Voltage protection circuit for multi-drop bus of an automated coin vending machine

[0002] This invention relates to a voltage protector, and more particularly, to a voltage protection circuit for a coin changer/validator unit in a vending machine with a multi-drop bus.

[0003] Automated coin vending machine manufacturers have adopted a set of standard interconnection protocols that control operations between machine controllers and various credit and money handling units in a vending machine. The standard, available from the National Automatic Merchandising Association, or NAMA, uses the multi-drop bus, or MDB, to connect the components of the vending machine, such as the coin changer/validator, to the machine controller. The bus supplies both power and communications data to the different components from the machine controller, thus allowing different components from different vendors to be interconnected and operate successfully together.

[0004] The NAMA voltage standard for the multi-drop bus establishes an acceptable range of voltages that can be supplied to the components of the vending machine without damage. However, the standard does not specify any power conditioning. As a result, spikes that occur on the power lines of the bus, especially when the AC power is suddenly cut off and then restored, can cause significant damage to the components of the vending machine, particularly the coin changer/validator. The internal power sources for the components, such as the coin changer/validator, afford only limited protection, and frequently fail adequately to protect the components against spikes on the power line. In addition, the multi-drop bus power supplies provide inadequate or no spike protection at all for the bus itself.

[0005] Accordingly, there is a need for a plug-in voltage protector for a multi-drop bus in a vending machine that can be added to any vending machine and protect the components of the machine from damaging spikes in power.

[0006] There is also a need for a voltage protector for a multi-drop bus in a vending machine that protects the multi-drop bus from damaging spikes in power.

[0007] There is a still further need for a voltage protector for a multi-drop bus in a vending machine that protects the coin changer/validator from damaging spikes in power.

[0008] In accordance with one aspect of the invention, a protective circuit for an electronic device includes two spike prevention circuits for preventing current spikes from flowing to the electronic device. The first spike prevention circuit prevents spikes above a first predetermined voltage from flowing to the electronic device. The second spike prevention circuit is connected in series after the first spike protection circuit and prevents current spikes above a second predetermined voltage from flowing to the electronic device.

[0009] In accordance with another aspect of the present invention, a voltage protective circuit for a component connected to a multi-drop bus in a vending machine includes an input terminal for inputting a first voltage from the multi-drop bus, an output terminal for outputting the first voltage to the component, and a line connecting the input and output terminals. Two spike prevention circuits are included in the line. The first spike prevention circuit prevents current spikes above a first predetermined voltage from flowing along the line to the component. The second spike prevention circuit is connected in series after the first spike protection circuit and prevents current spikes above a second predetermined voltage from flowing along the line to the component. The component may be a coin changer/validator in the vending machine.

[0010] In accordance with a further aspect of the present invention, a separate, plug-in voltage protector is connected between the DC power source and the multi-drop bus of a vending machine. The voltage protector protects at least one of the components of the vending machine from voltage spikes on the multi-drop bus from the DC power source. The protected component may be a coin changer/validator in the vending machine.

[0011] The attached drawings illustrate preferred embodiments of the invention, in which:

[0012] FIG. 1 is a block diagram of a vending machine; and

[0013] FIG. 2 is a schematic diagram of a voltage protection circuit in the vending machine of FIG. 1.

[0014] Referring to the drawings, and initially to FIG. 1 thereof, there is shown a vending machine 10 connected to an external source of AC power 12. AC power from the source 12 is supplied to an AC to DC power supply 14 that converts the AC power to DC power, and supplies the DC power through a multi-drop bus (MDB) 16 to a machine controller 18. DC power from the machine controller 18 is supplied via the multi-drop bus 16 to a voltage protector unit 20 and thence, to a coin changer/validator 22 and other units or components 22′, 22″.

[0015] In the illustrated embodiment, the components 22, 22′, 22″ of the vending machine 10 are connected to the bus 16 at a portion denoted by reference numeral 16′, and only one voltage protector unit 20 is included in the vending machine 10. In an alternate embodiment, each component 22,22′, 22″ can have a corresponding voltage protector unit 20, 20′, 20″ associated therewith. In such an instance, the voltage protector units 20, 20′, 20″ are connected to the bus 16 at a portion denoted by reference numeral 16″ in lieu of the portion 16′. It is to be appreciated that the voltage protector units 20, 20′, 20″ are separate from the other components 22,22′, 22″ of the vending machine 10, and can be added to any vending machine 10 with a multi-drop bus 16.

[0016] Turning now to FIG. 2, a voltage protector circuit 24 utilized in the voltage protector unit 20 is connected to the multi-drop bus 16 via a mating pair of standard bus connectors 26a, 26b, 28a, 28b. DC power is supplied on pins or lines 1 and 2 of the multi-drop bus 16. Pin or line 3 is a no connect line, while lines 4, 5 and 6 carry communications data. The standard bus connectors 26a, 26b, 28a, 28ballow the voltage protector circuit 24 to be a modular add-on in the vending machine 10, so that the components, such as the coin changer/validator 22, connected to the machine controller 18, can be easily plugged into the multi-drop bus 16.

[0017] DC power from the multi-drop bus 16 on line 1 is supplied to a first stage or shunt suppressor circuit 30 of the voltage protector circuit 24 and comprises a metal oxide varistor (MOV) transient suppressor 32 connected between line 1 and line 2 of the multi-drop bus 16. The metal oxide varistor 32 was, in a particular embodiment, selected to shunt to the power common or ground voltages that are greater than 60 volts. The metal oxide varistor 32 is normally not in conduction, but when a large voltage spike occurs on line 1, the metal oxide varistor 32 becomes conductive and shunts the spike to a power common or ground 34 on line 2.

[0018] A second stage or series regulator circuit 36 of the voltage protector circuit 24 follows the first stage 30 and comprises a resistor 38 coupled to the metal oxide varistor 32 and a base 40 of an NPN transistor 42. An emitter 44 of the transistor 42 is connected to the output of line 1. A transient suppressor Zener diode (TVS) 46 is connected in series with the resistor 38 to the base 40 of the transistor 42, and a power common or ground 48 on line 2.

[0019] The collector-emitter breakdown voltage of the transistor 42 has been selected to be greater than the normal clamping voltage of the metal oxide varistor 32. In one embodiment, the collector-emitter breakdown voltage of the transistor 42 was chosen to be 100 volts. The choice of a high breakdown voltage for the transistor 42 prevents input spikes on line 1 from breaking down the transistor 42 and flowing to the coin changer/validator 22 connected to the multi-drop bus 16.

[0020] It is to be appreciated that the beta of the transistor 42 amplifies the power for the multi-drop bus 16 when the power passes through the resistor 38 to the base 40 of the transistor 42. This is equivalent to the power on line 1 passing through a series resistor equal to the resistive value of the resistor 38 divided by the beta of the transistor 42.

[0021] The breakdown level of the transient suppressor Zener diode 46 is a preselected value, and in one embodiment, was chosen to be 38 volts, high enough to preclude the output power from the transistor 42 from rising above the maximum acceptable standard for the multi-drop bus 16. The value of the resistor 38 is determined from the difference between the breakdown voltages of the metal oxide varistor 32 and the transient suppressor Zener diode 46, and hence, the power handling capacity of the resistor 38.

[0022] In operation, voltage from line 1 of the multi-drop bus 16 is supplied to the metal oxide varistor transient suppressor 32. When a spike appears on line 1, and the voltage is above the shunt voltage of the metal oxide varistor transient suppressor 32, the metal oxide varistor transient suppressor 32 shunts the excess to the power common or ground 34. The first stage circuit 30 thus stresses a spike on input line 1 into the voltage limit of the metal oxide varistor transient suppressor 32.

[0023] However, the metal oxide varistor transient suppressor 32 may not remove all excessively high voltages from line 1, and a spike may still be on line 1. In this instance, the voltage of the spike is transmitted through transistor 42 until the voltage reaches the shunt voltage of the transient suppressor Zener diode 46. The transient suppressor Zener diode 46 then shunts the excess voltage to the power common or ground 48, thereby holding the voltage supplied to the base 40 of the transistor 42 to a constant level. Accordingly, the voltage appearing on line 1 at the emitter 44 of the transistor 42 remains constant, thereby eliminating the spike.

[0024] It will be appreciated that the second stage circuit 36 absorbs excess voltage on line 1 and converts it to heat. The second stage circuit 36 provides no additional stress to the voltage on line 1 when it is functioning.

[0025] It will be further appreciated that the second stage circuit 36 has two limits to its ability to protect. First, the transistor 42 can break down. Second, the heat from the second stage circuit 36, if not eliminated, can melt the transient suppressor Zener diode. So the second stage circuit 36 gently protects for small to moderate overvoltages, while the first stage circuit 30 protects the second stage circuit 36, the coin changer/validator 22 and the components 22, 22′, 22″ from higher voltages at the expense of the source.

[0026] It will be still further appreciated that the voltage protection circuit 24 of the present invention protects a component of the vending machine 10 such as the coin changer/validator 22 connected to the multi-drop bus 16 from large rapid transients and smaller, sustained over-voltage conditions. When the second stage circuit 36 is not eliminating a power spike, the second stage circuit 36 produces only a small voltage drop through the transistor 42.

[0027] The level for the shunt voltage for the first stage 30 is selected to be much higher than expected for a most spikes, and much higher than for a circuit that merely loads and stresses the DC power from the multi-drop bus 16. It is also selected to be higher than the shunt voltage for the second stage 36. As noted hereinbefore, in one exemplary embodiment, the shunt voltage for the first stage 30 was 60 volts, while the shunt voltage for the second stage was 38 volts. This is advantageous, as it permits the second stage 36 to protect at a lower voltage level than would otherwise be required. In addition, the second stage 36 does not add load to the supply while it protects against undesirable spikes.

[0028] It will be further appreciated that the second stage 36 of the voltage protection circuit 24 could be replaced by a suitable series regulator chip or circuit, and the invention would function as hereindescribed. In addition, the transistor 42 could be replaced by a Field Effect Transistor (FET) or other linear pass element.

[0029] For purposes of exemplification, particular embodiments of the invention have been shown and described according to the best present understanding thereof. However, it will be apparent that various changes and modifications in the arrangement and construction of the parts thereof may be made without departing from the spirit and scope of the invention as defined in the appended claims.