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:
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:
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