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.
(click to enlarge)
Broaching the INA168 / INA138, seen from above:
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