Fail-safe circuit for a control circuit

Abstract

 1 Fail-safe circuit (10) for a control circuit of a device to be operated, especially of a gas or oil valve, the fail-safe circuit (10) comprising: an input contact (11) by which the fail-safe circuit (10) is connect-able to a microprocessor, and an output contact (12) by which the fail-safe circuit (10) is connectable to a drive circuit of the control circuit, wherein the fail-safe circuit (10) is configured to operate the drive circuit on basis of a signal provided by the microprocessor at the input contact (11) of the fail-safe circuit (10) in such a way that the fail-safe circuit and thereby the drive circuit can only operate the device, especially open the gas or oil valve, when an input signal having at least two different frequency signals succeeding each other in time is provided at the input contact (11) of the fail-safe circuit (10); diodes (13, 14) connected in series; a first capacitor (15) connected in parallel to the series connection of diodes; a series connection of a resistor (21) and a second capacitor (22) connected between the input contact (11) of the fail-safe circuit (10) and a contact point (23) between two diodes (13, 14) of the series connection of diodes; a first transistor (24) connected to the input contact (11) of the fail-safe circuit (10), and a second transistor (25) connected to the output contact (12) of the fail-safe circuit (10). The first transistor (24) is a pnp bipolar junction transistor. The second transistor (25) is a n channel junction-fet transistor or a n channel metal-oxide-semiconductor field-effect transistor. The collector (26) of the first transistor (24) is connected to the gate (28) of the second transistor (25), wherein the source (29) of the second transistor (25) is connected to ground (17) and the drain (30) of the second transistor (25) is connected to the output contact (12) of the fail-safe circuit (10).

Description

[0001] The present patent application relates to a Fail-safe circuit.

[0002] Automatic control for a gas or oil valve needs to be fail safe. If an automatic control is based on a microprocessor, an additional fail-safe watchdog makes the total control fail safe. In case of any failure the gas or oil valve shall stop in a safe status, meaning that the valve shall not open and shall not be ignited. Similar requirements have to be fulfilled in other application is which a device to be operated is controlled on basis of a microprocessor.

[0003] EP 1 730 760 B1 discloses a control circuit for a gas valve. The control circuit according to EP 1 730 760 B1 comprises an input contact by which the control circuit is connectable to a microprocessor, output contacts by which the control circuit is connectable to the gas valve to be operated, a drive circuit and a fail-safe circuit. The microprocessor is connectable to an input contact of the control circuit, namely to an input contact of the fail-safe circuit, whereby the drive circuit can only open the gas valve when an input signal having at least two different frequency signals succeeding each other in time is provided at the input contact of the fail-safe circuit. The drive circuit of the control circuit known from EP 1 730 760 B1 comprises two Darlington transistor circuits.

[0004] Another control circuit for a gas valve comprises an input contact by which the control circuit is connectable to a microprocessor, output contacts by which the control circuit is connectable to the gas valve to be operated, a drive circuit and a fail-safe circuit is known from EP 1 730 432 B1.

[0005] The fail-safe circuit of the control circuit of EP 1 730 432 B1 is almost identical to the fail-safe circuit of the control circuit of EP 1 730 760 B1. The drive circuit of the control circuit known from EP 1 730 432 B1 comprises a comparator and a transistor.

[0006] The fail-safe circuits of EP 1 730 760 B1 and EP 1 730 432 B1, which are also often called watchdog circuits, can open the gas or oil valve only when an input signal having at least two different frequency signals succeeding each other in time is provided at the input contact of the fail-safe circuit. This means it is not possible to open in the gas or oil valve with one frequency signal, whatever the frequency of the signal is. In this way it is proved that the microprocessor, which generates the two specific frequencies in turn at the input contact of the fail-safe circuit, is running properly. If for any reason only one frequency signal with an unknown frequency is produced the gas or oil valves close immediately.

[0007] Against this background, an alternative fail-safe circuit for a control circuit of a device to be operated, especially of a gas or oil valve, providing the same function is provided.

[0008] The novel fail-safe circuit is defined in the claim 1.

[0009] The first transistor of the novel fail-safe circuit is a pnp bipolar junction transistor. The second transistor of the novel fail-safe circuit is a n channel junction-fet transistor or a n channel metal-oxide-semiconductor field-effect transistor. The collector of the first transistor is connected to the gate of the second transistor. The source of the second transistor is connected to ground and the drain of the second transistor is connected to the output contact of the fail-safe circuit.

[0010] The novel fail-safe circuit for a control circuit of a device to be operated, especially of a gas or oil valve, provides the same function like the fail-safe circuits known from prior art. However, the same is more reliable.

[0011] Preferably, the cathode of a first diode of the series connection of diodes is connected to a first contact of the first capacitor and to ground, wherein the anode of the first diode of the series connection of diode is connected to the cathode of a second diode of the series connection of diodes, and wherein the anode of the second diode of the series connection of diodes is connected to a second contact of the first capacitor and to the collector of the first transistor.

[0012] According to a first improved development, the base of the first transistor is connected to the input contact of the fail-safe circuit, wherein the emitter of the first transistor is connected to ground. In this case the base of the first transistor is connected to the input contact of the fail-safe circuit and to the anode of the second diode of the series connection of diodes.

[0013] According to a second, alternative improved development, the base of the first transistor is connected to the ground, wherein the emitter of the first is connected to the input contact of the fail-safe circuit. In this case

[0014] Preferred developments of the invention are provided by the dependent claims and the description which follows. Exemplary embodiments are explained in more detail on the basis of the drawing, in which:

Figure 1

shows a fail-safe circuit for a control circuit of a device to be operated, especially of a gas or oil valve;

Figure 2

shows another a fail-safe circuit for a control circuit of a device to be operated, especially of a gas or oil valve; and

Figure 3

shows the novel fail-safe circuit in combination with an exemplary the drive circuit.

[0015] The present patent application relates to a fail-safe circuit for a control circuit of a device to be operated, especially of a gas or oil valve.

[0016] Figures 1 and 2 both show fail-safe circuits 10 for a control circuit of a device to be operated, especially of a gas or oil valve, according to the present application.

[0017] The fail-safe circuit 10 comprises an input contact 11 by which the fail-safe circuit 10 is connectable to a microprocessor (not shown).

[0018] The fail-safe circuit 10 comprises further an output contact 12 by which the fail-safe circuit 10 is connectable to a drive circuit (not shown) of the control circuit.

[0019] The fail-safe circuit 10 is configured to operate the drive circuit on basis of a signal provided by the microprocessor at the input contact 11 of the fail-safe circuit 10 in such a way that the fail-safe circuit 10 and thereby the drive circuit can only operate the device, especially open the gas or oil valve, when an input signal having at least two different frequency signals succeeding each other in time is provided at the input contact 11 of the fail-safe circuit 12. The signal of the microprocessor having at least two different frequency signals succeeding each other in time is generated by the software of the microprocessor and provided by the microprocessor at the input contact 11 of the fail-safe circuit 10.

[0020] The fail-safe circuit 10 comprises diodes 13, 14 connected in series.

[0021] The fail-safe circuit 10 further comprises first capacitor 15 connected in parallel to the series connection of diodes 13, 14.

[0022] The cathode 16 of a first diode 13 of the series connection of diodes 13, 14 is connected to a first contact of the first capacitor 15 and to ground 17.

[0023] The anode 18 of the first diode 13 of the series connection of diodes 13, 14 is connected to the cathode 19 of a second diode 14 of the series connection of diodes 13, 14.

[0024] The anode 20 of the second diode 14 of the series connection of diodes 13, 14 is connected to a second contact of the first capacitor 15.

[0025] The fail-safe circuit 10 further comprises a series connection of a resistor 21 and a second capacitor 22 connected between the input contact 11 of the fail-safe circuit 10 and a contact point 23 between two diodes 13, 14 of the series connection of diodes 13, 14. The contact point 23 is positioned between the anode 18 of the first diode 13 and the cathode 19 of a second diode 14.

[0026] The fail-safe circuit 10 further comprises a first transistor 24 connected to the input contact 11 of the fail-safe circuit 10, and a second transistor 25 connected to the output contact 12 of the fail-safe circuit 10. The anode 20 of the second diode 14 of the series connection of diodes 13, 14 is connected to the second contact of the first capacitor 15 and to the collector 26 of the first transistor 24. A resistor 27 is connected between the anode 20 of the second diode 14 of the series connection of diodes 13, 14 and the collector 26 of the first transistor 24.

[0027] The first transistor 24 is a pnp bipolar junction transistor.

[0028] The second transistor 25 is a N channel junction-fet (JFET) transistor or a N channel metal-oxide-semiconductor field-effect (MOSFET) transistor.

[0029] The collector 26 of the first transistor 24 is connected to the anode 20 of the second diode 14 of the series connection of diodes 13, 14 and further connected to the gate 28 of the second transistor 25.

[0030] The source 29 of the second transistor 25 is connected to ground 17 and the drain 30 of the second transistor 25 is connected to the output contact 12 of the fail-safe circuit 10.

[0031] In the embodiment of Figure 1, the base 31 of the first transistor 24 is connected to the input contact 11 of the fail-safe circuit 10, wherein the emitter 32 of the first transistor 24 is connected to ground 17.

[0032] In this case, the base 31 of the first transistor 24 is connected to the input 11 of the fail-safe circuit 10 and further to the 20 of the second diode 14 of the series connection of diodes 13, 14.

[0033] A resistor 33 is preferably connected between the base 31 of the first transistor 24 and the input contact 11 of the fail-safe circuit 10. A further resistor 34 is preferably connected be-tween the base 31 of the first transistor 24 and the anode 20 of the second diode 14 of the series connection of diodes 13, 14.

[0034] The microprocessor connected to the input contact 11 provides an input signal at the input contact 11 of the fail-safe circuit 10.

[0035] The signal provided by microprocessor at the input contact 11 has at least two different frequency signals succeeding each other in time. The same can be a square wave signal with a high frequency signal and low frequency signal.

[0036] In the embodiment of Figure 1, the high frequency signal can charge first the capacitor 15 by the network of the resistor 21, the second capacitor 22 and the diodes 13, 14.

[0037] Voltage at the first capacitor 15 is lower than the ground voltage. By the resistor 27, the voltage is applied at the gate 28 of the second transistor 25. The second transistor 25 is a N-channel junction-fet (JFET) transistor or a N channel metal-oxide-semiconductor field-effect (MOSFET) transistor.

[0038] If the gate source voltage of the second transistor 25 is zero, the second transistor 25 is conducting. When second transistor 25 is conducting, the high frequency signal provides the voltage to shut off the second transistor 25. The high frequency signal is stopped by the first transistor 24. A parasitic capacitor between base and emitter and resistor 33 provides a low pass filter.

[0039] During high frequency signals, the parasitic capacitor of the first transistor 24 is charged at the average voltage value of the high frequency signal. The transistors 24, 25 are off. Low frequency signal is able to drive on and off the first transistor 24, so gate 28 of second transistor 25 can have by resistor 34 the negative voltage of the first capacitor 15 or a 0 voltage by collector of the first transistor 24.

[0040] The voltage across the first capacitor 15 should be bigger than the threshold voltage of the second transistor 25. The resistor values of resistors 34 and 33 should have a defined ratio, preferably R34:R33 = 1:1/2.

[0041] When the high frequency signal is provided, the voltage Vbe across the base and emitter of the first transistor 24 is negative to assure that the transistor 24 is not conducting. The transistor 24 shall be selected with a parasitic capacitor high enough to reduce the bandwidth of the input signal.

[0042] In the embodiment of Figure 2, the base 31 of the first transistor 24 is connected to the ground 14, wherein the emitter 32 of the first transistor 24 is connected to the input contact 11 of the fail-safe circuit 10. Preferably, the emitter 32 of the first transistor 24 is connected to the input contact 11 of the fail-safe circuit 10 through a resistor 35.

[0043] The function of the embodiment of Figure 2 is almost identical. The first transistor 24 is on while the microprocessor makes a high frequency signal. The first capacitor 15 is discharged just by the first transistor 24 and the resistor 27. If the low frequency switches off the first transistor 24, first capacitor 15 is not discharged.

[0044] The novel fail-safe circuit 10 is preferably a component of a control circuit comprising the fail-safe circuit 10 and a drive circuit. The input contact 11 of the fail-safe circuit 10 is connectable to a microprocessor (not shown).

[0045] The output contact 12 of the fail-safe circuit 10 is connectable to the drive circuit of the control circuit.

[0046] The novel fail-safe circuit 10 can be used for example in combination with the drive circuit 111 shown in Figure 3.

[0047] The drive circuit 111 of the control circuit comprises a first transistor 119 and a second transistor 120. The first transistor 119 of the drive circuit 111 and the second transistor 120 of the drive circuit 111 are both operated through the fail-safe circuit 10 on basis of a first signal provided by the microprocessor.

[0048] The first signal contains two signal components, namely a high-frequency signal component having a first duty cycle and a low-frequency signal component having a second duty cycle. The first duty cycle and the second duty cycle are preferably the same.

[0049] The first signal is provided by the microprocessor at the input contact 11 of the fail-safe circuit 10. The first transistor 119 of the drive circuit 111 and a second transistor 120 of the drive circuit 111 are both provided by MOSFET transistors.

[0050] A first parallel connection having a resistor 133, a capacitor 134 and a first and a second diode 135, 136 of the drive circuit 111 being serially connected is connected between the gate and the source of the first transistor 119.

[0051] The first and the second diode 135, 136 of the first parallel connection of the drive circuit 111 are serially connected in such a way that the cathode of the first diode 135 is connected to gate of the first transistor 119, that the anode of the first diode 135 is connected to the cathode of the second diode 136, and that the anode of the second diode 36 is connected to the source of the first transistor 119.

[0052] A second parallel connection also having a resistor 133, a capacitor 134 and a first and a second diode 135, 36 of the drive circuit 111 being serially connected is connected between the gate and the source of the second transistor 120.

[0053] The first and the second diode 135, 136 of the second parallel of the drive circuit 111 connection are serially connected in such a way that the cathode of the first diode 35 is connected to gate of the second transistor 120, that the anode of the first diode 135 is connected to the cathode of the second diode 136, and that the anode of the second diode 36 is connected to the source of the second transistor 120.

[0054] A first series connection 137 of the drive circuit 111 having a resistor 138 and a capacitor 139 is connected with a first contact point between the first diode 135 and the second diode 136 assigned to the first transistor 119 and with a second contact point to a first contact 12 of the fail-safe circuit 10. A further capacitor 141 is connected in parallel to one of the diodes of the first parallel connection, namely in parallel to the second diode 136.

[0055] A second series connection 40 of the drive circuit 111 also having a resistor 138 and a capacitor 139 is connected with a first contact point between the first diode 135 and the second diode 136 assigned to the second transistor 120 and with a second contact point to the first contact 12 of the fail-safe circuit 10. Another further capacitor 141 is connected in parallel to the fifth diode 316 of the second parallel connection.

[0056] The drain of the first transistor 119 of the drive circuit 111 is according to Figure 3 directly connected to a power supply contact 118.

[0057] The source of the of the second transistor 120 of the drive circuit 111 is according to Figure 1 directly connected to ground 27.

[0058] The source of the first transistor 119 of the drive circuit 111 is connected to the drain of the second transistor 120 through a third diode 130, wherein the anode of the third diode 130 is connected to the drain of the of the second transistor 120 and wherein the cathode of the third diode 130 is connected to the source of the first transistor 119. The source of the first transistor 119 of the drive circuit 111 is connected to a first output contact 131 of the control circuit and the drain of the second transistor 120 is connected to a second output contact 132.

[0059] The device, especially the gas or oil valve, to be operated is connectable to these output contacts 131 and 132. According to Figure 1, a resistor 124 is connected between the output contact 12 of the fail-safe circuit 10 and power supply contact 118.

[0060] As mentioned above, the microprocessor provides the first signal at the input contact 11. The signal provided at the input contact 11 comprises the high-frequency signal component and the low-frequency signal component. The high-frequency signal component has the first duty cycle and the low-frequency signal component having the second duty cycle. The first duty cycle and the second duty cycle of the two signal components of the first signal provided at the input contact 11 are non-variable. The first duty cycle and the second duty cycle of the two signal components of the first signal are preferably each 50%, meaning that the high-frequency signal component and the low-frequency signal component of the first signal are responsively present for a defined time period and responsively disappear for the same time period.

[0061] The drive circuit 111 shown in Figure 3 is of exemplary nature only. The fail-safe circuit 10 can also be used in combination with the drive circuits shown in EP 1 730 432 B1 and EP 1 730 760 B1.

List of reference signs

[0062] 

10

fail-safe circuit

11

input contact

12

output contact

13

first diode

14

second diode

15

first capacitor

16

cathode

17

ground

18

anode

19

cathode

20

anode

21

resistor

22

second capacitor

23

contact point

24

first transistor

25

second transistor

26

collector

27

resistor

28

gate

29

source

30

drain

31

base

32

emitter

33

resistor

34

resistor

35

resistor

111

drive circuit

113

transistor

118

power supply contact

119

transistor

120

transistor

121

input contact

122

series connection

123

series connection

124

resistor

130

diode

131

output contact

132

output contact

133

resistor

134

capacitor

135

diode

136

diode

137

series connection

138

resistor

139

capacitor

140

series connection

141

capacitor

Claims

1. Fail-safe circuit (10) for a control circuit of a device to be operated, especially of a gas or oil valve, the fail-safe circuit (10) comprising
an input contact (11) by which the fail-safe circuit (10) is connectable to a microprocessor, and an output contact (12) by which the fail-safe circuit (10) is connectable to a drive circuit of the control circuit, wherein the fail-safe circuit (10) is configured to operate the drive circuit on basis of a signal provided by the microprocessor at the input contact (11) of the fail-safe circuit (10) in such a way that the fail-safe circuit and thereby the drive circuit can only operate the device, especially open the gas or oil valve, when an input signal having at least two different frequency signals succeeding each other in time is provided at the input contact (11) of the fail-safe circuit,
diodes (13, 14) connected in series,
a first capacitor (15) connected in parallel to the series connection of diodes,
a series connection of a resistor (21) and a second capacitor (22) connected between the input contact (11) of the fail-safe circuit (10) and a contact point (23) between two diodes (13, 14) of the series connection of diodes,
a first transistor (24) connected to the input contact (11) of the fail-safe circuit (10), and a second transistor (25) connected to the output contact (12) of the fail-safe circuit (10),
characterized in that
the first transistor (24) is a pnp bipolar junction transistor,
the second transistor (25) is a n channel junction-fet transistor or a n channel metal-oxide-semiconductor field-effect transistor,
the collector (26) of the first transistor (24) is connected to the gate (28) of the second transistor (25), that the source (29) of the second transistor (25) is connected to ground (17) and the drain (30) of the second transistor (25) is connected to the output contact (12) of the fail-safe circuit (10).

2. Fail-safe circuit of claim 1 characterized in that the cathode (16) of a first diode (13) of the series connection of diodes is connected to a first contact of the first capacitor (15) and to ground (17), that the anode (18) of the first diode (13) of the series connection of diodes is connected to the cathode (19) of a second diode (14) of the series connection of diodes, and that the anode (20) of the second diode (14) of the series connection of diodes is connected to a second contact of the first capacitor (15) and to the collector (26) of the first transistor (24).

3. Fail-safe circuit of claim 1 or 2, characterized in that a resistor (27) is connected between collector (26) of the first transistor (24) and the anode (20) of the second diode (14) of the series connection of diodes (13, 14).

4. Fail-safe circuit of one of claims 1 to 3, characterized in that the base (31) of the first transistor (24) is connected to the input contact (11) of the fail-safe circuit (10), and that the emitter (32) of the first transistor (24) is connected to ground (17).

5. Fail-safe circuit of claim 4, characterized in that the base (31) of the first transistor (24) is connected to the input contact (11) of the fail-safe circuit (10) and to the anode (20) of the second diode (14) of the series connection of diodes (13, 14).

6. Fail-safe circuit of claim 6, characterized in that a resistor (33) is connected between the base (31) of the first transistor (24) and the input contact (11) of the fail-safe circuit (10), and that a further resistor (34) is connected between the base (31) of the first transistor (24) and the anode (20) of the second diode (14) of the series connection of diodes.

7. Fail-safe circuit of one of claims 1 to 3, characterized in that the base (31) of the first transistor (24) is connected to the ground (17), and that the emitter (32) of the first transistor (24) is connected to the input contact (11) of the fail-safe circuit (10).

8. Fail-safe circuit of claim 7, characterized in that the emitter (32) of the first transistor (24) is connected to the input contact (11) of the fail-safe circuit (10) through a resistor (35).

9. Control circuit of a device to be operated, especially for a gas or oil valve, the control circuit comprising an input contact (11) by which the control circuit is connectable to a microprocessor, an output contact (131, 132) by which the control circuit is connectable to the device to be operated, a drive circuit (111) and a fail-safe circuit (10), wherein the drive circuit (111) comprises a first transistor (119) and a second transistor (120) both being operated through the fail-safe circuit (10) on basis of a signal provided by the microprocessor, and wherein the signal is provided by the microprocessor at an input contact (11) of the fail-safe circuit, characterized by a fail-safe circuit (10) of one of claims 1 to 8.

Drawing