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I need to drive about twenty realys using ULN2803A. The MCU and the relays can't be locate on merely one PCB. So I separate the relays to another board, and the two boards are connected with wires. The wires may be longer than 30cm, and the relays are rated 24V. Then what about the drivers (ULN2803A), at MCU side or the relay side? Which one is better?

Any suggestions are appreciated.

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My opinion only - I would put the relay driver on the board with the controller and run wires to relays. This minimizes the chance of noise getting into the sensitive nodes around the controller.

Relay coils are just not as sensitive to external disturbances as logic signals.

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The higher voltage is going to be passing through the ULN', therefore it should be as close to the relays as possible. The MCU lines should be the long ones, properly terminated at both ends if needed.

For bonus points, use a '595 (or TPIC6C595) to reduce the number of lines further.

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  • \$\begingroup\$ Thanks. TPIC6C595 is serial-in parallel-out chip, it can only reduce the wires from MCU to the drivers. Any method to reduce the wires from drivers to relays ;). \$\endgroup\$ – diverger Feb 11 '15 at 4:09
  • \$\begingroup\$ Nope. The driver has to connect directly to the relay. Which is also why you want a very short connection. Also, the TPIC' is intended to replace the driver for relays needing 100mA or less. \$\endgroup\$ – Ignacio Vazquez-Abrams Feb 11 '15 at 4:10
  • \$\begingroup\$ What about the driver is located at the MCU side? Then I don't need to worry the low level logic signal are distorted. Can you explain more about "The higher voltage is going to be passing through the ULN', therefore it should be as close to the relays as possible. "? \$\endgroup\$ – diverger Feb 11 '15 at 5:35
  • \$\begingroup\$ Thanks to Ohm's Law, we know that the higher a current is through a conductor the higher the voltage loss across it will be. Since we want to minimize voltage loss, the high current lines should be short. The MCU side will be using low current compared to the relay side. We can infer this based on the fact that a driver is required in the first place. \$\endgroup\$ – Ignacio Vazquez-Abrams Feb 11 '15 at 5:43
  • \$\begingroup\$ Yes, you are right. In my situation, because the high voltage (24V), and I think the voltage drop won't affect the relay action, if the wire resistance isn't so biggggggger. \$\endgroup\$ – diverger Feb 11 '15 at 6:01

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