Using Solid State Switches with Time Delay Relays

Issue:

I have been told not to use solid state input devices such as proximity or photoelectric switches with solid state time delay relays. Why not and what can I do?

Solution/Resolution:

Ever since the introduction of solid state input devices, designers have had difficulty using them with certain solid state time delay relays. That’s because solid state switches have an inherent problem of leakage current, which could either falsely trigger a solid state time delay relay or keep a unit from resetting.

Therefore, time delay relay manufacturers usually have issued a warning not to use solid state input devices with their triggered time delay relays. This problem certainly limits the choices for a design engineer, probably leading them to use something less optimal than they would like.

If you have run into this problem, then Macromatic has a solution for you. To use a solid state switch with a solid state time delay relay, simply follow these guidelines:

• if you are using a time delay relay that is initiated by the application of input voltage (On Delay, Interval On, Flasher or Repeat Cycle), a resistor must be added to your circuit as shown in Figure 1. Please contact Macromatic for help in determining the
  required resistor value.
• if you are using a triggered time delay relay (Off Delay, Single Shot, Watchdog, etc.), it cannot be a unit triggered by a dry contact switch (as are products from most manufacturers in these functions). Instead, it must be a power triggered unit connected
  as follows:
  • for input devices with leakage currents of 0.5 ma or less, use it as you would normally apply an input device with a power triggered time delay relay (Figure 2)
  • for input devices with leakage currents >0.5 ma, two resistors must be added to your circuit as shown in Figure 3. Please contact Macromatic for help in determining the required resistor values.

Following these recommendations will allow you to use solid state input devices with time delay relays, and avoid utilizing an interposing relay or other means to isolate the switch from the time delay relay. Macromatic offers power triggered time delay relays in functions such as Off Delay, Single Shot and Watchdog. For more information on power triggered units, click here.