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I'm preparing for a project that will interface an FPGA with many 3v relays on its gpio. Additionally, some of the relays will be powered with the same power supply that the FPGA uses. The FPGA will actuate certain relays, as well as read signals that are in series with relay coils.

I know that a closing relay can create an inductive voltage spike that might damage the FPGA. Also, I'm not confident that the gpio pins can supply enough current to actuate several relays in parallel.

I've encountered fly-back diodes, voltage followers, and optocouplers before, and I'm pretty sure one or all of these could help here. What should I do to protect my FPGA and make sure it can drive the relays?

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    \$\begingroup\$ What are the relays switching? \$\endgroup\$ – Spehro Pefhany Sep 10 '14 at 3:33
  • \$\begingroup\$ Other relays, mostly. It's an exercise in relay-based digital logic. \$\endgroup\$ – davidglanzman Sep 10 '14 at 7:19
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I'd use both a transistor between the FPGA and the relays to reduce the currents through the FPGA, and add a flyback diode across the relays to protect the transistor.

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Since the FPGA gpio pins only will be used as outputs an easy solution would be to use N-MOS transistors as a simple low side switches to control the relays.

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  • \$\begingroup\$ I'm planning on having relays attached to inputs as well though. What should I do in that circumstance? \$\endgroup\$ – davidglanzman Sep 14 '14 at 3:27

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