High Side Current Measurements


It’s always a bit
difficult to measure the current in the positive lead of a power supply,
such as a battery charger. Fortunately, special ICs have been developed
for this purpose in the last few years, such as the Burr-Brown INA138
and INA168. These ICs have special internal circuitry that allows their
inputs to be connected directly to either end of a shunt resistor in the
lead where the current is to be measured. The shunt is simply a
low-value resistor, across which a voltage drop is measured whenever a
current flows. This voltage is converted into an output current Io by the
IC.

This current can be used directly, or it can be converted into a
voltage by means of a load resistor RL. In the latter case, the
‘floating’ measurement voltage across the shunt is converted into a
voltage with respect to earth, which is easy to use. The value of RL
determines the gain. A value of 5 kΩ gives 1×, 10 kΩ gives 2×, 15 kΩ
gives 3× and so on. It all works as follows. Just like any opamp, this
IC tries to maintain the same potential on its internal plus and minus
inputs. The minus input is connected to the left-hand end of the shunt
resistor via a 5-kΩ resistor.

When a current flows through the shunt, this voltage is thus lower
than the voltage on the plus side. However, the voltage on the plus
input can be reduced by allowing a small supplementary current to flow
through T1. The IC thus allows T1 to conduct just enough to achieve the
necessary lower voltage on the plus input. The current that is needed
for this is equal to Vshunt / 5 kΩ. This transistor current leaves the
IC via the output to which RL is connected. If the value of RL is 5 kΩ,
the resulting voltage is exactly the same as Vshunt. The IC is available
in two versions.

The INA138 can handle voltages between 2.7 and 36 V, while the
INA168 can work up to 60 V. The supply voltage on pin 5 may lie anywhere
between these limits, regardless of the voltage on the inputs. This
means that even with a supply voltage of only 5 V, you can make
measurements with up to 60 V on the inputs! However, in most cases it is
simplest to connect pin 5 directly to the voltage on pin 3. Bear in
mind that the value of the supply voltage determines the maximum value
of the output voltage. Also, don’t forget the internal base-emitter
junction voltage of T1 (0.7 V), and the voltage drop across the shunt
also has to be subtracted.


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