Short handbook of current sensing in power DC supply lines – Part two
Previously, in Part one, we discussed low cost and efficient circuits for high side sensing in positive power supplies.
Introduction
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 at low cost in many cases.
In fact the solution of the task consists of building an appropriate current to voltage converter (CVC) producing an output voltage Vc proportional to the DC or AC power supply current over a particular power line.
Description of the circuits
Figure 2.1 shows probably the two simplest circuits for sensing of the current over the negative power supply lines.
Figure 2.1: Simplest circuits for current sensing in negative power supplies with an NPN transistor. a/ without initial current for the transistor. b/ improved amplifier with NPN transistor with initial current.
The circuit from Figure 2a uses NPN transistor without initial current. The resistor R1 converts the current into a voltage which controls the transistor T1. The resistor R2 protects the base of the transistor. The resistor R3 converts the collector current of T1 into a voltage Vc which can be measured. The capacitor C5 reduces the probability to have unwanted oscillation from the transistor T1 in active mode. The output signal Vc can be used by an ADC after appropriate signal conditioning.
Some of the advantages of this circuit are:
* The circuit is operational and we may use it for approximate evaluation of the low, middle and even high voltages and all sorts of power supply currents (after using appropriate components).
* The circuit is very simple and low cost.
* We may connect R3 to the ground or to other voltages according to design needs
* When properly designed the circuit is difficult to damage.
* The calibration is not always needed because sometimes we should know only if the output current is above a predefined value or when the current rises or falls when we activate or deactivate particular modules or when we change the mode of operation a module, etc.
Some of the disadvantages of the circuit are:
* The voltage drop over R1 can be significant, e.g. 0.5-1V depending on the transistor T1.
From that point of view it is good to use Germanium transistors because they need lower base-emitter voltage to operate but nowadays these transistors are difficult to acquire.
* The circuit has a voltage threshold dependent on temperature and selected transistor T1.
That voltage threshold changes at around -2.2mV/C.
* The function between the load current and the voltage Vc is not linear
* The reproducibility of the circuit is low and if we change the transistor we may need recalibration.
* If higher accuracy is needed we should do a calibration procedure of the circuit at several points, with several loads.
We can improve the circuit and obtain the circuit from Figure 2.1a, e.g. we can make it more linear and less dependent from the change of the transistor T1.
Figure 2.1b shows the improved circuit for sensing of negative power supplies with analog amplifier by the NPN transistor. The resistor R4 stabilizes the gain of T1. The gain of the circuit is approximately equal to –R3/R4. The trimmer potentiometer R5 is used to adjust the initial current for the transistor T1. If needed we can use a Zener diode D1 to limit the output voltage Vc up to Vz.
Frequently we have a bipolar power supply and we wish to monitor the direction of the current in the common ground line. That is important even we wish to keep power supply the positive and the negative power supply currents in certain proportions.
We can use the circuits from Figure 2.1b to solve that issue as shown on Figure 2.2.
Figure 2.2: Ground side sensing for bipolar power supplies with NPN and PNP transistors.
In the Figure 2.2 we can use the resistor R1* or the resistor R4* to convert the ground current into a voltage, Vcsi1 or Vcsi2, and we will amplify and measure that voltage.
The difference between the voltages Vcsi1 and Vcsi2 is that the Vcsi1 includes the currents of the positive regulator (PR) and the negative regulator (NR) and Vcsi2 does not include that current.
Depending on the polarity of the captured voltage it is amplified by T1 or by T2. If needed the output voltages are limited with the Zener diodes D5 and D6 and/or by resistor dividers R2, R9, R7 and R10. If not needed D5, D6, R9 and R10 can be omitted.
The gain of T1 is fixed by R3 and R2 and the gain of T2 is fixed with R7 and R6. The produced output voltages Vc1 (> 0) and –Vc2 (< 0) can be measured with an ADC after appropriate signal conditioning.
If we are in need of higher accuracy of the CVC we should use operational amplifiers (OAs) to build the converter circuit as shown on the Figure 2.3.
Figure 2.3: Ground side sensing for bipolar power supplies with differential amplifier with operational amplifier and with bipolar power supplies.
In Figure 2.3 a differential amplifier is built around an operational amplifier (OA) IC1. We may need to use a general purpose single OA such as LM741, TL061, TL071 or half of a dual OA such as RC4558, MC1458 and similar to amplify the voltages Vcsi1 and/or Vcsi2.
The gain of the OA can be set to any applicable value with R3, R4, R5 and R6. The power supply for the OAs can be taken before or after the regulators PR and BR depending on the conditions. The output voltage of the OA, +-Vcs, is bipolar and should be limited if needed with Zener diodes as shown in Figure 2.3.
Sometimes we should connect in parallel several electrolytic capacitors with large capacity, e.g. 10000uF, 22000uF and larger. This is needed to obtain large total capacity with relatively low serial resistance and low serial inductance. The problem in this case is that some of the paralleled capacitors may deteriorate and we may discover that too late.
Consequently in some of the cases it is appropriate to monitor the charge and the discharge currents of the electrolytic capacitors in order to be sure that they take equal quanity of the total current.
We may solve that problem by connecting in series with each capacitor a small resistance to monitor the voltage drop over that resistance. The value of the resistance is usually one resistor of 0.1 Ohm or two resistors of 0.1 Ohm in parallel (in total 0.05 Ohm). Apart from the current sensing role these resistors work also as equalization resistors for the series resistances of the capacitors in parallel.
Figure 2.4 presents a circuit for sensing the voltage drop over the resistors connected in series of the monitored capacitors.
Figure 2.4: Circuit for sensing the charge and discharge currents for bipolar power supplies with two differential amplifiers and with additional power supply.
C1, C3 and C5 are paralleled capacitors with high capacitance for the positive power supply. C2, C4 and C6 are paralleled capacitors with high capacitance for the negative power supply.
Between each of these capacitors and the ground there are resistors of 0.1 Ohm or 0.05 Ohm. The voltage drops over these resistors are amplified with individual non-inverting amplifier with gain of ten. Only the amplifying stages for C5 and C6 are shown.
Vref1 and Vref2 are external reference voltages which can be used to shift (offset) the output voltages Vc1 and Vc2. R7, C11, D1 and D2 protect the input of IC1.1. R8, C12, D3 and D4 protect the input of IC1.2. D1, D2, D3 and D3 are low voltage Zener diodes, e.g. for 2.4v to 3.3V.
Also we may use other diodes instead of Zener diodes as shown in the small circuit on the bottom of Figure 2.4. R1 and R2 depend of the surge current of the monitored capacitors. If the surge charge and discharge current is +-10A the voltage drop over R1=0.1 Ohm will be maximum +-1V. In that case the output voltages Vc1 and Vc2 will be in the range of +-10V.
We may have two or more diodes charging the same filtering capacitor. And we need to monitor the current between the capacitor each of the diodes.
Figure 2.5 shows the circuit for sensing the individual currents from diodes working over the same capacitor.
Figure 2.5: Circuit for sensing the currents from each of the two power supply diodes working over the same load.
The circuit is with two differential amplifiers build around operational amplifiers. The OAs use additional power supplies. The circuit captures and amplifies the voltage drop over the resistors R1 and R2 which is proportional to current through the diodes D1 and D2.
Conclusions
Part 2 of the short handbook presents simple, low cost and efficient circuits for sensing the current over negative power supply lines, ground lines and capacitors with large capacitance.
The circuits are built around NPN transistors and operational amplifiers. The circuit can be adapted to a large variety of applications. The protection components for the active components are not shown on all circuits.
Part three will discuss circuits for sensing currents in paralleled rectifiers
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