MIL-STD-202H.pdf - 第290页

MI L - S TD - 202 -3 10 4.2.2 Test cir cuit B . Th e m o ni t or - c i r c ui t s how n on f i gur e 3 p er m i t s d et ec t i o n of c on t ac t - c h a tter of cl osed contacts and false c losure of op en c ont ac t s…

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MIL-STD-202-310
C
1
, C
2
- Choose for specified time (see note 1) R
6
- 1,000 ohms 1/4W, 5%
DS
1
, DS
2
- No. 344 Lamp R
7
- 100 ohms 1/4W, 5%
R
1
- 750 ohms 1/4W, 5% R
8
- 200 ohms pot.
R
2
- 2,000 ohms pot. Q
1
, Q
2
- 2N332A or equivalent
R
3
, R
5
- 10,000 ohms 1/4W, 5% SCR
1
, SCR
2
- 2N1595 or equivalent
R
4
- 2,500 ohms 1/4W, 5% S
1
- SPST NC Push
NOTE:
1. Use .0022 µF for 10 microsecond time-duration. Other time-duration will require larger capacitors.
FIGURE 3. Test-circuit B; monitor circuit for contact-chatter and false closures.
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MIL-STD-202-310
4.2.2 Test circuit B. The monitor-circuit shown on figure 3 permits detection of contact-chatter of closed contacts
and false closure of open contacts, independently or simultaneously. The low contact-load levels (see 4.1.1.2) insure
that there will be no arcing of the contacts during monitoring.
a. The chatter portion of figure 3, resistors R3 and R4 form a voltage divider with their junction at +2 volts. The
closed contacts of the component under test, short-circuit R4 and place the base of transistor amplifier Q1 to
ground potential. When the contacts under test "chatter" (open), resistor R4 is no longer short-circuited and
capacitor C1 starts to charge through R2 and R7 to +2 volts. The time necessary for C1 to charge to the
correct bias-level is determined by the resistance of R2 and R7 and the capacitance value of C1. As
transistor Q1 draws current through the gate of SCR1, the unit will fire and turn-on lamp DS1. Since in a
silicon-controlled rectifier, the gate loses control after it is turned "on", the contacts can reclose at any time
thereafter without affecting the monitoring circuit. The time-delay, before turn-on, can be adjusted by varying
R2 and selecting the capacitance value of C1. (For example: C1 = .002 µF gives a 10-microsecond open-
contact time.)
b. In the false-make portion of figure 3, transistor-amplifier Q2 is normally "on" with the gate of SCR2 being
effectively held at ground potential by the low-output impedance of transistor Q2. When a "false-make"
occurs, the base of Q2 transistor is grounded, turning Q2 "off". This allows the gate of the SCR2, which is
tied to the collector of transistor Q2, to rise to +12 volts. The rate of increase is determined by the value of
C2 and R8. (For example: C2 = .002 µF gives a 10-microsecond false-make time.) When the voltage
reaches the gate turn-on level of SCR2, lamp DS2 will light, indicating a false closure of the open contacts.
c. When this circuit is being used to simultaneously monitor both the open and closed contacts of a double set
of contacts:
(1) If DS1 "lights", it is an indication of contact chatter.
(2) If DS1 and DS2 "lights", it is an indication of false transfer or possible bridging, i.e., the movable contact
of the open circuit "closes" but the closed circuit has not opened.
(3) If DS2 "lights", it is an indication of bridging.
d. Restoration of the circuit for an indication of failure is accomplished by the operation of S1.
4.2.2.1 Calibration procedure for test-circuit B. The calibration-circuit shown on figure 4 may be used to calibrate
the monitoring-circuit shown on figure 3 by using the following procedure:
a. Make the proper connections of the monitoring-circuit to the calibration-circuit.
(1) BP1 and BP2 for contact-chatter calibration.
(2) BP1 and BP3 for false contact-make calibration.
b. Select the appropriate 5 volt square-wave "pulse-polarity" and "pulse-width" to be furnished by the pulse
generator and monitor the pulse on the oscilloscope, as follows:
(1) For contact-chatter calibration: Negative pulse.
(2) For false contact-make calibration: Positive pulse.
(3) Pulse width for either of the preceding (1) or (2) equal to the required detection time.
c. If DS1 or DS2 (as applicable) "lights", adjust R2 or R8 until the light is extinguished.
d. Slowly adjust R2 and R8 (as applicable) to the time-duration specified in the individual specification, as
indicated by the first point at which DS1 or DS2 "lights".
6
北测（上海）电子科技有限公司
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MIL-STD-202-310
NOTES:
1. The square-wave pulse generator and oscilloscope shall have an accuracy of ±3 percent or better.
2. The ratio of off-time to detection-time shall be 10:1 or better.
FIGURE 4. Calibration circuit for test-circuit B.
7
北测（上海）电子科技有限公司
联系方式：xuyj@beice-sh.com 13917165676