Electricity: electrical systems and devices – Control circuits for electromagnetic devices – Systems for magnetizing – demagnetizing – or controlling the...
Reexamination Certificate
2002-01-25
2004-01-06
Dinkins, Anthony (Department: 2836)
Electricity: electrical systems and devices
Control circuits for electromagnetic devices
Systems for magnetizing, demagnetizing, or controlling the...
C361S152000, C361S154000
Reexamination Certificate
active
06674628
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to relays and in particular to a relay having a pulse-width modulated control voltage.
2. Description of Related Art
FIG. 1
is a simplified sectional elevation view of a typical prior art normally open relay
10
including a glass tube
12
containing a pair of conductive reeds
14
,
15
serving as the relay's contacts
16
for selectively providing a signal path between two circuit nodes. A wire
17
wrapped many turns around tube
12
forms a coil
18
. Reeds
14
,
15
are normally spaced apart, but when a current passes through coil
18
, coil
18
produces magnetic flux causing reed
14
to contact reed
15
so that a current may flow through the relay contacts
16
.
FIG. 2
is a schematic diagram illustrating a typical relay control system
25
for driving relay
10
. Control system
25
includes a switching power supply
28
that can produce a DC signal VS of a desired voltage from an AC signal. As illustrated in
FIG. 2
, power supply
28
includes a rectifier
28
A and capacitor
28
B for converting the AC signal to a high voltage DC signal. A switch
28
C controlled by a pulse width modulation circuit (PWM)
28
D couples the DC signal across the primary winding of a transformer
28
E. The transformer's secondary winding is connected across a diode
28
F and a capacitor
28
G. The power supply's DC output voltage VS developed across capacitor
28
G is a function of the duty cycle with which pulse PWM circuit
28
D closes switch
28
C. PWM circuit
28
D monitors VS and adjusts the duty cycle to keep VS at a desired level.
Relay control system
25
also includes a switch
26
linking a power supply
28
to a circuit node
23
. A limiting resistor
24
links node
23
to a node
27
. Relay coil
18
is connected between node
27
and ground. (Limiting resistor
24
may be a separate discrete component as shown in
FIG. 2
or may be implemented as the resistance of the wires forming coil
18
.) Relay
10
includes a diode
29
connected across coil
18
. When signal SW is applied, switch
26
closes, a current I begins to flow in coil
18
and power supply
28
begins to supply energy to coil
18
and resistor
24
at a constant rate. Initially, current I is very low and resistor
24
dissipates little of the energy output of power supply
28
. Coil
18
stores most of the energy output of power supply
28
in a magnetic field. However the current is proportional to the strength of the coil's magnetic field and the strength of the coil's magnetic field is proportional to the amount of energy stored in the field. Hence the coil current increases and the magnetic field becomes stronger as coil
18
continues to receive energy from supply
28
and store it in the magnetic field.
The rate at which resistor
24
dissipates energy supplied by power supply
28
is proportional to I
2
R, where R is the resistor's resistance. Little current passes through resistor
24
when switch
26
initially closes and therefore coil
18
stores little energy in its magnetic field. Therefore resistor
24
initially dissipates little of the energy produced by supply
28
. However as coil
18
begins to store increasing amounts of energy in its magnetic field, it permits an increasing amount of current I to flow through resistor
24
, and the resistor begins to dissipate an increasing proportion of the output energy of power supply
28
. Coil
18
stops adding energy to the magnetic field when coil current I reaches a substantially constant steady state level I
ss
after all initial transients or fluctuating conditions have settled. At that point resistor
24
will dissipate all of the energy being produced by supply
28
. The resistance of resistor
24
(in combination with the inherent resistances of coil
18
, switch
26
and supply
28
) limits the steady state level I
ss
at which the coil current I levels off after switch
26
closes. Resistor
24
therefore acts as a “current limiting” resistor inserted into the circuit to limit the flow of current, thereby preventing excessive current from damaging other parts of the circuit.
When switch
26
opens, the magnetic field surrounding coil
18
continues to induce a current I within coil
18
and there is no instantaneous change in its amplitude. However rather than circulating in the loop including coil
18
, supply
28
, switch
26
and resistor
24
, coil current I instead circulates in the loop including coil
18
and the diode
29
connected across the coil's terminals. As the inherent resistances of diode
29
and coil
18
dissipate the energy stored in the magnetic field, the field collapses and the coil current amplitude tapers off to zero.
The intensity of the magnetic field coil
18
produces is proportional to the product of the amplitude of the current passing through coil
18
and the number of turns of the coil about tube
12
. A typical relay coil
18
will include a large number of turns to minimize the amount of steady state current I
ss
needed to operate relay
10
because this also minimizes the power resistor
24
dissipates. The power P that resistor
24
dissipates in response to the steady state current is as follows:
P=I
ss
2
R [1]
By doubling the number of coil turns we can reduce the required steady state current I
ss
while still maintaining the same steady state field intensity. To reduce I
ss
, we double R. While equation [1] tells us doubling R increases power dissipation by a factor of two, it also tells us that reducing I
ss
in half decreases power dissipation by a factor of four. Thus, the net effect of doubling the number of coil turns and doubling the size of resistor
24
is to cut the resistor's power dissipation in half. Hence to reduce power consumption, relay coils typically have many turns. However as we add turns to a relay coil, we not only add to the cost of making the relay, we also add to the relay's physical size. A relay's coil typically contributes more than half of its thickness.
Thick relays can be problematic in applications where large numbers of relays must be packed into a small volume. For example, an integrated circuit (IC) tester typically uses relays to route signals between test circuits and terminals of an IC device under test (DUT). It is helpful to position the test circuits as closely as possible to the DUT terminals so that signal paths between the test circuits and the DUT's input/output terminals are very short. Since a large number of relays must reside between the test circuits and the DUT, we want to be able to pack as many relays as possible into a small space. However since relays having many turns are thick, large numbers of them cannot be packed into a small space.
We could use thin relays having fewer coil turns, but as discussed above, conventional controllers for such relays would generate substantial amounts of heat in their current limiting resistance. Therefore what is needed is a relay controller that can drive a high current relay having relatively few coil turns without incurring substantial power loss in current limiting resistance.
BRIEF SUMMARY OF THE INVENTION
The present invention is directed to a relay controller using pulse-width modulation to control the current in a relay coil. Rather than continuously linking a power supply to a relay coil and using resistance to limit coil current when a control signal indicates the relay coil is to produce the magnetic field, a relay control system in accordance with the invention intermittently connects the power supply to the coil with a controlled frequency and duty cycle. No current limiting resistor is required because the frequency and duty cycle with which the power supply is connected to the coil limits the steady-state amplitude of the current passing through the coil.
In some embodiments of the invention the frequency and duty cycle with which the power supply is connected to the relay coil is fixed to a level that
Bedell Daniel J.
Credence Systems Corporation
Smith-Hill and Bedell
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