Semiconductor ac static power switch has long life and high reliability, contains no moving parts, and operates satisfactorily in severe environments, including high vibration and shock conditions. Due to their resistance to shock and vibration, static switches are used where accidental switching caused by mechanical vibration or shock cannot be tolerated

Vrancik, J.

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June 1968	 Brief 68-10224 
I- &
NASA TECH BRIEF	 /& 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Semiconductor AC Static Power Switch 
OSCILLATOR CIRCUIT
	
TRANSFORMER, RECTI- 	 DRIVE CIRCUIT	 POWER-HANDLING 
FIER AND FILTER
	
CIRCUIT 
The problem: 
Switches for aerospace power systems must have 
long life and high reliability and must operate satis-
factorily in severe environments, including high vibra-
tion and shock conditions. Conventional electro-
mechanical switches and relays often have undesirable 
characteristics for these applications; contact bounce, 
contact pitting, arcing, slow switching speed, etc. 
The solution: 
The construction of an AC (400 cps) static switch 
using semiconductors such as transistors and silicon 
controlled rectifiers (SCR's) which are reliable, capa-
ble of long life, and contain no moving parts.
How its done: 
Minimum power drain from the control signal is 
achieved by having the control signal turn on a small 
switch which in turn supplies turn-on power to the 
main SCR's. Since the control signal may or may not 
be grounded, an oscillator-transformer-rectifier cir-
cuit provides isolation between the control signal 
and the switch, and permits the use of a single control 
signal to operate all three phases of the switch. 
The four main sections of the switch are the oscil-
lator circuit, rectifier and filter, drive circuit, and 
power-handling circuit. 
The oscillator is an astable multivibrator using two 
signal transistors (Qi and Q2) that alternately conduct 
(continued overleaf) 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19680000224 2020-03-12T05:34:20+00:00Z
and cut off. The input is 10 volts DC at 10 milli-
amperes (100 mW) and the output power is 60 milli-
watts. The switch will operate with control signal input 
as low as 5 volts at 5 milliamperes over its temperature 
operating range of —40 0
 to 800 C. The oscillator trans-
former has three output windings, designed for 3.5 volts 
at 6 milliamperes each. Each winding feeds a full-wave 
bridge rectifier arid filter, the output of which is 2 volts. 
This voltage is applied to the drive transistor (Q) which 
saturates and in turn supplies power from the power 
line to the power handling SCR's through D5 to D6, 
D7 to D8, and R6 to R7. 
The switch has positive turn-on over its tempera-
ture operating range when the power line voltage is at 
least 8 volts. Under normal conditions, the drive 
transistor dissipates less than 0.1 watt. However, if the 
switch is turned on when the AC supply voltage is at 
its maximum, the base drive will not be sufficient to 
hold the transistor in saturation. For this particular 
application, as much as 225 watts peak power ma' be 
dissipated in the drive transistor; thus, a high power 
transistor is used. The switch has a design rating of 
4 amperes and has a continuous overload capability of 
14 amperes for a range of heat-sink temperatures from 
—40° to 80°C.
Notes: 
I. The desirable characteristics of this static switch 
are the absence of moving parts (and the associated 
wear), and the high gain possibilities between the 
control power and the power transferred. 
2. Due to their capacity for a large number of opera
-
tions, static switches are particularly suited to ap-
plications such as on—off type regulation, position 
controls for the orientation of solar panels, radio 
antennas, flashing lights, etc. They are also useful 
in applications which require fast response time 
such as the interconnection of alternators for 
parallel operation. Due to their relatively high 
resistance to shock and vibration, static switches 
are well suited to applications where accidental 
switching caused by mechanical vibration or shock 
cannot be tolerated. 
3. Inquiries concerning this innovation may be di-
rected to:
Technology Utilization Officer 
Lewis Research Center 
21000 Brookpark Road 
Cleveland, Ohio 44135 
Reference: B68-10224 
Patent status: 
No patent action is contemplated by NASA. 
Source: James Vrancik 
(LEW- 10344) 
Brief 68-10224	 Category 01


Semiconductor ac static power switch

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