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002 Ammeter

Electronics> Achievements> Display / measurements> 002 Ammeter

Overview

This ammeter, simple to achieve if the CMS do not be afraid (no reason for it is not), can measure the intensity of an electric current with high precision.



It is based on the use of a dedicated circuit from Texas Instrument / Burr-Brown, the INA168 (for a voltage range up to 60 V) and INA138 (for a voltage range up to 36 V ). The measurement principle is the same as those for the ammeter 001.
See also Theory - Measure of a current.

The diagram

Relatively simple scheme, is there any need to convince you?


General Operation

Integrated circuits INA168 and INA138 are specialized circuits for the accurate measurement of currents. They have two entries (one non-inverting input and an inverse) is applied between which the voltage drop caused by a shunt resistance measurement. The output is current, which requires the use of a resistor (R1 here) to convert voltage. The output voltage at point Umes (terminal 1 of the IC) is defined by the following formula:
Umes = IS * RS * R1 / 5000
If we give the value of R1 KO 5 (in practice two 10 KO resistor in parallel or a single resistor precision of 4.99 KO), then the output voltage Umes is defined by the following simplified formula:
Umes = IS * RS
This simply means that there is a voltage output identical to the one developed at the terminals of the shunt resistor RS. If one wants to have an output voltage greater for the same current through RS, it is sufficient to adopt a value for R1 resistance more important. Thus, a gain of 10 is obtained if R1 = 50 KO (two resistors 100 KO in parallel or a single resistor accuracy of 49.9 KB). The maximum value of R1 is 500 KB, which achieves a maximum gain of 100 (output voltage 100 times greater than the voltage across RS).

Choice of the shunt resistor RS

The value of this resistance depends on the current range to be measured. It must be a compromise between the desired accuracy for low current values, and the voltage drop introduced on the line alment circuit under control. A high value is recommended for measuring low currents with a value as low voltage chutée is low and the measurement error is larger (due to offset voltages). At the same time, the value of RS is low and the voltage drop can be considered negligible or non-obtrusive. The best results are obtained when the voltage drop on RS is a few tens of mV. The manufacturer recommends a maximum value between 50 mV and 100 mV for full scale, with a maximum set at 500 mV. We can therefore deduce the following values of RS, for a maximum voltage drop of 100 mV:

currant Max measured	Value of RS (ohms)	Voltage drop on RS (for max)
1 mA	100	100 mV
10 mA	10	100 mV
100 mA	1	100 mV
1A	0.1	100 mV

Filtering output

The output voltage can be used as is, at the terminals of R1. It can also be filtered if somewhat slight fluctuations in current occur and are annoying to the viewer (on a digital voltmeter, for example). The value of C1, added here to limit these fluctuations should be based on the desired cutoff frequency and the value of R1, as follows:
F (-3dB) = 1 / (2 * Pi * R1 * C1)

Feeding circuit

Feeding can be done directly on the circuit which measures the current, but it can also be independent. In the first case, the jumper JP1 must be set up, and in the second case it should be removed. In both cases, food must not exceed the value specified by the manufacturer, ie 36 V for the INA138 or 60 V for the INA168. Note that it is possible to measure current, a voltage source whose value is higher than the voltage of the integrated circuit. Thus, no danger when using an independent power supply of +5 V to pin 5 of IC and a voltage of 36 V (if INA138) or 60 V (INA168 if) is applied to the input terminal 3 of IC.

The printed circuit

Directed single sided.

Typon to PDF, EPS and bitmap 600 dpi

Do not forget the IC SMD (SOT23) copper side ...

(click to enlarge)

Broaching the INA168 / INA138, seen from above:

001 Ammeter

Electronics> Achievements> Display / measurements> 001 Ammeter

Overview

This ammeter, simple to implement, allows to learn the extent of current low values ranging from 1 nA to 100 mA. Modest accuracy does not allow it to compete with laboratory equipment that is not its purpose. This is just to show how to disrupt as little as possible in the circuit which performs the measurement of current, while enjoying a good measurement accuracy. The current display is done on a galvanometer or voltmeter with a full scale of 1 V. Another gauge of the same type but implementing a dedicated circuit type INA168 is shown on page 002 Ammeter.

Measurement of intensity

It does not seem reasonable to show the pattern before making a quick reminder on how to measure the intensity that circulates in a circuit, and to recall the limitations of such a process. I invite you to consult first page Theory - Measure of a current, if you feel the need.

The diagram

It is composed of two parts, one is completely optional and only used for rough indicator overflow range.

The meter itself

Lower part of the scheme
RS resistance shunt inserted between the power circuit under measurement and the measurement circuit in itself, and the extent to which voltage is made to infer the current that the cross is made of one or more resistors RS1 to RS8. For example, the size 100 mA for which the switch K2 is set to 8, only the resistance of 0.11 ohm RS8 is in the measuring circuit. On size 1 mA for which the switch K2 is set to 6, the three resistors RS6, RS7 and RS8 (11.11 ohms total) are in use. And the range of the lowest values (10 nA, K2 in position 1), all resistance in RS1 are RS8 circuit (total value of 1.111111 MB). As you can see, we have for each class of a shunt resistance whose value is of type "1.1". This means that for a current of 1 mA, and if the RS resistance is 11.11 ohms, there is a voltage input measurement of 11.11 mV. The input voltage to be increased to the value of 1.0 V (full scale value of the range) must be amplified by a ratio of 90. What luck, it is precisely the gain of the voltage amplifier consisting of U1: A, first half of the TL072. This gain is in fact defined by the ratio of resistors R2 and R3, using the following formula:
Gain = 1 + (R3 / R2) = 1 = (89000 / 1000)

The display circuit

This is a simple voltmeter needle or digital, including the full scale is 1.0 V. However, it is quite possible to use a model whose full scale is 2.0 V, it will simply be "in half". The important thing is that the device, whatever it is, has an input resistance of 1 KO. Which fortunately is the case for many devices.

The indicator range is exceeded

Upper diagram
This is a simple voltage comparator, with two red and green LEDs. Until the voltage at the terminals of RS remains below 20 mV, the green led is lit and the red LED is off. Above this threshold, the red LED lights up. The threshold voltage of 20 mV, which might be lowered to 15 mV, is obtained through the diode D3 polarized into direct and therefore provides a voltage around 0.6 V, and the voltage divider consisting of R7 and R8 which followed.

power

The power of the ammeter is entrusted to two batteries of 4.5 V plates, so as to retain the independence of the circuit under measurement. To minimize the overall consumption, you better be used for D1 and D2, high brightness of the LEDs or low power consumption, with resistors R4 and R5 series of value at least ten times higher than the values shown in the diagram (you can try values between 2K2 and 10K).

Improvement

The PDO of the TL072 meter section can be advantageously replaced by a TL071, which offers extra opportunity to adjust the voltage offset. This allows you to calibrate the device and have a really display "0.00" when the two key points remain in the air (not connected anywhere). If you choose this "option", set the following mounting:

Measuring a courant

Electronics> Theory> Measuring a courant

Overview

Measuring the intensity of an electric current may seem simple when you use any device that works well. But it was also quick to make a measurement error with a voltmeter wrong ... As with any measure, it is well known for its measuring device and to know its limits. There are several ways to measure a current:
- Intrusion (insertion) of the measuring circuit in the power
- By coupling electromagnetic current transformer style
- Hall effect
This page describes briefly the first two methods.

Measurement of current intrusion

This first method, recommended for low current or average value, is simple to implement for measuring a current flowing into an integer, but is more difficult to apply for the measurement of current flowing in a portion of mounting . It involves inserting a resistor of known value in series with the power, and measuring the voltage drop it causes.

Example

On the block diagram below, we see a circuit (represented by the blue rectangle) powered by a source voltage of 12V BAT (battery lead, for example). We would like to know the course I charged by the voltage source BAT, on the one hand to check that everything goes well, and partly to more precisely estimate the possible autonomy that can be expected of the system.



The insertion of an ammeter or a universal controller set to ammeter in series with the food, is the first idea that may come to mind, as shown below:



As may already have an order of magnitude of current drawn without knowing exactly the value (otherwise the question would be meaningless), the ammeter is placed on the gauge for measuring the most accurate possible. In this case it is the caliber 200 mA, the lowest in the device used. This requires opening the power circuit for the connection of the measuring device, which generally does not pose too much problem. But it may also prefer to put a measurement system to remain independent from the meter that can be used in a lot of other things. The idea is to insert a resistor in series with the power, in order to continuously measure the voltage developed at its terminals, as shown below.


The measure now a unit "volt" or "millivolt" which requires the conversion unit "amperes" or "milliamp. Where previously it read 12 mA, it now reads 12 mV. The conversion unit is easy, as long as you know the phrase "U = R * I", which can easily derive the formula "I = U / R". The choice of the value of the series resistance RS seems evident, as giving the value of 1 ohm, the value of I is equal to the value of U: the value displayed in Volt corresponds exactly to the actual mA value: 12 mV displayed correspond to a current of 12 mA.

Miraculous!

Things are as simple as this: add a 1 ohm resistor in series with the feed assembly, and reading directly the voltage at its terminals, instantly assume the current drawn ... It is possible that this operation "universal" problem in some cases ... To get an idea of the results obtained in other contexts with the process very simple, look at the thing with two other circuits, one that consumes very little current (12 uA), and one that consumes much more (1.2 A).

1 ohm resistor with a circuit consumes 12 uA

In the previous circuit, we could assimilate the resistance of the power electronic circuit to a value close to 1 KO, since there was an intensity of 12 mA at a voltage of 12 V (ratio of absolute mile). RS added resistance of 1 ohm value was low compared to the internal resistance, and could be considered very little pertubative, even if it caused a small reduction of current consumption (a fall of about 0.1%). By measuring a consumption of 12 uA under a single source voltage of 12 V, it is reasonable to assume that the internal resistance of the circuit is close to 1 MB. RS shunt resistance of 1 ohm (the term shunt is often used to describe a low resistance value used for current measurement) placed on the course of food will have a more significant influence, and we can expect a value read more just. Lets look at it closely.

Ah ... the measured voltage is 0.01 mV, and the caliber of the voltmeter is the lowest you can have. We are therefore facing a lack of precision in the measurement itself. Because if the owner is well aware of 12 uA, we read a value of 10 uA: measurement error is 20%, a value totally unacceptable. The solution is to use a voltmeter to measure and display microvolts, which is not necessarily a simple solution for everyone. Another approach: increasing the value of the series resistance RS until the voltage chutée be used with a conventional voltmeter. In bringing this series resistance RS to a value of 1 KO, or 1000 times larger than the previous value, the voltage should be chutée thousand times larger ... in theory. Because obviously it's not quite true. By increasing the resistance in the power circuit, the current drawn is necessarily weakened. Everything is, as previously, not the affable little (compared to the current consumed in the absence of resistance) on the one hand not to disturb too far, but more importantly, not to disrupt the operation of the assembly itself. On the diagram below, we find our strength SR 1 KO, whose value is compared to 1 MB of internal resistance of the circuit.


That's it. By adopting a value of 1 KO for RS, it is 12 mV corresponding to a value of 12 uA (I = U / R = 12 mV / 1 KO). The assembly operation is not disturbed, because the current drop caused by the addition of RS is 0.1% and can be considered negligible. The first conclusion to be drawn from this experience, the series resistance RS should have a low value against the resistance of the mounting power, while having a high enough voltage to develop easy to measure.

1 ohm resistor with a 1.2 A circuit consuming

What will happen now if you insert a resistance of 1 ohm RS, in the supply of a montage of a nominal current of 1.2 A? If the current through RS is 1.2 A, it should in theory have a voltage drop of 1.2 V at its terminals. The assembly will be somewhat more power than under 10.8 V, but this should not pose a problem, is not it. Let the measure.

Oops ... 1.09 V at the terminals of the resistor RS. One might think that the assembly until a voltage of 12 V and who receives only 10.9 V not work perhaps not quite as expected: the current drawn is increased from 1.2 A to 1.09 A, approximately 10% less. RS and resistance, which dissipates a power of more than 1 W, chauffe much because the author's head was elsewhere and had chosen a 0.5 W. Without a high school (which is my case), we can conclude that the measurement method is not good. Or if the method itself is correct, the value given to RS would probably be revised. If we had the same reasoning as just now, we would have known that the internal resistance of the circuit is close to 10 ohms (1.2 to 12 V), and was found later that the value of RS n was not quite small in proportion. Therefore adopt as a moment ago a report about a thousandth, or RS value of 0.01 ohms (10 milli-ohms).

This time, we measure a voltage drop of 12 mV in the RS resistance of 0.01 ohm, which corresponds to a current of 1.2 A. The assembly is fed a loss of voltage of 12 mV, which is still more acceptable that a loss of 1.1 V. The conclusion can be drawn from this experience with that of previous experience: the value of the resistor RS must be much smaller than the resistance of the mounting power, so that the resulting voltage drop is limited to few mV. If one wishes to make measurements on a wide range of currents possible, use several different resistance values, for each class of measurement.

My dealer does not have a resistance of 0.01 ohm

Admittedly, a small resistance value is not as common as resistance to 1 or 47 KO KO. You can easily find the 0.1 ohm resistors (see photo below) but below it is more difficult.



Because generally, the resistance of low value are not what you might think. It may indeed be a piece of track on a printed circuit board, a rigid piece of metal shaped tongue (long and flat) or bridge, or wire used for the resistive heaters ( Kanthal wire to 1 ohm per meter, for example).



In a test, it can also be a resistance "electronic" consisting of a power transistor providing a short circuit and frank ordered PWM (resistance to the terminals of the transistor can have a value that can be programmed).

Note: The values of resistivity of agenda milliohm also correspond to the residue of electrical contacts, the minimum values of certain power MOS transistors in passing mode. These are the ones we hunt (because we want to avoid or at least control or offset) in the mountains with high power consumption.

Making this measurement method by inserting a shunt resistance

001 analogue ammeter
002 analogue ammeter

Measurement of current coupling

This second method is perhaps more widely used for measuring strong currents, but it is also suitable for low currents of a few tens of microamperes (measurements of leakage current) or a few hundred mA. It makes use of current sensors or current transformers, which consist of a coil (single or multiple coils) through which we place the electrical conductor carrying the current to be measured.

When the electrical conductor carrying the current to be measured is covered by a current, it creates around it a magnetic field that is sufficient to capture and convert voltage function performed by the coil and more or less associated electronics. When the driver in question can not be cut for the purpose of the measure, it needs to have a system where the measurement loop can be opened (for the setting up around the driver) and close (so far can take place). Such a system can be in the form of rigid cable electrical contact or as a clamp.

Pince ampermétrique

Measurement of current Hall effect

This type of measurement is based on the use of a Hall effect sensor, which produces a voltage which is a reflection of the current linear detected.

sommaire

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