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Jim Nordin asked how to use solid state relays in place of the
stock electromechanical relays. First, the problem with the
stock relays is that they have moving parts, and sometimes
the parts don't move when they should (bad coil or welded contacts)
or move when they shouldn't (vibration or high-G). The solid state
relay is immune to those failure modes.
The stock wiring diagram is reprinted from the Lancair 320 build manual.
The factory design uses two SPDT (single-pole double throw) relays.
Not shown are the diodes that should be added across the relay coils
to shunt the inductive current when breaking the circuit. Those
won't be needed on the solid state relays.
The solid state version of SPDT relays is expensive,
so I selected SPST (single pole single throw) relays in
NO (normally open) and NC (normally closed) configurations.
Used together, one NO SPST and one NC SPST make an SPDT - almost.
The control switch could be simplified to an SPDT instead of DPDT
as shown in the second diagram, though this is optional. In the
stock design, motor current passes through one side of the switch
while relay coil current passes through the other side of the switch.
This provides some protection against welded relay contacts,
but increases the risk of flaky switch contacts.
There is a flaw in the solid state design as shown. There is no
guarantee that one relay opens before the other closes. The design
should be tested to insure that the SSRs break faster than they make.
Otherwise the battery is briefly shorted to ground during the switching.
There are SPDT solid state relays with guaranteed break-before-make
(BBM), but the versions I have found are more expensive
(e.g. Micropac 53208).
Alternatively, some circuitry can be added to delay the switch closings.
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