Chris Litwin 5 years ago in Master modules / MD4 updated by Gustav Widén (System support) 5 years ago 4

The MD4 documentation states to use 1.5K ohm resistor on the +RTC circuit. Is the 1.5K ohm the only type that is usable, or is there a range that will work?


There are probably others that will work. We chose the 1.5k value because it is a common address idTag from Parker (address 3). The purpose is to limit the current in the RTC wire so it could not be shorted to another wire that is controlled by the ignition and inadvertently power up the module. The resistor should be as close to the battery as possible to protect the entire length of the RTC wire.

If you choose another resistor, the current supply needed for the RTC sets the limit on how high the resistance can be. 

You'd want to minimize head dissipation in the resistor, so higher resistance is better as long as it doesn't limit the current too much for the RTC. 

The Appendix A info sets the limits:

The MD4 RTC needs about 3.7 mA in a 24 V system, meaning the resistance shouldn't be higher than 24V/3.7mA = 6.5 kOhm.


Ok, that makes sense. Thank you.