Design and production of Millivolt tablets, Millivoltmeter
Keywords: design and production of alternative millivolt gauge
Design and production of alternating millory
When e-experiment or production, in addition to the signal source, it will be convenient to work if there is a communication millivolt table. For example, measuring the frequency characteristics and gain of the amplifier. Attachment distortion and channel balance detection, the cutoff frequency adjustment of the filter, etc., can be worn. But generally commercial meter or digital table. It does not have the "MV" gear, and the frequency of the measured frequency is more than 1 kHz, and the above requirements cannot be satisfied.
The AC millivoltic table made here, the design goal is 3mV to 100V, and the frequency range is 12Hz-60KHz. The measurement error is less than 1%. For amateur electronic enthusiasts, it is like a tiger. The whole machine circuit is shown in Figure 1.
In Fig. 1, the input terminal is selected by the two-knife eleven band switch, and the input impedance is 300kΩ. The first level amplifier selects a single shipping LF357H, and its gain belt is widely gb = 20 MHz, which can also ensure the bandwidth of 200kHz (-3dB) under 100 times gain, which can fully meet the requirements of the audio range. The second level is connected to an active detector and performs AC-DC transformation. In order to drive the input number millivoliv, the third level is connected to a 10x amplifier. However, the gain can be changed by switch K1-2.
As can be seen from Fig. 1, the input stage attenuator only uses five precision resistors, and is not like the attenuator of the general voltage meter, and each gear is required to have an attenuation resistance, the reason is to reduce the number of precision resistors (especially low). The wire wind resistance of the resistance is to prevent the frequency characteristics of the input stage from deteriorating. However, in order to reach the range of 10 gear, a 43 kΩ resistor is added to the 10 magnification gear with the input stage attenuating switch K1-1, and 43 kΩ resistors are added to change the gain of the end level to obtain suitable attenuation.
The final input voltage is read with a current table of 1mA. The power supply voltage of the millivolt gauge is positive and the negative 15V dual power supply is powered.
Components Choose three plasmounts using metal packages, C1, C2 use non-polar capacitance, and C2 can also be used in series with two 10μF tantalum electrolytic capacitors, and the electrical resistance in the figure is a metal film resistance. The resistance of the precision is 1% requires a number of digital tables, and 30Ω resistors can be used in parallel, and they are obtained in parallel, try not to wire resistance. The input socket uses a common lotus socket (RCA) on the audio device, and the wire of the test cover is used as a metal shield line, and the shield is grounded.
Installation and adjustment This form is very sensitive, it should be installed in the metal chassis. To avoid external interference; the input terminal and the range switch K1 are as close as possible to the mounting, the input capacitor (0.22μF) and five attenuated resistors are directly soldered on the switch K1. The power transformer should be installed at a location farthest distance.
Draw a general finished current table of the current table, which can only be used for one magnification. This table should be adapted to 3 times the magnification and 10 times. Need to re-draw. The steps are as follows: 1) Unpack the current table cover, remove the dial, measuring the length of the ephesus and the radians of the whole scale (ie, the table needle rotation range.]. 2) A arc of two diameters of about 100-200 mm (high diameter mapping accuracy), the arc length is consistent with the angle of rotation of the table, and one of the arcs is 50 classes (10 times), and another The equal is 30 classes (3-fold); 3) overlapping the print paper that is slightly greater than the actual size of the dial is overlapped on the above large-scale paper. The center is coincident, and the scale is drawn on the arc with a transparent ruler. 4) The drawn scale is in the dial, pay attention to the center of the center, and there is no air between the paper and the dial. Use a knife to cut an extra paper around the dial. Finally, the dial paper quickly passed through the alcohol lamp flame, burning the "hair" flying from the paper to avoid hindering the clockwork. If you draw a computer, you can save steps 2, 3.5) to install the dial back to the current table. Pay attention to the adjustment of the dial of the dilatation
Location.
Turn on the power supply, in the input of the input, the meter pointer should not move; if there is a slight jitter in each of the gear gauges, the offset adjustment circuit can be added to the end level. As shown in Figure 2; if only the low-inclusion end (such as millivollation) has a phenomenon, it should be an external electromagnetic interference, and the input portion (including input terminals, K1, and capacitance, resistance) can also be isolated in the chassis. Get up and the metal plate is ground. Finally, enter 1V (1 kHz) AC signal at the input, in the "1V" gear, adjust the 5kΩ potentiometer, make the table needle indicate fullness. Then enter different AC voltages. Detect each gear accuracy. If the error is large, the attenuation resistance and gain conversion resistance (100kΩ, 43kΩ) should be reconstructed. Be
To this end. A high cost-effective exchange millivolt production was successful.
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