Monitor the current with an operational amplifier, a BJT and three resistors
This article explains the functionality of an intelligent circuit that accurately measure the supply current.
First of all, I must admit that the title is slightly misleading. The circuit presented in this Article requires only an op-amp, a transistor and three resistors. It is not, however, an independent power monitor, perhaps “measuring current” it would be more accurate “current monitor”, but even this definition is accurate. Can I assume that in the end the circuit is just over a “current-voltage converter”, but keep in mind that converts the current into voltage in a manner compatible with the supply current monitoring applications. So maybe we should call it a “current to voltage converter for the output current monitoring applications”
There are various situations in which you want to measure current consumed by your project.
Among the many comes to mind to me dynamically adjust the functionality of a subsystem based on the current consumption of another subsystem, estimate the battery life, or to establish the smallest possible IC regulator capable of providing adequate output current. It is also possible to use the measurements of current consumption recorded as a minimally invasive way of tracking the transitions between states of a microcontroller of the upper and lower power.
As discussed in the opening paragraphs, this circuit converts the current into voltage. If you need a circuit that is more autonomous in its ability to record and / or respond to the current consumption behavior, you probably want to digitize the measurements using a microcontroller. If you are required only basic functionality and do not need any other processor, you can use a comparator or an analog detector windows.
The circuit presented in this article is based on a circuit found in an application note entitled “Op Amp Circuit Collection”, published (in 2002) da National Semiconductor. My version looks like this:
And here is my implementation LTspice:
This may seem a little confusing at first sight, but the system is really quite simple:
The current flows from the supply, through R1, To the load. R1 works like a typical resistor current sensor, as such has a very low resistance so as to reduce the power dissipation and minimize its effect on the measurements and on the load circuit.
The voltage applied to the input terminal of the operational amplifier non-inverting input is equal to the supply voltage minus (supply current × R1).
Do not let the PNP transistor will detract from the fact that the operational amplifier has a negative feedback actually.
Since the upper end of both the resistors R1 and R2 is connected to the supply voltage, to bring the operational balance, there must be an equal tension on both these resistors, consequently the current through R2 is equal to the current through R1 divided by the ratio between R2 and R1. In the circuit shown above, R2 is 1000 times larger than R1, which means that the current through R2 will be 1000 times smaller than the current through R1.
The base current of the BJT is very small, then we can say that the current through R3 is more or less equal to the current through R2. Then, R3 we use to create a voltage that is directly proportional to the current flowing in R2, which in turn is directly proportional to the current flowing through R1.
This is the scheme that should help to clarify and reinforce this explanation:
The final equation for VOUT is
What exactly he is doing the PNP?
You can think of the transistor in two ways: as an adjustable valve that allows the operational amplifier to increase or decrease the current flowing through R2 and R3, or as a device with a variable voltage drop that the operational amplifier can be used to establish the correct voltage VOUT on node. In both cases the end result is the same: the transistor is the means by which the operational amplifier can force the voltage on the inverting input terminal to match the voltage on the noninverting input terminal.
The transistor is really the most interesting part of this circuit. We often use in applications BJT “on/off” and it is important to recognize that the situation is completely different in this circuit. The operational amplifier (with the help of negative feedback, obviously) He is actually making small and precise adjustments to the emitter-base voltage of the PNP (WEBSITE). The following graph shows VEB for a range of load currents (corresponding to the load resistors 50 Ω a 300 Oh).
Note how all these voltages are close to the typical threshold of ignition (~ 0,6 V) for a pn junction silicon. This shows that the op-amp is very carefully operand in the region of the BJT threshold to produce the changes required to produce a large voltage drop between emitter and collector. The entire range of values is about VEB 50 mV
Compare this variation around 50 mV with the change of approximately 4 V in the emitter-collector voltage:
If use is made of resistors low tolerance and a good operational amplifier, this circuit can be considered quite accurate. I treated an interesting and effective circuit that accurately converts the supply current into a voltage that can be measured, digitized or used as input to a comparator.
https://www.elettroamici.org/wp-content/uploads/2018/03/IMG_3896.jpg230307Amilcarehttps://www.elettroamici.org/wp-content/uploads/2017/08/FAVICON-1-300x271.pngAmilcare2018-03-18 11:31:242019-10-03 16:44:10Monitor the current