Circuit, based on use of power transistors which do not display second breakdown under valve loading, drives inductive loads. Peak voltages, power dissipations, heat sink requirements, and thermal stability considerations can be obtained by theoretical analysis

Cooke, W. A.

English

NASA Technical Reports Server

1972	 B72-10268 
NASA TECH BRIEF 
Ames Research Center 
NASA Tech Briefs announce now 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. 
Driver Circuit for Inductive Loads 
The problem: 
To design a circuit which can drive inductive loads 
such as are presented by the solenoids of cold-gas 
thrust valves used in rocket attitude control systems. 
Since the fine adjustment of attitude requires that 
the thrust valves deliver very small increments of gas 
on command, the very rapid on-off valve operation 
makes it impractical to diode-clamp the turn-off tran-
sient voltage of the valve coil. 
The solution: 
A circuit based on the use of power transistors 
which do not display second breakdown under valve 
loading. 
How it's done: 
The figure shows two valve driver circuits con-
trolled by an inhibit circuit. Each drive circuit sup-
plies power to its valve solenoid when about +3 volts 
appear at the valve command terminal and turns off 
power when about +0.5 volt appears at the terminals. 
The drive circuits are inactivated by an inhibit signal 
of the order of +12 volts. 
Transistors Qi and Q2 are in a Darlington con-
figuration for power gain; resistor R2 allows Q2 to 
saturate completely, and R3 provides a leakage path 
to ground. Both Qi and Q2 are off when a negative 
or zero volt input appears at the input terminal 
leading to the base of Qi, and since no valve current 
flows, the valve is in the off position. When a posi-
tive input command of more than one volt is applied, 
both Qi and Q2 turn on and the resulting current 
turns on the valve. The current through the valve 
increases exponentially, and typically has a 10-milli-
second time-constant. Now, if the transistor drive
circuit is turned off abruptly, a voltage is induced 
across the inductor of a magnitude and duration 
which keeps the collector current flowing in Q2, thus 
driving it into avalanche and high power dissipation, 
Valve Coil 
Equivalent 
Valve 
Command 
DI	 R3
+V 
D2 
D3	 Q3	 Valve Coil 0
	
it	
Equivalent 
Inhibit
Q^I^Rl 
Valve	 R6 
Command
co 
i.e., the so-called second breakdown where the base 
no longer controls normal collector characteristics. 
Physically, second breakdown is , caused by very 
localized current concentrations, resulting in the un- 
controllable generation and multiplication of carriers, 
with a sudden reduction of collection impedance. 
(continued overleaf) 
Thisdocumentwas prepared underthe sponsorship oithe 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=19720000268 2020-03-17T03:27:21+00:00Z
Because the duration of this phenomenon exceeds the 
thermal time constant of the small area in which 
current is flowing, the transistor is damaged. 
The usual methods for preventing damage of tran-
sistors connected to inductive loads use "free wheel-
ing" diodes, Zeners, or combination of Zeners and 
diodes; however, these methods increase the time re-
quired for a solenoid to unload its magnetic field 
when current is interrupted. 
The solenoid driver circuit depicted in the figure 
performs successfully because Q2 (and Q5) is se-
lected from a small group of transistors which have 
adequate instantaneous power dissipation and which 
do not readily exhibit second breakdown. Peak volt-
ages, power dissipations, heat sink requirements and 
thermal stability considerations can be obtained by 
theoretical analysis; the effectiveness of the circuit is 
proven by tests of the assembled components.
Note: 
Requests for additional information may be di-
rected to: 
Technology Utilization Officer 
Ames Research Center 
Moffet Field, California 94035 
Reference: TSP 72-10268 
Patent status: 
No patent action is contemplated by NASA. 
Source: Walter A. Cooke of 
Lockheed Missiles & Space Company
under contract to

Ames Research Center 
(ARC-10073) 
B72-10268	 Category 01


Driver circuit for inductive loads

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server