The abstract introduced that one kind take the PIC16C57 monolithic integrated circuit as core low resistance measuring instrument, what is different with the ordinary low resistance measuring instrument: This measuring instrument’s test electric current is only 05 mA and Canada has many kinds of protection circuits, has the very high safety performance. Mainly applies in profession and so on war industry, national defense, civil demolition electricity igniters direct-current low resistance surveys.
Key word PIC monolithic integrated circuit low resistance igniters オ
The low resistance measuring instrument may divide into two kinds according to its test electric current’s size: Kind of test electric current is big, mainly uses in product and so on connector, switch, conductor direct-current low resistance surveys; Another kind of test electric current is very small (generally is about 1 mA), uses in the electric fuse, the flame igniter or other dangerous explosive primary device’s and so on situation connector, switch direct-current low resistance surveys. The low resistance measuring instrument is very high to the safety performance request, must increase many kinds of protection circuits; When PCB board design wiring must consider the security and the reliability. in 1989 until now, we improve and the perfect circuit design, the design have produced 4 model low resistance measuring instrument, the following are the newest 〥 ZC-4 intelligence low resistance measuring instrument unceasingly.
1 overall concept and technical specification
ト bin the mark other shows, this instrument is composed of 5 parts: Power source supply, precise constant current, precise voltage amplifier, A/D switch, monolithic integrated circuit controller.
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Its major technique target is as follows:
Its major technique target is as follows: Test scope: 0~20 ω0~200ω0~2 k (three keep off measuring range automatic cut over);
Most hr: 0.001;
Test electric current: 0.5 mA;
Test precision: ± (0.2% 2);
The complete machine consumes the electricity: <30 mA.
2 hardware designs
(1) power unit
The complete machine uses 6 5 nickel hydrogen battery power supply (7.2 V) and the general DC/DC converter, transforms the stable ±5 V DC voltage the battery voltage. This part also has the battery voltage observation circuit and the charging circuit, the battery fills the electricity to be possible to supply the instrument long-term usage the approximately 50 hours.
(2) constant current
Transport by the precise datum potential source and the high performance put are composed, to was measured that the resistance provides the precise test electric current. The test electric current elects is 0.5mA.
Because the precise current supply part is the entire instrument safety performance weak link, must consider that each kind of interior and the exterior possible factor the damage which creates to the instrument, thus affects the safety performance. Mainly has taken the following measure:
The limiting current use ties the field effect tube’s constant current characteristic limit test electric current ITEST size, generally takes IDSS≈2*ITEST. In order to increase the reliability, uses two field effect tube series.
The limit voltage uses the zener diode parallel in the test end limiting pressure.
(3) precision amplifier
By cuts wave steady transporting piecemeal to put makes the spike the synchronism enlargement, because a/D switch’s full scale voltage is 2 V, test electric current 0.5mA, corresponds the 20,200,2K three measuring ranges the synchronism enlargement factors is 200,20,2. Turns on two in the negative feedback return route simulation electronic switching SW1 which, SW2 controls by the monolithic integrated circuit, uses in 3 measuring range different enlargement factor the cut: When the measuring range is 20 guarantees 琒 W1, SW2 to close bravely; For 200 brave 盨 W1 opening; 2k brave 盨 W1, SW2 opens. The instrument verifies when should start from the lower range, otherwise cannot calibrate all measuring ranges. Moreover, Ri, R1, R2, R3 elected with the precision resistor and the precision resistor, causes the temperature the influence to drop to lowly.
(4)A/D switch
As shown in Figure 2, uses 4 1/2 ICL7135 chips, meets the full scale is 2 V voltmeter’s patterns. ICL7135 provides the data with 5 BCD code’s form to the monolithic integrated circuit, and has provided the measuring range (OV) and owes the measuring range (UN) the signal to use in the measuring range automatic cut over to the monolithic integrated circuit. ICL7135 clock rate from T5 (CD4060) the Q5 end, the frequency is 1.25 kHz, happen to is the power frequency 50Hz integral multiple, can enhance the instrument anti-power frequency disturbance ability. A/D inversion frequency approximately is: 3.3 /s. The T5 COUT end also provides 4MHz the clock rate to give the monolithic integrated circuit.
(5) monolithic integrated circuit controller
This part of functions include: Data acquisition, processing, demonstration, measuring range cut, voltage monitoring and so on. This aircraft display module has two piece of 74LS164 to use in the LED nixietube separately the position actuation and the section actuation, altogether has 5 numerical codes to demonstrate, as shown in Figure 2.
The monolithic integrated circuit T2 25 feet use in closing the display module the demonstration, in order to avoid the nixietube demonstrates the additive device in the data transmission process. The T2 24 feet are the battery undervoltage examination input, T2 23, 22 foot control precision amplifier’s simulation electronic switching, has enlargement factor which needs.
3 software designs
This instrument most major characteristic is realizes the automatic zero setting and the measuring range with the software transforms, omits the failure rate high potentiometer and the measuring range cut switch. The software zero alignment’s method is: After the starting, monolithic integrated circuit carries on self-checking, if the system work normal reads the result which A/D transforms. When reads 5 A/D transformation result after continuously, judges them whether to be smaller than 0.2Ω, otherwise, thought that the operator will not have measured the testing rod short-circuits reliably, the instrument continues to demonstrate the zero alignment prompt symbol. If the continual 5 values are smaller than 0.2Ω, by now discovered the minimum value takes the starting value, later the measurement result each time must subtract the starting value.
The measuring range and owes the measuring range signal according to A/D transformation chip ICL7135, carries on the measuring range cut automatically by the monolithic integrated circuit. The measuring range cut must complete 3 work: First is the cut precision amplifier’s enlargement ratio, is again adjusts the starting value the significant digit, finally is adjusts the decimal point the position.
The PIC16C57 monolithic integrated circuit look-at-me function, has not used the inquiry the method and a/D switch communication, transforms the gap demonstration data using A/D.
The PIC monolithic integrated circuit and in display module’s 74LS164 uses the serial data communication, each time only demonstrates 1 bit data, to prevent the demonstration to have the twinkle, refurbishing rate should be bigger than 30Hz.
Procedure flow as shown in Figure 3.
4 conclusions The instrument with was measured that the resistance uses 4 system diagrams, can eliminate the connector resistance the influence. In addition, because the instrument resolving power is very high, therefore the request test jig carries on silver-plating processing, this point is very important, will otherwise cause the test result drifting not to decide.
The instrument volume production’s test result indicated: Instrument’s technical specification achieves the design requirements, can satisfy the production, the scientific research need. If the instrument improves slightly, may treat as the high keen direct-current voltmeter and the ampere meter use.
