Electrical isolation circuit

A circuit provides electrical isolation from an input signal which has a leading edge and a trailing edge. On the leading edge of the input signal, a first signal is transmitted across an isolation device to provide a first isolated signal representative of the leading edge of the input signal. On the trailing edge of the input signal, a second signal is transmitted across the isolation device to provide a second isolated signal representative of the trailing edge of the input signal. The first and second isolated signals are electrically isolated from and representative of the input signal.

BACKGROUND OF THE INVENTION 
The present invention relates to electrical isolation circuits and in 
particular an isolation circuit which utilizes at least two signals to 
represent the signal to be isolated. 
Isolation circuits have utilized many different isolation devices, such as 
electromagnetic coils and optical couplers. Electromagnetic coils have 
been used mainly to isolate analog signals by coupling the entire analog 
signal. Optical couplers generally have a light emitting device coupled to 
a photo sensitive device and have been used to couple digital pulses by 
techniques including optical coupling of the entire pulse. Optical 
couplers have also been used to isolate analog signals by measuring 
received light intensity and by converting analog signals to pulse width 
modulated, pulse amplitude modulated, and pulse frequency modulated 
digital signals where the entire signal is coupled through the optical 
coupler. Coupling an entire pulse through an optical coupler required that 
the light emitting device and photo sensitive device have substantially 
equal, time stable and sharply defined turn-on and turn-off time 
characteristics. 
SUMMARY OF THE INVENTION 
An isolator circuit provides electrical isolation from an input signal 
which has a leading edge and a trailing edge. A first means receives the 
input signal and provides a first signal representative of the leading 
edge of the input signal. The first means also provides a second signal 
representative of the trailing edge of the input signal. An isolation 
means has an input coupled to the first means and has an output. The 
isolation means provides a first isolated signal responsive to the first 
signal and a second isolated signal responsive to the second signal at its 
output which are electrically isolated from and representative of the 
input signal. 
In one preferred embodiment, the isolation means is an optical coupler 
directly transmitting the first and second signals and receiving the first 
and second isolated signals. The input signal in this embodiment is a 
pulse width modulated signal which in turn is representative of an analog 
input signal. The first and second signals are preferably dc pulses 
wherein one of the pulses has a duration which is longer than the other, 
thus distinguishing the leading and trailing edges of the input signal. 
Thus, the first and second isolated signals are similarly distinctive of 
the leading and trailing edges of the input signal by having different 
pulse widths and with suitable logic circuitry are converted to a pulse 
width modulated signal with a pulse width substantially equal to the pulse 
width of the input signal. 
In a further preferred embodiment, the input signal is a sine wave wherein 
the leading edge corresponds to the sine wave having a first predetermined 
level with a positive slope and the trailing edge corresponds to the sine 
wave having a second predetermined level with a negative slope. 
In yet a further preferred embodiment, the isolation means is a single 
transformer having accurate repeatable turn-on times responsive to the 
first and second signals. It is desirable that the particular isolation 
means utilized have fast and repeatable turn-on times and response 
characteristics suitable for distinguishing between the first and second 
signals. Use of a single isolation device ensures that the turn-on times 
in response to the first and second signals are substantially equal, thus 
increasing the accuracy of the isolator circuit. 
One advantage of the present invention is derived from converting the input 
signal into two signals representative of the leading and trailing edges 
respectively. This takes advantage of relatively stable turn-on times of 
isolation devices and places less importance on turn-off times which are 
usually different than turn-on times causing distortion of the width of 
the input signal in devices transmitting an entire pulse. While turn-on 
and turn-off times may vary with time and temperature, the turn-on time of 
the isolation device will still be substantially equal relative to 
immediate successive first and second signals, while the turn-off time 
remains stable enough to distinguish between the first and second signal 
pulse widths thus maintaining high accuracy over time.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A preferred embodiment of the present invention is represented in block 
diagram form of a circuit 8 in FIG. 1. An input signal indicated at 10 is 
provided by an input circuit 11. Input signal 10 preferably comprises a 
pulse width modulated square wave signal representative of a signal from a 
sensor, such as an industrial standard 4 to 20 milliampere signal. Input 
signal 10 has a leading edge 12 and a trailing edge 14. Input signal 10 is 
provided along a line 16 to a first means 18, also referred to as signal 
generator 18. Signal generator 18 receives input signal 10 and provides a 
first signal 19 on a line 20 and a second signal 21 on a line 22 which are 
representative of leading edge 12 and trailing edge 14 of input signal 10 
respectively. The first signal 19 and second signal 21 are preferably 
electrical pulses having different pulse widths to distinguish between 
leading edge 12 and trailing edge 14 of input signal 10. Signal generator 
18 preferably provides the first and second signals at substantially equal 
intervals from receiving the leading edge 12 and the trailing edge 14. 
Lines 20 and 22 are coupled by a line 23 to an input 24 of an isolation 
means 26, also referred to as isolator 26. Isolator 26 is preferably a 
6N139 opto-coupler, which is photo diode package which provides electrical 
insulating or dielectric isolation at its output 29 from its input 24. 
Isolator 26 thus provides distinctive first and second isolated signals 27 
and 28 on a line 30 which are electrically isolated from and 
representative of the first and second signals 19 and 21. First and second 
isolated signals 27 and 28 are coupled to a suitable output means 32 also 
referred to as output circuit 32 for processing to provide for example a 1 
to 5 volt dc signal representative of the input signal 10. 
In further preferred embodiments, output means 32 comprises a visual 
indication device such as a voltmeter. 
In a further preferred embodiment, input signal 10 comprises a sine wave 
which has a leading edge corresponding to a predetermined level at which 
the sine wave has a positive slope and a trailing edge corresponding to a 
predetermined level at which the sine wave has a negative slope. First and 
second signals 19 and 21 are then representative of the frequency or the 
amplitude of the sine wave input signal dependent upon the levels chosen, 
as desired. 
A detailed schematic diagram of circuit 8 indicated in FIGS. 2A and 2B on 
separate sheets is numbered consistently with FIG. 1. Input circuit 11 in 
FIG. 2A comprises a pair of input terminals 40, 42. Input terminal 40 
receives a field input signal such as a 4 to 20 milliampere signal from a 
sensor. The field input signal is provided through an input impedance 44 
and back through terminal 42. The field input signal is also provided 
through a resistor 45 which is coupled to terminal 40 to a first input 46 
of a buffer amplifier 47. In further preferred embodiments, the field 
input signal comprises a voltage or frequency signal for which isolation 
is desired. 
Buffer amplifier 47 provides a buffered field signal along a line 48 to a 
first input 50 of a comparator amplifier 52. A ramp generator 54 provides 
a ramp signal along a line 56 through a buffer amplifier 58 through a 
resistor 59 along a line 60 to a second input 64 of comparator amplifier 
52. Comparator amplifier 52 provides a square wave alternately having a 
high level and a low level at its output 66. When the ramp signal at a 
second input 64 attains an amplitude proximate to the amplitude of the 
buffered field signal at first input 50, the square wave at output 66 
changes level. Output 66 is also coupled through a resistor 63 to second 
input 64 of comparator amplifier 52. The ramp generator starts a new ramp 
each time the ramp signal reaches a predetermined maximum level thus again 
causing a change in level of the square wave. 
The square wave has a width representative of the field input signal at 
terminal 40. Output 66 is coupled by a line 68 to a squaring device 71. 
Squaring device 71 provides input signal 10 having precisely defined 
leading and trailing edges 12 and 14. The time between the leading and 
trailing edges 12 and 14 is representative of the field input signal at 
terminal 40. Line 16 carries input signal 10 through a resistor 72 to a 
second input 73 of buffer amplifier 47 for stabilization of the square 
wave at output 66. Second input 73 of buffer amplifier 47 is also coupled 
to line 48 through a capacitor 74. 
Line 16 is also coupled to signal generator 18 which comprises a first 
one-shot 75 and a second one-shot 76. First one-shot 75 is configured in a 
conventional manner to provide first signal 19 on line 20 upon detection 
of leading edge 12 of input signal 10. Second one-shot 76 is similarly 
configured to provide second signal 21 on line 22 upon detection of 
trailing edge 14 of input signal 10. The first signal 19 and second signal 
21 preferably are pulses and are distinguishable as by having different 
durations and respective leading and trailing edges. The first signal 19 
preferably has a duration distinguishably longer than the duration of 
second signal 21. 
First one-shot 75 and second one-shot 76 are preferably matched one-shots 
such that the time between first one-shot 75 receiving the leading edge 12 
of input signal 10 and providing first signal 19, and the time between 
second one-shot 76 receiving the trailing edge 14 of input signal 10 and 
providing second signal 21 are substantially equal so that the time 
between the leading edges of first and second signals 19 and 21 accurately 
represents the input signal 10 pulse width. To preserve this time 
relationship between the first and second signals 19 and 21, lines 20 and 
22 are preferably of equal length and have consistent signal propagation 
characteristics. Line 20 is coupled to line 23 through a resistor 77, and 
line 22 is coupled to line 23 through a resistor 79. 
A transistor 78 has a base 80 coupled to line 23 and has a collector 82 and 
an emitter 84. Emitter 84 is coupled to input terminal 42 by a line 86. 
Base 80 is coupled to line 86 through a resistor 87. A light emitting 
means 90, also referred to as light emitting diode 90 and a resistor 92 
are series coupled between a source V1 and collector 82 of transistor 78. 
First signal 19 or second signal 21, when received at base 80 of 
transistor 78, cause a conductive path between source V1 and emitter 84 
which forward biases light emitting diode 90 which then emits a light 
signal represented by arrows 94 having a substantially constant intensity 
and duration substantially equal to the duration of the first signal 19 or 
the second signal 21. Light is defined as including electromagnetic 
radiation in the infrared, visible and ultraviolet portions of the 
spectrum. 
First and second signals 19 and 21 preferably have amplitude and transient 
characteristics which cause transistor 78 to be conductive from collector 
82 to emitter 84 at substantially equal times to preserve the time 
relationship between the first and second signals 19 and 21. A single 
light emitting diode 90 is preferably used such that the emitted light 
preserves the time relationship between first and second signals 19 and 21 
as thermal drift effects on the time relationship are substantially 
eliminated. Use of two light emitting diodes invites different turn-on 
times due to turn-on time dependence on environmental parameters such as 
temperature, as well as specification tolerances for a given lot of light 
emitting diodes which can adversely affect the time relationship between 
first and second signals 19 and 21. 
A light responsive means 100, also referred to as a photo diode 100 in FIG. 
2B is optically coupled to light emitting diode 90 to receive the light 
signals indicated again by arrows 92. Isolator 26 is shown partially in 
FIG. 2A and partially in FIG. 2B and comprises light emitting diode 90 and 
photo diode 100. Isolator 26 in FIG. 2A is coupled to the remaining 
portion of isolator 26 in FIG. 2B by coupling broken lines 26A and 26B in 
FIG. 2A to corresponding broken lines 26A and 26B in FIG. 2B. Use of a 
single photo diode as opposed to separate photo diodes further preserves 
the time relationship between the first and second signals 19 and 21. 
Photo diode 100 is reverse bias coupled between a source V2 and a first 
transistor 101 at its base 102. Transistor 101 also has a collector 103 
coupled to source V2 and an emitter 104 coupled to a second transistor 105 
at its base 106. Transistor 105 has an emitter 107 coupled to source V2 
through a capacitor 108 and has a collector 109 coupled to source V2 
through a resistor 110. When photo diode 100 receives the light signals, 
it conducts thus turning on transistors 101 and 105 and provides the first 
and second isolated signals 27 and 28 at isolator output 29 which is 
coupled to collector 109 of transistor 105. The first and second isolated 
signals 27 and 28 are preferably negative going pulses of substantially 
the same duration and time relationship as first and second signals 19 and 
21 respectively with corresponding leading and trailing edges. A line 114 
is coupled to emitter 107 of transistor 105 and serves as a ground which 
is electrically isolated from terminal 42 and field ground. Isolator 
output 29 is coupled by line 30 to a first one-shot 116 which on receiving 
the leading edges of the first and second isolated signals 27 and 28 
provides a first pulse of a predetermined duration having a leading and 
trailing edge along a line 118 to a second one-shot 120. Line 30 is also 
coupled to both inputs of an inverter 122 which has an output coupled by a 
line 124 to a first input 125 of a nand gate 126. A second input 127 of 
nand gate 126 is coupled by a line 128 to the output of second one-shot 
120. Second one-shot 120 provides a second pulse of predetermined length 
along line 128 to nand gate 126 and is triggered by the trailing edge of 
the first pulse from first one-shot 116. Nand gate 126 is coupled to both 
inputs of an inverter 130 by a line 132. The output of inverter 122 is 
also coupled by line 124 to a flipflop 140. Thus the first and second 
isolated signals on line 30 are inverted by inverter 122 and trigger 
flipflop 140 on their respective leading edges. Flipflop 140 then provides 
a pulse width modulated output signal on line 142 as a function of the 
first and second isolated signals 27 and 28. The output of inverter 130 is 
coupled to flipflop 140 by a line 144 to reset flipflop 140 when flipflop 
140 is in an incorrect state with respect to the leading edges of first 
and second isolated signals on line 124. The durations of the first and 
second pulses provided by first and second one-shots 116 and 120 are such 
that if flipflop 140 is being triggered to wrong states by first or second 
isolated signal 27, 28, such as on startup of circuit 8, nand gate 126 and 
inverter 130 will cause flipflop 140 to be reset and thenceforth be 
triggered to correct states. 
The pulse width modulated output signal from flipflop 140 on line 142 
preferably has the same pulse width as input signal 10 and is electrically 
isolated therefrom. A squaring device 146 is coupled to line 142 and 
squares the edges of the pulse width modulated output signal on a line 
152. The pulse width modulated output signal on line 152 has an amplitude 
with respect to isolated ground on line 114 which is consistent with the 
predetermined maximum amplitude of the ramp generated by ramp generator 
54. 
A low pass filter 150 is coupled to squaring device 146 by a line 152. 
Filter 150 comprises a first resistor 154 coupled between line 152 and a 
second resistor 156. A capacitor 158 is coupled between isolated ground 
114 and first and second resistors 154, 156. The end of resistor 156 not 
directly coupled to resistor 154 is coupled through a resistor 158 to a 
first input 160 of an amplifier 162. Amplifier 162 provides the dc voltage 
signal at its output on line 164 representative of input signal 10. First 
input 160 of amplifier 162 is also coupled to isolated ground 114 through 
a capacitor 166. The output of amplifier 162 is coupled to a second input 
of amplifier 162 through a resistor 168. The output of amplifier 162 is 
also coupled through a capacitor 170 to a junction 172 between resistors 
156 and 158. 
Filter 150 is preferably a 3 pole low pass filter to smooth the pulse width 
modulated output signal to provide the voltage signal on a line 164, 
preferably a 1 to 5 volt dc signal which is representative and preferably 
substantially equal to the field input signal across input impedance 44. 
A partial parts list of the preferred embodiment of FIGS. 2A and 2B 
comprises: t,0130 
An idealized timing diagram of the preferred operation of output circuit 32 
is represented in FIG. 3, traces a through h. Signals appearing at the 
circled letters a through h on FIGS. 2A and 2B correspond to the traces of 
FIG. 3. The input signal 10 on line 16 is represented in trace a. Trace a 
shows the input signal 10 on line 16 as a pulse width modulated signal 
having a leading edge indicated at t1 and a trailing edge indicated at t2. 
Trace b represents the first and second isolated signals 27 and 28 on line 
30 as a series of inverted pulses. Leading edges of input signal 10 in 
trace a, such as at t1 correspond to first isolated signal 27 which 
comprises pulses of longer duration than second isolated signal 28 which 
comprises pulses in trace b representing the trailing edges of input 
signal 10 in trace a. First isolated signal 27 should not be longer than 
the pulse width of input signal 10, and second isolated signal 28 should 
not be longer than the time between pulses of input signal 10. Trace c 
shows a series of first pulses from first one-shot 116 which is triggered 
by leading edges of the first and second isolated signals 27 and 28 of 
trace b. Trace d represents a series of second pulses from second one-shot 
120 which is triggered by the trailing edges of the first pulses of first 
one-shot 116 in trace c. The first pulses of trace c have a selected 
duration longer than the duration of the second pulses in trace d, the sum 
of the first pulse duration and the second pulse duration preferably being 
less than the time between pulses of input signal 10 in trace a. Trace e 
represents inverted first and second isolated signals 27 and 28 on line 
124 which is preferably an inversion of trace b. Trace f represents the 
AND logical condition of the second pulse from second one-shot 120, trace 
d and the inverted first and second isolated signals, trace e, and is used 
to reset flipflop 140 such as during startup of the circuit as indicated 
by a time frame tsu on traces g and h. Trace g represents the output 
signal on line 142 and as can be seen, at time t1 circuit 8 is turned on 
and at time t3, the logical AND of trace d and trace e is true so flipflop 
140 is reset. In normal operation, flipflop 140 changes state as indicated 
in trace g upon receipt of leading edges of the signal in trace e, as 
indicated at t4, t5, t6 through ti. Trace h is simply the pulse width 
modulated output signal on line 152, an inversion of trace g. The first 
pulses g and h are representative of the startup time of the circuit 
indicated by time frame tsu. The total time t2-t1 between leading and 
trailing edges of trace a should be greater than the sum of the pulse 
durations in trace b which are representative of the leading and trailing 
edges in trace a. 
In a further preferred embodiment of the present invention, the first and 
second signals 19 and 21, and corresponding first and second isolated 
signals 27 and 28 are equal duration pulses whose amplitude distinguishes 
between leading edge 12 and trailing edge 14 of input signal 10. The 
advantage of representing the leading and trailing edges by two 
distinctive signals to utilize turn-on time stability of isolation devices 
such as optical, electromagnetic or mechanical isolation devices is 
obtained so long as the signals have distinguishable characteristics 
representative of the leading and trailing edges respectively of input 
signal 10. Low cost and simple circuitry is used to implement the present 
invention because great accuracy of turn-off times of isolator 26 is not 
required nor do widths of the signals need to be precisely measured. 
In yet a further preferred embodiment in FIG. 4 wherein the numbering is 
consistent with FIG. 2A and 2B, at least a portion of first signal 19 is 
optically coupled directly to flipflop 140 by an isolator 210 to directly 
reset flipflop 140. First and second signals 19 and 21 are then of shorter 
and preferably equal duration and are optically coupled directly to 
flipflop 140, still utilizing the immediately successive repeatability 
characteristics of isolator 26. The shorter durations of first and second 
signals 19 and 21 permit shorter duration pulse width modulated input 
signals to be isolated since the durations need not be distinguished. 
In still a further preferred embodiment in FIG. 5, a pulse width modulated 
input signal on a line 316 is coupled to a first edge triggered monostable 
vibrator 320 and a second edge triggered monostable vibtrator 326 which 
output on lines 330 and 332 respectively a first pulse and a second pulse 
corresponding to the leading and trailing edges of the input signal 
respectively. The first pulse is preferably of shorter duration than the 
second pulse. Lines 330 and 332 are coupled to a logical "OR" gate 336 for 
squaring the edges of and combining the first and second pulses on a line 
340. Line 340 is coupled through a current limit resistor 344 and through 
a light emitting diode 348 to a first ground 350. Light emitting diode 348 
provides light in response to the first and second pulses, preferably with 
a minimum delay which is equal relative to the first and second pulses. 
The light is received by the base of a photo transistor 360 having a 
collector 362 coupled through a resistor 364 to a voltage V and an emitter 
368 coupled to a second ground 370 isolated from first ground 350. On 
receipt of the light from diode 348, the first and second pulses are 
inversely reproduced at collector 362. Collector 362 is coupled by a line 
372 to a Schmitt trigger 374 for squaring the edges and inverting the 
first and second pulses. Trigger 374 provides the first and second pulses 
on a line 378 which are of the same polarity as the pulses on line 340 and 
are electrically isolated therefrom. A pulse discriminator 380 is coupled 
to line 378 and set such that the second pulse triggers a reset signal at 
the discriminator 380 output on a line 382. Line 378 is also coupled to a 
flipflop 386 having an output on a line 390. The leading edges of the 
first and second pulses cause the flipflop output to change state, while 
the reset signal on line 382 causes the flipflop to be reset during the 
second pulse, thus distinctly identifying the second pulse. Thus, a pulse 
width modulated output signal responsive to the first and second pulses on 
line 378 and the reset signal on line 382 is provided on line 390 
electrically isolated from and preferably having an equal pulse width as 
the pulse width of the input signal on line 316. 
While the present invention has been described with reference to preferred 
embodiments, it will be recognized by those skilled in the art that 
further preferred embodiments such as use of the turnoff characteristics 
of an isolation device to represent the input signal are also within the 
scope of the present invention.