Ground Faulty
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A circuit to detect ground faults
Features
- "one shot" check on startup or continuous checking
- self test by turning on both test faults at the same time
- alarm when and while fault detected or continuous alarm after fault first detected
- 60s or 1s delay between checks
Design
Source code & schematic available via git at http://carrott.org/git/ground-faulty.git
I've prototyped most of the design, but haven't got the whole thing working yet. There are likely some changes needed:
- power transistor for buzzer
- relay for error output
- input reverse polarity protection
- input capacitor
- replace voltage regulator with http://www.digikey.com/product-detail/en/LM2940T-5.0%2FNOPB/LM2940T-5.0%2FNOPB-ND
- indicators on all inputs and outputs
- do the sense wires to the battery require fuses or do the high value resistors suffice?
- pick component values
- add switch to choose between 60s and 1s delay between checks
- Mechanical Relays:
- Solid State Relays:
Sense Resistor Selection
The H11AA4-M opto-isolator has a 60mA maximum current, so we need to select sense resistors to remain below this current. The sense resistor is in series with the fault resistance.
Enter system voltage and desired ground fault test current to obtain test resistor specifications (note this doesn't work yet):