I have a DPDT relay and I would like to create a H-bridge.
How do I implement a H-bridge with a DPDT relay?
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You can wire a DPDT relay as a reversing relay but you cannot get more than two independent states from a device with only one input.
An H-bridge has four switches, so potentially there are 16 states. In practice, you won't likely want to use the two states that short the power supply and many are redundant for a floating load, so you really have three or four useful states-
If you are only interested in states 1 and 2, the DPDT relay is wired as a reversing relay (it will always be driving, just the polarity changes). If you also need one or both of the other states you will need at least two relays total. Two SPDT relays or one DPDT relay plus a SPST relay are useful for giving you states 1,2,4 or 1,2,3 respectively.