Tuesday, February 21, 2012

Project ST5000: Report #4

  • I have ordered 4 Solid State Relays (SSR's) via eBay.  They are Mager brand, rated 5-220 VDC load voltage, 3-32 VDC control voltage, 40 Amps capacity and 0.035 ohms resistance in the on state.  That resistance means that the heat generation within the relay will be at a rate of 8 watts with locked rotor conditions on the drive, and about 0.9 watts during drive actuation under normal operation.  This should put heat generation well within the capacity of the cast aluminum case to absorb it.  Yes, they are Chinese - but Mager publishes all the specs.
  • Worked up the wiring path for the ST5000 drive output to the H-Bridge inputs, including the limit switches in the drive unit, using diodes to select the flow path and to protect the inputs of the SSR's from back voltage.  (As always, click on the image for a full-sized version.)
  • Hooked up the drive and my single SSR (left over from another project), and was able to actually observe the autopilot running the drive at slow speed when small course corrections are requested, and at very, very slow speed when only a 1° change in course was requested.  I am very much looking forward to seeing this in actual operation. 
Now I am (not very) patiently waiting for the relays to arrive, and looking forward to beginning the assembly.   I plan to document and write up the wiring changes that I make in the drive unit, in a step-by-step fashion... for myself as well as for others who may wish to adapt a Benmar drive to a modern autopilot control head.



Dougm said...


I'm really looking forward to the step-by-step write up. Don't be afraid to "dumb it down" for those of us who might be electronically challenged!

Doug M

Robert Salnick said...

Doug -

Stand by - coming in a week or three...


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