appropriate to the desired voltage. The devices also come preset with Normally Open (NO) or Normally Closed (NC) connections. Know whether your device is NO or NC before proceeding to step 2.

  2. 2

    Insert the desired SPDT device into the test board. These boards--also called breadboards--have small insertion holes that will grab the relay's pins. Select one of the two output throws of the SPDT device to carry the signal or voltage. This will be the main voltage path. You will not use the second throw.

    How to use spdt relay as spst
    An evaluation breadboard allows for quick experimentation before using a printed circuit board. (testing a circuit image by Albert Lozano from
  3. 3

    Apply voltage and ground to the breadboard's power and ground busses. Keep the power source off until ready to test your circuit. Connect these two according to the suggested application diagram provided with the relay's datasheet. Also, read the manufacturer's datasheet to understand the voltages required to switch between the output throws. A Normally Open device will require control voltages to connect between the desired open and closed paths. A Normally Closed device needs no control signal until opening (disconnecting) the intended SPST path.

    How to use spdt relay as spst
    Follow the manufacturers's suggested circuit layout for using your SPDT device. (repair of an old radio receiver 1. image by wrangler from
  4. 4

    Pick between the following options for converting your SPDT device into an SPST switch:

    Leave the second throw (output) in an open state. Drive the control input to close the relay to the desired SPST output in a Normally Open switch. When switched into the unused or "open" position, the input will see an open circuit and nothing will occur. This can simulate an "off" state for the relay. A Normally Closed device will require no control input until the device must open to switch "off" the SPST.

    In the second method, pull the second throw (output) to ground through a very large resistor (greater than 100 mega ohms). This works best when the input voltages are around 5 volts. Large input power with greater than 100 volts makes this an inconvenient solution because it forces the need for a rather large pull-down resistor.

  5. 5

    Test your reconfigured SPDT by driving input voltages into the device. Use a multimeter or oscilloscope to check the outputs of the device. Apply the minimum power at the SPST signal input. The intended SPST output will see a voltage (minus some resistive loss), while the open or pulled-down SPDT pin will see zero volts. If you are using the pull-down method, probe the output at the points before and after the resistor for power. The larger the resistor, the less the power through the device.

    How to use spdt relay as spst
    Use a multimeter to test input and output voltages of your SPST circuit. (electronic multimeter image by Albert Lozano from
  6. 6

    Switch between throws to check whether the SPST output turns on or off. This will prove that the SPDT is now an SPST.

Tips and warnings

  • Solid state switches can often be used to replace relays. Know the power requirements of the devices before selecting the appropriate replacement.
  • Applying large amounts of power can be deadly. Use caution when voltages exceed 100 volts or when dealing with currents in the single- or double-digit ampere range. Ensure that proper grounding is available regardless of the power used.

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