better algorithm description for setting desired current

1 files changed, 9 insertions, 2 deletions

diff --git a/qolab/hardware/power_supply/keysight_e3612a.py b/qolab/hardware/power_supply/keysight_e3612a.py
index 7b1dee2..fc13bc0 100644
--- a/qolab/hardware/power_supply/keysight_e3612a.py
+++ b/qolab/hardware/power_supply/keysight_e3612a.py
@@ -89,11 +89,18 @@ class KeysightE3612A(PowerSupplySCPI):
         """
         Tuning Vout to achieve desired Iout.
         Generally setting current limit will maintain current near but not exact to desired.
-        This function will try to guess the correct Vout value
+        Since Vlimit can be set with good precision,
+        this function will try tune Vlimit until the Idesired is reached.
         """
         iDesired = val
         self.setChanIlimit(chNum, val+currentHeadRoom)
-        # here we assume that hook up is already made, so we can estimate source resistance
+        # Here, we assume that hook up is already made, so we can estimate source resistance
+        # So the protocol is the following:
+        #  find R -> calculate required Vout for the desired Idesired
+        #  -> set Vlimit to reach desired Vout and Iout
+        # In general, once we estimate resistance of the load + source, we do not need to
+        # anything extra. But there is a problem: for a given Vlimit setting, the actual Vout
+        # is slightly off.
         for i in range(10):
             iOut=self.getChanIout(chNum)
             if abs(iOut-iDesired) <= currentPrecision: