|My testing workbench|
- The MOV's have arrived (part no. V33ZA70)
- I have determined that feeding the control inputs to a SSR with reverse voltage results in a 76 mA current flow - probably unacceptable for long-term use. I got a couple of diodes (Radio Shack, $1.19) which will prevent back-feeding the inputs.
- The MOV's are temporarily installed - they're the two red disks attached to the terminal strip in the drive... one for the clutch relay, and one for the motor.
- I have successfully actuated the clutch in the drive unit, using the ST5000+
- I have successfully activated the drive motor from the ST5000+, using the one SSR that I have.
- Irony: reviewing the schematic shows that the output circuit of the ST5000+ is actually an H-bridge. Which I will be using to feed another H-bridge (albeit one that can handle a lot more current).
A Crydom D1D20 (20 amps rated capacity; 0.10 ohms internal resistance) would produce twice the heat. Both Crydom units far exceed the needed current rating, but considering heat output, I am leaning toward the 40-amp unit. My plan is to bolt the SSR's to the inside of the cast aluminum drive case cover, using it as a heat sink. If I stick with 40-amp units, and given the intermittent nature of the motor's duty cycle when the autopilot is in use, heat should not be a problem.
There are a large number of Chinese SSR's on eBay with suitable advertised current carrying capacity. However, none of these units show internal resistance in their specifications (on purpose?), and many of the ads show the SSR's bolted to large exotic heat sinks. Tho they are more expensive, I am strongly tempted by the Crydom units.