Silicon controlled rectifier opens circuit with currents flowing up to values of 30 amperes. Switching concept halves both current and voltage in middle of commutating cycle thereby lowering size and weight requirements. Commutating circuit can be turned on or off by command and will remain on in absence of load due to continuous gate

Mcconville, J. H.

English

NASA Technical Reports Server

From 
source
Li 
0 SCR1
To 
load 
March 1973	
B73-10004 
NASA TECH BRIEF 
.0	 Lewis Research Center 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service. Springfield. Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
IMPULSE COMMUTATING CIRCUIT WITH

TRANSFORMER TO LIMIT REAPPLIED VOLTAGE 
Figure 1. - Basic commutating circuit. 
n
A new circuit to achieve commutation of a switching 
silicon controlled rectifier (SCR) has been developed to 
open a circuit with currents flowing up to values of 30 
amperes. This circuit utilizes a new concept of switching, 
which halves both current and voltage in the middle of the 
commutating cycle thereby lowering the size and weight 
requirements. 
The commutating circuit has performed successfully 
throughout the temperature range of 218 to 373 K (.67 to 
2120 F). It can be turned on or off by command and will 
remain on in the absence of a load due to the continuous 
gate. 
A schematic diagram of the commutating circuit is 
shown in Figure 1; its operation is illustrated in Figure 2. 
In operation, SCR 1 carries the load current. Capacitor C 
is precharged to 450 volts in the polarity shown. A 
manual shut down or trip level overcurrent is sensed at the 
time t0 . SCR 2 is triggered on at this time dumping the 
capacitor across Li and driving the anode of SCR 1 
negative so that it stops conducting almost instantly. The 
current flowing through Li continues to flow but now 
goes through SCR 2 .. When the voltage on SCR 1 comes 
up to zero at time i t , the voltage has been negative on 
SCR I long enough to keep SCR 1 from conducting with 
no gate even though the voltage on the anode is now 
reapplied in the positive direction.
At time t2 , the voltage on the capacitor is zero and the 
current through the inductor is at maximum. SCR 3 is 
now gated on. Until this time, L2 has been in an open 
circuit with no current flow. Now that current can flow 
through L2, the magnetic field in the inductor is the same 
but the number of windings has doubled so that the 
current, almost instantly, goes to half the peak value. The 
oscillation of the circuit continues at half the frequency. 
The current flows through Li and L2 in series, which have 
capacitor voltage across them, so that SCR 2 now has half 
the capacitor voltage across it, in reverse. Thus, it is 
turned off. The same voltage is applied to SCR 1; i.e., half 
of the voltage that would be applied if SCR 3 did not 
conduct. This permits lower ratings for SCR 1. 
As the oscillation continues, the current goes through 
zero and SCR 4 is gated on to permit the current to flow 
in the opposite direction. This is at time t 3 . The capacitor 
is now charged to a voltage representing the energy of its 
initial charge plus the energy stored in the inductor Li at 
time to, exclusive of losses. At time t 4 , the voltage across 
SCR 1 is back down below source voltage, the capacitor 
voltage is going through zero, and SCR 4 is carrying 
maximum current. By time t 5 , the capacitor is nearly 
recharged (except for losses), SCR 3 is turned off, current 
ceases to flow and the voltage on SCR I goes back to 
source voltage. The circuit is now reset and is nearly ready 
(continued overleaf) 
This document was prepared under the sponsorship of the National
	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States
	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States
	 will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19730000004 2020-03-17T06:52:27+00:00Z
. 
+300 V 
SCRI 
voltage -150 V 
Capacitor 
current	 C 
Capacitor 
voltage
-450 V
to	 t i	 t 2	 t3	 t4	 t5 
Figure 2. - Principal waveforms of commutating circuit. 
to be turned on again. In reality, the charge on the 
capacitor will be built up to its final precharge value by 
the capacitor recharge power supply before it is desirable 
to turn the circuit on again. The whole commutation 
cycle takes place in less than 16-millisecond and occurs 
with no reference to the return circuit of the power 
source so that minimum transient disturbances are caused. 
NOTES: 
1. The commutation circuit is ideal for use in direct 
current switching where SCR's are used as the principle 
switching element, because the network interrupts the 
supply and load current immediately upon initiating 
the commutation cycle. The circuit does not induce 
transient current in the supply or load and limits the 
transient voltages. This solid state innovation should 
have potential use in spacecraft and aircraft electrical 
systems, in transportation systems, and in hazardous 
areas such as mining.
2. The commutation scheme can be extended to chopper 
circuits where the voltage is controlled by switching 
and filtering. 
3. No additional documentation is available. Specific 
questions, however, may be directed to: 
Technology Utilization Officer 
Lewis Research Center 
21000 Brookpark Road 
Cleveland, Ohio 44135 
Reference: B73-10004 
PATENT STATUS: 
NASA has decided not to apply for a patent. 
Source: J.H. McConville 
Martin Marietta Corp. 
under contract to
Lewis Research Center
(LEW-I 1849) 
B73-10004	 Category 01


Impulse commutating circuit with transformer to limit reapplied voltage

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