The invention relates to a protective circuit for the induction
coil of a magnetically inductive type flow meter. If the leads of
a flow meter used in an explosive environment are cut there can
occur a high voltage and resulting sparking which could cause an
explosion. The invention provides a circuit in which a short circuit
path is provided for the induction coil if the leads thereof are
severed to prevent the development of a high voltage which could
cause dangerous sparking.
1. A magnetically inductive flow meter installation, comprising,
an explosion risk zone and an explosion risk-free zone, an induction
coil, a current reception circuit in said risk zone which includes
said induction coil, first and second transistors each being effectively
in parallel with said coil and arranged in mutually oppositely passing
directions, a current supplying circuit in said risk-free zone connected
to said current reception circuit for supplying positive and negative
current pulses to said induction coil, control means for said transistors
connected in parallel with said induction coil, said control means
being operable during each of said positive and negative pulses
to first bias a corresponding one of said transistors to a short
circuiting conducting state and then to a blocking state, said control
means being similarly operable to effect short circuiting of said
induction coil through one of said transistors whenever the voltage
in said current reception circuit rises due to an interruption of
current flow in said current supplying circuit; and bidirectional
current limiting means in said current supplying current being operable
during normal operation whenever said short circuiting occurs by
reason of said transistors being biased to short circuit conducting
states by said positive and negative pulses.
2. A magnetically inductive flowmeter installation according to
claim 1 wherein said first and second transistors have mutually
opposite pass directions and are connected with the collector-emitter
paths thereof in series, and diode means connected respectively
in anti-parallel to each of said transistors.
3. A flow meter installation according to claim 1 or 2 wherein
said control means comprises a resistor-capacitor series circuit
and first and second resistors connected to opposite ends thereof
in series therewith, said first and second transistors having respective
bases connected to said opposite ends of said resistor-capacitor
4. A flow meter installation according to claim 1 wherein in said
blocking state conditions said positive and negative current pulses
are applied to said coil to facilitate measurements relative to
said coil during said blocking states.
The invention relates to a protective circuit for the induction
coil fed with A.C. pulses in a magnetically inductive flow meter.
BACKGROUND OF THE INVENTION
Magnetically inductive flow meters such as known from U.S. Pat.
No. 4614121 work on the principle that a magnetic field is produced
of a given size and the voltage induced in the flowing liquid is
measured transversely to the magnetic field.
For this purpose, the induction coil is alternately fed with positive
and negative current pulses of a predetermined value. The pulses
may be separated from each other by a passage through zero and/or
by a pause between them. Measurement always takes place after the
end of a pulse, after all transitory conditions have decayed. For
example, the polarity of the current changes from eight to ten times
per second. The induction coil has an inductance of, for example,
100 to 600 mHy. The energizing current is in the order of .+-.0.1
to 0.2 A.
If, during the operation of such a flow meter, the supply line
is interrupted, for example because a plug is removed, a high voltage
is set up at the point of interruption by reason of the inductance
of the coil and this can lead to sparking. This is particularly
so if the leads are first short circuited and then separated again,
as is the case when the leads are accidentally cut by pliers. In
this case, because of the small space between the leads, even lower
voltages will produce a spark. It was therefore not possible to
place a magnetically inductive flow meter in an explosion risk zone.
SUMMARY OF THE INVENTION
The invention is based on the problem of providing a protective
circuit of the aforementioned kind which permits a magnetically
inductive flow meter also to be used in rooms where there is a risk
This problem is solved according to the invention in that the induction
coil is firmly connected to a current receiving circuit which bridges
same and has two Miller integrators of which, depending on the direction
of the current, one is active and the other is inactive by means
of a bridging diode, and that in the leads in the non-explosion
risk zone there is a two-part current limiting circuit of which,
depending on the direction of the current, one part is active and
the other is inactive by means of a bridging diode.
By reason of the respectively active Miller integrator, upon an
interruption in the leads, the current receiving circuit initially
acts as a short circuit path for the induction coil and then assumes
a high resistance depending on the charging of the condenser which
belongs to the Miller integrator and which gradually controls the
associated transistor arrangement into the non-conductive state.
The time dependencies can readily be designed so that no dangerously
excessive voltages are produced at the point of interruption. However,
the current receiving circuit which is designed for both directions
of current will also act as a short circuit when the polarity of
the current pulses changes. For this reason, there is the current
limiting circuit which ensures that this short circuit will not
overload the current generator. Since these procedures occur on
commencement of the current pulse, the actual measurement taking
place at the end of the current pulse is not influenced. The bridging
diodes ensure that the circuits are equally effective for both polarities
of the current pulses.
It is particularly favourable if the two Miller integrators have
a common condenser. This affords a saving of material. In addition,
a comparatively small condenser can produce a short circuiting period
sufficient for the decay of the coil current because this condenser
will, upon an interruption in the conductors, first become discharged
and then charged in the other direction to operate the associated
In a preferred embodiment, in the current receiving circuit the
collector-emitter paths of two transistor arrangements form a first
series circuit, have mutually opposite pass directions, and are
each bridged by a bridging diode which is conductive in the opposite
direction, and the bases of the transistor arrangements are disposed
at the tappings of a second series circuit which consists of a first
resistor, a second resistor in series with a condenser, and a third
resistor, and bridges the induction coil in the same way as the
first series circuit. This gives a very simple symmetrical construction.
By allocating the bridging diodes to the respective transistor arrangements,
one ensures in a simple manner that the current receiving circuit
is effective in both directions.
It is also advisable for two current receiving circuits to be firmly
connected in parallel. This double safety factor permits the protective
circuit to be allocated to a high quality grade in which proper
functioning is not impeded even if one of the circuit components
Preferably, in the current limiting circuit the collector-emitter
paths of two transistor arrangements are in series with each other
and with at least one current measuring resistor, have mutually
opposite pass directions and are each bridged by a bridging diode
which is conductive in the opposite direction, and the voltage drop
at the current measuring resistor controls that transistor arrangement
which is active. Here, again, the allocation of the bridging diode
to the respective transistor arrangement ensures in a single manner
that the current limiting circuit will be effective for positive
and negative current pulses.
The two parts of the current limiting circuit may have a common
current measuring resistor. This enables the number of resistors
to be reduced.
It is particularly favourable if the current limiting circuit is
also provided with a current increase limiter. This can, for example,
occur with the aid of a condenser which charges from commencement
of the pulses and, with an increase in voltage, increasingly controls
a transistor arrangement into the conductive state. If, upon commencement
of the current pulse, the current receiving circuit acts as a short
circuit, the current can increase only according to a predetermined
increase function. When the final value of the current limiter has
been reached, the current receiving circuit will likewise have approached
its high final resistance value.
A particularly simple embodiment is obtained if each transistor
arrangement is associated with a control transistor of which the
base-emitter path is in parallel with the current measuring resistor,
the collector-emitter path is bridged by a condenser and the collector
is connected to the base of the transistor arrangement and, by way
of a resistor, to its collector. The condenser ensures the desired
gradual increase in current.
Desirably, each lead contains a two-part current limiting circuit.
If one fails, the other remains effective.
It is also favourable for the voltage between the leads to be limited
in the non-explosion risk zone by a voltage limiting circuit which
is effective in both directions. The current limiting circuit may,
for example, consist of Zener diodes.
Advantageously, the transistor arrangements each comprise two transistors
in Darlington circuit and form an integrated circuit together with
the associated bridging diode. Such integrated circuits can be readily
purchased and can therefore by easily incorporated in the circuit.
In particular, the integrated circuit may also comprise the base-emitter
resistors of the two transistors and define the first and third
resistors of the second series circuit.
BRIEF DESCRIPTION OF THE DRAWING
A preferred example of the invention will now be described in more
detail with reference to the drawing, wherein:
FIG. 1 is block diagram of the protective circuit according to
FIG. 2 is a diagram of the current pulses fed to the induction
FIG. 3 is a diagram showing modified current pulses.
FIG. 4 shows one embodiment of the current limiting circuit.
FIG. 5 shows one embodiment of the current receiving circuit, and
FIG. 6 shows an integrated circuit that can be used in accordance
with the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
According to FIG. 1 an induction coil 1 of a magnetically inductive
flow meter is alternatively fed with positive and negative current
pulses. The pulses may follow each other directly (FIG. 2) or exhibit
a pause when passing through zero (FIG. 3). The individual pulses
have, for example, a duration of 60 ms and an amplitude of 125 mA;
the pause may also be 60 ms. The connection of a current supply
circuit 2 may be by way of two leads 3 and 4.
The induction coil 1 is in an explosion risk zone 5 whereas the
current supply circuit 2 is disposed in a non-explosion risk zone
In the lead 3 there is a current limiting circuit 7 consisting
of two parts 8 and 9 which can each be made inactive by a bridging
diode 10 or 11 depending on the direction of the current. A corresponding
current limiting circuit 107 with parts 108 and 109 each bridged
by a bridging diode 110 or 111 is disposed in the lead 4. Between
the leads 3 and 4 there are two voltage limiting devices 12 and
112. In the same way as the current limiting circuits 7 and 107
these are disposed in the non-explosion risk zone 6. The lead 3
therefore extends between the terminals 13 and 14 and the lead 4
between the terminals 15 and 16. By way of cables 19 and 20 the
terminals 17 and 18 of coil 1 are releasably connected to the terminals
14 and 16 and fixed to two current receiving circuits 21 and 121.
Each current receiving circuit consists of two Miller integrators
22 23 or 122 which, by means of a respective bridging diode 24
and 25 or 124 and 125 can be made inactive depending on the direction
of the current.
The current limiting circuit 7 and 107 as well as the voltage limiting
circuits 12 and 112 may have the construction shown in FIG. 4. Between
the terminals 13 and 14 there is the series circuit of a current
measuring resistor R1 and the collector-emitter paths of two transistor
arrangements T1 and T2. These collector-emitter paths have mutually
opposite pass directions and are bridged by the bridging diode 10
or 11 of opposite pass direction. A control transistor T3 has its
base-emitter path in parallel with the current measuring resistor
R1. Its collector-emitter path is bridged by a condenser C1. Its
collector is connected to the base of the transistor arrangement
T1 and, by way of a resistor R2 to its collector. Similarly, there
is a control transistor T4 of which the base-emitter path is in
parallel with the current measuring resistor R1 its collector-emitter
path is bridged by a condenser C2 and its collector is connected
to the base of the transistor arrangement T2 and, by way of a resistor
R3 to its collector. The current limiting circuit 108 has the same
construction. Reference numerals increased by 100 are employed.
The voltage limiting circuit 12 consists of two series-connected
Zener diodes Z1 and Z2 having opposite pass directions. Similarly,
the voltage limiting circuit 112 consists of two Zener diodes Z101
FIG. 5 illustrates an embodiment of the measuring head arranged
in the explosion risk zone 5 with the induction coil 1 which here
consists of two series-connected individual coils 1a and 1b, and
the two current receiving circuits 21 and 121. The current circuit
receiving 21 comprises two transistors 75 and 76 of which the collector-emitter
paths form a series circuit, have mutually opposite pass directions,
and are each bridged by a bridging diode 24 or 25. The bases of
the transistor arrangements are connected to tappings 26 and 27
which are formed by a second series circuit consisting of a first
resistor R4 a second resistor R5 in series with a condenser C3
and a third resistor R6. The current receiving circuit 121 has the
In this way, two oppositely acting Miller integrators are formed
in each current receiving circuit and comprises a common integration
condenser C3. If, for example, a positive voltage is applied to
the thermal 17 a short circuit is practically produced by way of
the bridging diode 24 and the collector-emitter path of a transistor
arrangement T6. Simultaneously, however, the condenser C3 is charged
by way of the second series circuit and the voltage drop across
the resistor R6 decreases so that the transistor arrangement T6
is blocked after a short period and hence the entire current receiving
circuit assumes a higher resistance. The Miller principle is based
on the fact that with the aid of a comparatively small integration
condenser C3 one controls a substantially larger integration current.
With a voltage acting in the opposite direction, the first short
circuit current flows through the bridging diode 25 and the collector-emitter
path of the transistor arrangement T5.
FIG. 6 shows a conventional integrated circuit 28 comprising two
transistors T7 and T8 in Darlington circuit, a diode D and two base-emitter
resistors R7 and R8. This circuit 28 can be employed instead of
the combinations shown in broken lines in FIGS. 4 and 5 and consisting
of a transistor arrangement and associated diode. If the circuit
parameters are chosen accordingly, the first and third resistors
R4 and R6 of the second series circuit may even be dispensed with
because they are respectively replaced by the resistors R7 R8.
It will be assumed that during normal operation the cable 19 is
interrupted during a positive current pulses. The current in the
induction coil 1 will then tend to continue to flow using the short
circuit path through the bridging diode 25 and the collector-emitter
path of the transistor arrangement T5 (the same applies to the second
current receiving circuit 121). The previously positively charged
condenser C3 is discharged through the coil 1 and is finally charged
in the opposite direction. This reduces the voltage drop at the
resistor R4 until the transistor arrangement T5 finally blocks.
All this takes place without any considerable voltage build-up between
the terminals 17 and 18 so that no excessively high voltage that
might cause sparking occurs at the point of interruption. If the
interruption occurs during the negative current pulse, the current
receiving circuit will operate in an analogous manner but this time
the short circuit path is formed by the bridging diode 24 and the
transistor arrangement T6.
The current receiving circuit 21 or 121 will also produce a short
circuit each time the polarity of the current is changed. This short
circuit would impermissibly overload the current supply circuit
2 which is prevented by the current limiting circuit 7 and 107.
When a positive current pulse is to be supplied, it flows through
the collector-emitter path of the transistor arrangement T1 the
current measuring resistor R1 and the bridging diode 11 to the induction
coil 1 and through the collector-emitter path of the transistor
arrangement T102 the current measuring resistor R101 and the bridging
diode 110 back to the current control circuit 2. The condenser C1
which charges only gradually ensures that the current increase is
limited. The current through the transistor arrangement T1 does
therefore not immediately reach its final value but only after a
certain period. The latter is such that in the meantime the condenser
C3 of the current receiving circuit 21 is charged and the latter
therefore assumes a high resistance. During further operation, the
transistor arrangement T1 is brought by way of the control transistor
T3 from the voltage drop at the current measuring resistor R1 to
the desired amplitude value of, for example, 125 mA. This limiting
value of the current is maintained during a short circuit. In some
cases, this limitation will alone suffice to keep the short circuit
current low when charging the Miller integrator. The same function
is also fulfilled by the transistor arrangement T102 of the second
current limiting circuit 107. Upon a negative current pulse, the
current passes through the collector-emitter path of the transistor
arrangement T101 the current measuring resistor R101 the bridging
diode 111 the induction coil 1 the collector-emitter path of the
transistor arrangement T2 the current measuring resistor R1 and
the bridging diode 10.
The functions of current limiting control and current increase
limiting control could also be separated from each other. For example,
each part 8 9 108 109 of the current limiting circuit 7 107
may contain a conventional current regulator in series with a further
transistor of which the base-emitter path is bridged by the condenser
that charges on commencement of the pulse.
The Zener diodes Z1 Z2 may also be so connected that on the one
hand they are connected by way of a diode of opposite pass direction
to the one lead and on the other hand to the base of a transistor
arrangement in the other lead.
Such a protective circuit permits the measuring head of a magnetically
inductive flow meter also to be employed in rooms in which there
is a risk of explosions.