Eric Page
Joined: 15 Feb 2017
Posts: 253
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 Posted: Sat May 17, 2025 11:16 am Post subject: Overvoltage Protection Relay Driver |
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This post and the attached PDF document describe a heavily modified version of Bob's OVM-14 Mk III Overvoltage Protection Module. Instead of working as a crowbar, this design switches relay coils to interrupt the feed from a permanent magnet stator to the rectifier/regulator on an engine like the Rotax 9-series or Verner radial. It can switch one relay for a single phase stator or two for a three phase stator.
I added three features to the original OVM-14 Mk III design:
1. A PUSH ISOLATE switch gives the pilot manual control of the stator feed (similar to the ALT FIELD switch in legacy designs). This also allows a tripped OVM to be reset (this version doesn't blow a breaker).
2. A 1.5Hz, 20mA flashing output directly drives an OVP TRIP indicator LED. This is combined with the PUSH ISOLATE switch in an illuminated latching pushbutton.
3. A current-limited output drives an EFIS alarm input via a 10V active-high signal.
This design uses all through-hole components and the circuit board is designed to fit into a small enclosure with mounting flanges. Wires exit the enclosure through a small rubber grommet.
I'm not wise enough in the ways of legacy charging systems to know if interrupting a traditional alternator's field circuit with a relay is a good or bad idea. The documentation for this device includes a drawing for this application, but as the notes state, it's probably best to use a regulator with integral overvoltage protection or the OVM-14 Mk III crowbar module.
I'm not suggesting this design as an alternative or improvement to Bob's work. As he has rightly pointed out (see the thread linked below), his crowbar circuit could just as easily control relays as it does an alternator field, and the features that I added are functionally unnecessary. Bob's design is a one-size-fits-all solution. I just had a slightly different application and wanted to add LED and alarm outputs. Perhaps others have a similar interest.
Finally, I want to be clear that I'm not an electrical engineer; I'm a hobbyist. I designed the changes in this module to the best of my ability, based on my exchanges with Mr. Nuckolls in the following thread, where we discussed an early revision of his OVM-14 Mk III...
http://forums.matronics.com/viewtopic.php?p=511451
I've tested this design on the bench and I'm confident enough in it that I'm installing it in my Kitfox, but it hasn't been tested in a flying aircraft yet. Please consider it experimental and use it at your own risk.
PDF documentation is attached. Links for circuit boards and components are on page 6.
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| Description: |
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Download |
| Filename: |
OVP Relay Driver for PM Stator (Rev G-TH).pdf |
| Filesize: |
2.31 MB |
| Downloaded: |
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