I have 2 external signals arriving to my board, A and C. B is generated inverting A. They 3 will be used for generating 3 switch control signals: A', B', C' at every point (there will be 8x3 24 switch). Every one will be 12V or 0V. 12V state means to activate the coil of a relay, because of coil needs 12V for closing contacts.

According to the application, there can only be one active switch at the same time I thought a way for ensuring that only A', B' or C' will be ON and the other two signals: OFF.

I though about muxes and logic gates that supports input and output voltages of 12V. So I found useful products. But after thinking that I was having a good solution, I realized that:

  • these coils have a resistance = 150 Ohms. So Icoil = 12V/150 = 80mA -relay datasheet don't say anything about cutoff current, but drop-out voltage: 1V -
  • the most of existing logic gates provide a max output current = 10mA.
  • so, I'm afraid that coils will not be energized because 10mA < 80mA


  1. I dont' know if my explanation and situation is correctly deduced. Is it generally only needed to have 12V or may the coils need 12V + I > 80mA?
  2. If I can' t find AND and NOR gates with this current output features, how do you solve the situation?
  3. Must I say bye to modifying control signals using logic gates? or can be applied some electronics to the outputs? I don't consider using current elevators due to the quantity of them that I would need.

Any information will help.


1 Answer 1


The tidiest cheapest solution is likely to be to use a relay driver like the ULN2803. They have a logic-compatible input, an output capable of switching 500mA and 50v, there are 8 channels in a small package, they are used by the gazillion so are very obtainable and cheap, and they even incorporate the catch diodes you need to drive relays without blowing up your driver.

Think of it this way, eight 12v 80mA relays plus a single ULN2803 driver IC is equivalent to eight 'logic level input' relays.

  • \$\begingroup\$ So, @Neil_UK are you telling me to put the drivers after logic gates and before coil relays: between Gates Vout and Vcoil, is it? Thank you so much! \$\endgroup\$
    – Suvi_Eu
    Commented Jul 11, 2019 at 11:12
  • 2
    \$\begingroup\$ @EugeniaSuarez Yes, check the applications information in the 2803 datasheet, and in particular the connection for the pin that's connected to the catch diodes, it's an easy one to miss. \$\endgroup\$
    – Neil_UK
    Commented Jul 11, 2019 at 12:44
  • \$\begingroup\$ Lot of thanks! I will take all the suggestions into account! I think it could be a good way. \$\endgroup\$
    – Suvi_Eu
    Commented Jul 11, 2019 at 12:55

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