Solution
Background
There are several protection mechanisms built into the user-accessible 3.3V and 5V fixed output to protect the NI myRIO from misuse. This protection circuitry works in three different modes:
- Mode 1 (user is drawing 10mA or less):
In this mode, the current flows through a diode and the user will see the output voltage drop as more current is applied according to the diode's properties. As current increases, the output voltage will decrease.
- Mode 2 (user is drawing between 11-30mA):
Once the 10mA total output current threshold is reached, the protection circuitry will turn on the MOSFET (acting as a switch), shorting the diode. The forward voltage drop across the diode will be removed as the diode is now shorted.
- Mode 3 (user is drawing greater than 300mA total from both connectors):
In this mode, the NI myRIO will go into over-current protection and turn off the user rail.
Possible Situation
There is a potentially undesirable effect from this protection scheme. If the user increases the total output current by dynamically switching from
Mode 1 to
Mode 2, over time they will see the output voltage dip until the 10mA threshold is crossed. When switching from
Mode 1 to
Mode 2, the voltage will snap back to nominal.
Note: Even with this dip, the output stays within the specified voltage tolerance as called out in the manual.
Workaround
It is desirable to operate in
Mode 1 when possible because it offers the most protection to the NI myRIO against accidental misuse of the user voltage rails.
If the fluctuation of the rail is too much of an issue for a particular application, the user can force the NI myRIO to operate in Mode 2 entirely by using a 250Ω resistor from the user output to ground. This will pull 20mA of static current from the user voltage which will effectively disable Mode 1 operation.