Short handbook of current sensing in power DC supply lines - Part two

June 03, 2014 // By Petre Petrov
Short handbook of current sensing in power DC supply lines - Part two
Petre Petrov in this second article of the series considers simple circuits for sensing currents in negative and bipolar power supplies.

Previously, in Part one , we discussed low cost and efficient circuits for high side sensing in positive power supplies.
Some of the embedded systems have negative or bipolar power supplies (PSs). These PSs are used mainly for some peripheral blocks which may be switched ON and OFF.

The topic about current sensing of positive power supplies is largely discussed but is rarely employed in the following practical cases:

* negative lines of the power supplies

* ground lines of the power supplies

* individual current in the large paralleled electrolytic capacitors

* individual current of the rectifying diodes/bridges, including paralleled diodes/bridges.
Here we will discuss several solutions of some of these, not so largely discussed but important, practical cases. The accuracy of the measurement will be not our first objective.
The importance here is that sometimes the MCU should have an idea about:

* the presence of the power supply

* the power consumption (the current) from the negative power supply

* the power consumption (the current) over the ground lines,

* the equality of the current through the paralleled electrolytic capacitors with high values, etc.
For example we should know approximately how the power consumption changes when:

* we switch ON and OFF particular blocks of the system

* we change the mode of operation of particular parts of the system

* and how the peripheral devices as DC motors react to the changes in the loads/environment, etc.

* and how the capacity and the quality of the large electrolytic capacitors change in the time.

Usually the accuracy of these measurements is not very high and we can accept a tolerance of +-5% and sometimes higher than that.
In order to reduce the cost of the solutions we will use bipolar transistors such as PN2222A/PN2907A and popular OAs such as RC4558, NE5532, RC4560, MC1458 and similar which can offer good solutions

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