Current Measurement Apparatus

The present invention relates to current detection techniques and particularly relates to a current measurement apparatus and power distribution apparatus utilising such a technique.

Background of the Invention

Power distribixtion systems are known and are particularly suited to distribute power to several loads on a vehicle such as a car. It will be apparent that power distribution systems can be used with any type of vehicle in which power is required to be distributed to electrical loads and/or to provide overload protection.

A power distribution system usually comprises a power distribution unit that controls a variety of loads on the vehicle. Fig. 1 shows a simplified view of a power distribution system in a vehicle.

In this particular example, a power distribution unit 10 has sixteen input channels 11, sixteen output channels 12, and a serial configuration port 13. The unit 10 also has positive and negative power supply terminals that are connected to the vehicle's battery 15. The power supply terminals 14 provide electrical power for the unit's internal circuits, and the positive terminal provides a high current feed that is switched to external loads 16 by means of solid-state semiconductor switches (not shown) within the unit 10.

The unit 10 is configured by means of software downloaded from an external computer 17. The unit's sixteen output channels 12 may be connected to a variety of different electrical loads 16 within the vehicle, such as lights, motors and pumps. Each of the sixteen output channels 12 has a current limit that can he preset to a particular value by the system designer using me coiifiguration software. ^" When the unit is in operation, the current drawn from each active output is constantly monitored by

circuits (not shown) within the unit 10 and compared to the preset value. In the event of a fault that causes the output current to exceed the preset value, the unit 10 turns off the relevant output, thereby providing a fully configurable fuse function that limits the fault current to safe levels.

Up to sixteen input channels 11 may be used to control the power switched to the vehicle's loads. Any number of switches (not shown) may be connected to the input channels 11 according to the requirements of the overall system. It will be appreciated that any type of input device may control the input signal into each respective channel. For example one channel may comprise a switch to control the input whereas another channel may have means for providing a digital input signal into the channel. In simple terms, any input channel can be arranged to control any output channel, and the particular relationship between inputs and outputs is established by means of the configuration software.

Note that although the functions and behaviour described in this example refer mainly to the sixteen-channels they are not limited to such and may be utilised with any number of channels.

In order for the unit 10 to function properly, it is necessary to measure the full current range on each channel with good accuracy and resolution. There currently exist techniques for the measurement of DC current. The techniques typically rely upon one of the following methods:

(i) A 'current sense' resistor (usually low value) is placed in the path of the current to be measured. The 'sense voltage' developed across this resistor is directly proportional to the current and can be measured using any conventional voltage measurement technique.

(ii) The magnetic field developed by the flowing current can be sensed and converted into a voltage. An example of this approach is the Hall Effect

Current Sensor, whereby the magnetic field is used to generate a 'Hall Voltage' proportional to the current.

Of these techniques, the sense resistor approach, is not suitable for the unit 10 due to the high load currents that must be accommodated. The presence of a low-value sense resistor (even a fraction of an ohm) in the current path can lead to significant power dissipation. This is undesirable as it leads to unacceptable heat rise inside the unit 10. For example, a sense resistance as low as lmω will dissipate roughly half a watt at a peak load current of around 2OA. Furthermore, the relatively low sense voltage (around lOOμV) generated by a low current level of around 0.1A would require considerable amplification before it could be measured properly. Although such amplification is feasible, it incurs penalties in terms of cost, electrical noise, circuit complexity and PCB (printed circuit board) space.

The second technique mentioned above has similar disadvantages in that a Hall sensor device must be located in, or near, the load current path. Again, this introduces cost and board space implications, and has complications associated with the circuitry required to process the Hall voltage. Furthermore, as with the first technique, there is a significant power dissipation which leads to heat rise within the unit 10.

It should be remembered for a sixteen channel unit that the disadvantages outlined above are effectively multiplied by sixteen given the need to measure the load current on every channel.

Accordingly, it is an object of the present invention to overcome the problems associated with the above mentioned techniques and to provide an alternative technique for current detection which accurately measures the load current over a wide range whilst producing minimal power dissipation.

Summary of the Invention

A first aspect of the present invention provides a current measurement apparatus comprising: an output channel for outputting current to a load; a first switching means arranged to operate in a first current range; a second switching means arranged to operate in a second current range different to the first current range; a processing means for measuring a first value for the current delivered to the load; and a control means for controlling the activation and the deactivation of the first and second switching means, wherein if the first current value measured by the processing means is within the second current range, the control means is arranged to cause the second switching means to he activated and the first switching means to be deactivated and to cause the processing means to measure a second value for the current. Preferably, the switching of the first and second switching means is performed mutually exclusively from each other as the second switching means is connected electrically in parallel to the first switching means.

Furthermore, there may be a small overlap time during which both the first and second switching means are activated so that one of the devices is allowed to be switched on fully before the other is deactivated.

Preferably, the first range is typically 5A - 25A and the second range is below the first range i.e. typically less than 5A. Different ranges may be appropriate to different embodiments. The switching means may be high side drivers which are chosen such that they operate efficiently within the appropriate ranges. The control means is preferably arranged to cause the first switching means to be deactivated after a time delay, typically between Vz and lms.

A second aspect of the present invention provides a detection apparatus for detecting the presence of a load connected to a power distribution unit, the apparatus comprising; current control means for controlling the current delivered to an output of the apparatus;

voltage measurement means for measuring a value of the voltage between the output and a processing means, wherein the processing means is arranged to receive the value of the voltage from the voltage measurement means, and determine whether or not a load is connected to the output depending on the value of the voltage.

Preferably, if the processing means receives a voltage value that is greater than a predetermined value which is stored in advance by the processing means, it determines that a load is not connected to the output. If the value is less than or equal to the predetermined value, it determines the load is connected to the output.

A third aspect of the present invention provides a power distribution system for a vehicle comprising a current measurement apparatus of the first aspect and a detection apparatus of the second aspect.

Brief Description of the Drawings

Ia order that the present invention be more readily understood, embodiments thereof will be described by way of example with reference to the accompanying drawings in which:

Fig. 1 shows a schematic diagram of a vehicle electrical system.

Fig. 2 shows a high side driver utilised in the power distribution unit of Fig. 1.

Fig. 3 shows a circuit of a preferred embodiment of the invention. Fig. 4 shows a timing diagram of the circuit in Fig. 3.

Fig. 5 shows a circuit diagram of a load detection technique used with the circuit of

Fig. 3.

Fig. 6 shows a modification to the circuit shown in Fig. 5.

Fig. 7 shows a current limit time delay function that may be incorporated into the unit.

Fig. 8 shows a single-wire indication technique that may be incorporated into the unit.

Detailed Description

The preferred embodiment of the present invention is implemented in a system as shown in Fig. 1. The reference numerals of the features already described in the . background will be used to identify the features in the following description and function of the features already mentioned will be omitted to avoid duplication.

The unit 10 employs special-purpose semiconductor devices usually called 'high-side drivers' or high current drivers to control the load current on each channel. Also Imown as 'solid state relays', 'smart power switches' and 'high-side power switches', these devices incorporate a power MOSFET device together with protection and diagnostic functions integrated into a single package.

In addition to power switching and protection functions, several high-side drivers also provide a 'sense current' function, whereby the device outputs a low-level current that is directly proportional to the load current conducted by the internal power MOSFET. This function is utilised by the unit 10 to measure the load current on each channel.

The high-side drivers can be classified in terms of their 'on' resistance, R _0N , and their maximum current rating, l _L < _m a _x > The R _0N rating relates to the maximum resistance introduced into the current path by the power MOSFET when turned fully 'on'. The ILC _πMKC) rating determines the maximum current that can safely be handled by the device.

The unit 10 comprises at least one high-side driver in each channel for switching the load currents.

A simplified representation of the basic arrangement of the unit 10 is shown in Fig.2 which illustrates the high-side driver for one channel only (all other channels are identical).

The unit 10 derives its power from the vehicle's battery 15 (nominally +12V ₅ although other voltages could be accommodated), A microcontroller 10a is responsible for turning high-side driver 10b on and off in response to a signal from the appropriate input channel (not shown). The high-side driver 10b is so-called because it is connected to the positive ('high') side of the battery supply (+12V) and switches power. to a 'low-side' load connected to the negative GND potential (the vehicle's chassis).

Essentially, the high-side driver 10b has four terminals: IN (a low-level digital input that switches the power MOSFET on and off); VBB (the positive supply connection); OUT (the output terminal that sources current to the load); and IS (the sense current output). The simplified view of the high-side driver 10b shows only the power, or 'load' MOSFET ₅ Q _L , and the 'sense' MOSFET ₅ Q _s : all other internal functions have been omitted for clarity.

When the driver 10b is driven on by the microcontroller, the common gate potential, V _G , is driven to a high level that turns on both Q _L and Q _s and the device switches load current, I _L , to the external load. Due to the 'mirror' connection of Q _L and Qs ₅ a sense current, Is, flows out of the IS terminal. The ratio of sense current to load current depends on the relative size of the sense MOSFET, Q ₈ , and the load MOSFET, Q _L . Since Q _s is much smaller than Q _L , the ratio Ii/I _s , denoted K, is very large.

The low-level sense current is fed to a sense resistor, R _s , which, converts I _s to a sense voltage, V ₈ , that is directly proportional to the load current. The sense voltage is fed to the microcontroller's internal analog-to-digital converter (ADC) 10ai which converts the analog quantity V _s to a digital equivalent.

A preferred embodiment of the present invention will now be described with reference to Fig. 3.

The measurement technique according to the preferred embodiment employs two high-side drivers 10b, 10c physically connected in parallel. Fig.3, illustrates the concept for a single channel - all other channels are essentially identical.

Here, the term 'parallel' applies to the devices' internal power MOSFETs (not shown) which are effectively connected in parallel by virtue of the common OUT connection. A first high side driver 10b is chosen such that it operates for high current applications (i.e.> 5A) where its sense function behaves well. At current levels below this, it is not possible to guarantee accurate measurement. A second high side driver 10c is used for low or medium current applications and is chosen as it exhibits a far superior current sense function at low load current levels (i.e. < 5A). The second high side driver 10c has a relatively high 'on' resistance that can be as much as an order of magnitude greater than the 'on' resistance of the first high side driver 10b and is therefore not suited for use with high load currents as it would result in excessive heat rise within unit 10.

The high-current driver 10b is selected for its ability to handle relatively large load currents. For example, a device such as the BTS555 can handle load currents up to 165A or above, and has very low R _ON (typically 2.5mω); therefore, at a load current of, say, 2OA the BTS555 would typically dissipate just IW. The low R _0N rating of the high-current driver 10b is essential in ensuring minimal power dissipation within the unit 10 when conducting maximum load currents.

The low-current driver 10c is selected primarily for its ability to sense relatively low load currents with a good degree of accuracy. For example, a device such as the

VND600SP provides a current sense function that is fully characterised down to about

0.5A, and samples of the device have been found to exhibit very good current mirror performance at load currents as low as 0.1A. However, the low-current driver 10c usually has a relatively high 'on' resistance that can be as much as an order of magnitude greater than the On' resistance of the first high side driver 10b and therefore cannot be used to replace the high side driver 10b directly since the

associated high power dissipation at high load currents would result in excessive heat rise within the unit.

In this scheme, each device can be turned on and off individually by separate signals derived from the microcontroller 10a. Similarly, the sense currents output by each device are connected to individual sense resistors R _SH and R _SL - Thus, the sense current I _SH derived from the high-current driver 10b is fed to sense resistor R _SH and develops a sense voltage V _S H ₅ whereas the sense current I _SL derived from the low-current driver 10c is fed to sense resistor R _SL and develops a sense voltage V _SL .

A timing diagram shown in Fig.4 illustrates how the parallel technique works. Initially, at point T, the high-current driver 10b is the only device turned on and is therefore conducting the full load current for that channel. The microcontroller 10a makes an analog-to-digital conversion on the sense voltage, V _SH , to determine the magnitude of the load current. If the result indicates a value above 5 A (the "threshold level"), the reading is taken as valid and no further action is necessary. However, if the result indicates a value below the threshold level, the microcontroller 10a invokes the parallel measurement technique. Thus, at point '2', the microcontroller 10a turns on the low-current driver 10c and, after a short delay, turns off the high-current driver 10b. The brief overlap where both devices are simultaneously on ensures there is no interruption to the load current.

At point '3', the low-level sense voltage V _SL has settled and is measured by the microcontroller's ADC 1Oa ₁ . At point '4', when the measurement is complete, the result is an accurate value for the sub-5A load current. The microcontroller 10a now turns on the high-current driver 10b and, after a short delay, turns off the low-current driver 10c. Again, the brief overlap ensures no interruption to the load current. Since the low-current driver 10c is only turned on for a fraction of a second (typically lms) the power dissipated by its relatively high on resistance is negligible. Furthermore, as it is only necessary to switch on the low-current driver 10c at current levels below 5A, the device 10c is never required to conduct the maximum load current (25A, or more)

so its inferior current handling (compared to the high-current driver 10b) is not an issue.

With the above configuration, the low-current driver 10c does not need to be continuously activated and as such is only used as a supplementary device which is activated briefly. The device 10b is the primary device which is used mostly and is only deactivated at low current levels. The maximum current level to be measured by device 10b is about 25 A in this embodiment but this can vary and is only limited by the other factors used in the unit. In particular, the capability of the wiring used in the unit 10 will have an effect on the maximum current level.

In a modification to the basic arrangement shown in Fig. 3, the unit may be provided with a calibration means (not shown) to vary the threshold level by determining the K value of each high side driver devices and then calibrating and setting the threshold level accordingly depending on the characteristics of the particular devices.

In a further embodiment, the unit 10 is provided with a load detection technique which may be used in addition to the current measurement technique. In particular, the load detection technique described hereinafter may be utilised initially to determine whether a load is present before the current measurement technique described hereinbefore is commenced. Alternatively, the load detection technique may be utilised independently of the current measurement technique..

The load detection technique will be described with reference to Fig. 5 which shows a simplified circuit diagram of the implementation of the technique.

The open load detection function enables the unit 20 to detect the absence of a load 21 on one or more output channels caused by a fault (such as a broken cable) before applying power to the load. In other words, the function allows the unit 20 to detect an open load wHle the high-side driver is switched off. This is a useful diagnostic function in that it allows the integrity of the wiring system to be checked before applying power to the load 21. It will be appreciated that each channel of the unit 20 may be provided with the detection function.

In order to detect an open load condition, it is necessary to source a relatively small ^' current to the load 21 in order to detect its presence. The simplest way to achieve this is to use a 'pull-up' resistor (R _PU ) connected between the +12V supply rail and the channel's output terminal. The detection function is invoked while the High-Side Driver 20b is switched off.

If the load 21 is present, the output voltage (the voltage appearing at the 'OUT' terminal of the High-Side Driver 20b) will be 'pulled down' by the relatively low impedance of the load 21. On the other hand, if the load 21 is missing, the output voltage will be 'pulled up' toward the +12V rail by the pull-up resistor R _PU . The low or high voltage is detected by a simple voltage comparator circuit connected between the output terminal and the microcontroller 20a.

Assuming that the voltage comparator 2Od has relatively high input impedance (which will normally be the case), practically all of the pull-up current (I _PU ) sourced via the pull-up resistor R _PU will flow into the load 21 (if present). Ideally, the value of I _PU should be very small (on the order of microamperes) such that its presence will have no detrimental effect on Hie load 21 when the High-Side Driver 20b is in the 'off state. For example, selecting a value of 1Mω for R _PU would produce a nominal value of Ipu = 12μA, which would be so small as to have negligible effect on the load 21.

However, such, a small value of I _P u could result in the detection function being adversely affected by leakage currents (caused, for example, by dampness or dirty connections) which might cause the unit 20 into believing that the load 21 is present when actually it is not. Therefore, although relatively very low, the current sourced to the load 21 (when present) by the pull-up resistor R _PU should be large enough to ensure that the detection function is not affected by leakage currents. Selecting a larger value of I _P u has an additional advantage in that the detection function is not adversely affected by excessive electrical noise that could be present at the output terminal under certain, conditions. Ideally, the value of I _PU should be in the region of several hundred, or possibly several thousand, microamperes.

This could be achieved simply by selecting a smaller value for the pull-up resistor R _PU which would result in a larger value of I _PU . However, this approach has a significant disadvantage in that it results in an unacceptably large 'off state' current that flows into the load 21 when the channel's High-Side Driver 20b is switched off. Not only could a large 'off state' current have an adverse effect on the load 21 (if present), it would also lead to unacceptable current drain on the vehicle's power source (not shown). This latter problem is particularly detrimental when the vehicle's engine is not running, since it could result in a continuous drain on the vehicle's battery (this problem would effectively be 'multiplied' by sixteen for a 16-channel unit).

To circumvent these disadvantages, the unit 20 employs a 'switched' pull-up scheme. The basic elements are shown in Fig. 6, where an electronic switch 20e activated by the microcontroller 20a is used to connect the pull-up resistor R _P u to the positive +12V rail (although any other positive rail, e.g., the +5V rail, could be used instead).

It will be appreciated that although Fig. 6 shows a pull-up resistor R _PU being used to set the pull-up current, the technique is not restricted to a resistor, and the pull-up current could be set equally well using an electronic constant-current source.

Under normal working conditions, the electronic switch 2Oe is open, such that the pull-up resistor Rpu plays no part in the circuit and has no effect on the channel's output. However, when it is necessary to check for an open load (before turning on the high-side driver 20b), the microcontroller 20a closes the electronic switch 2Oe. The ^c pull-iιρ' current, I _PU , now flows through the pull-up resistor R _PU towards the channel output. If the load 21 is present, the low voltage at the output is detected by the voltage comparator 2Od which in turn signals this condition to the microcontroller 20a. The value of I _PU can be made fairly large to avoid the leakage and noise problems mentioned above. However, since the electronic switch 2Oe is normally open, Ipu has no effect on the load 21 when the high-side driver 20b is off. Furthermore, the drain on the vehicle's battery is reduced enormously (compared to the simple scheme shown in Fig.5) since the pull-up current flows only very briefly whenever the open load detection function is activated.

It will be apparent from the above that the open load detection function sources such a relatively small current to the load 21 that it does not energise the load 21. In other words, any loads 21 present when the function is invoked would not be activated and would remain 'dormant'.

Other features which may be incorporated into the unit 10 will now be described.

Sensor Monitoring Function

The unit 10 may feature a sensor monitoring function that allows the unit to respond in a configurable manner to the output(s) of one or more internal or external sensors. In this way, the unit can change the state of one or more outputs when the sensor(s) output passes a pre-set threshold.

As an example, the unit can activate or deactivate one or more loads in response to Hie output of an internal temperature sensor. The threshold at which the load(s) is affected

(in this example, the internal temperature level) is fully configurable by means of the

configuration, software mentioned previously. For instance, the unit could be configured to turn, on a cooling fan connected to output channel five when the internal temperature exceeded +5O ⁰ C.

The unit is not restricted to monitoring temperature sensors. Other sensors such as accelerometers, light sensors, pressure sensors, etc., may be accommodated. These devices may be located internally within the unit, or externally via means of a suitable interconnecting bus.

Data Streaming

In addition to functioning as an intelligent power control unit, the unit has the facility to 'stream' data via the communication port (or some other suitable port) to an external monitoring or logging device. In this way, operating parameters such as load current, battery voltage, temperature, load power, and vehicle acceleration can be monitored and recorded in real time. The provision of this function incorporated into a power control unit provides advantageous data management functionality to the unit.

Selectable Configuration Function

As mentioned previously in section (ii), the relationship between input devices and output loads, and the current or power level at which a load is shut down, are fully configurable parameters which may be set and changed by the system engineer using the configuration program. Any number of different configurations may be set up by the user and downloaded to the unit. Usually, the unit will operate with a single configuration. However, there may be instances where it is necessary or desirable to switch quickly and simply between different configurations in order to alter the unit's behaviour to suit different applications. To achieve this, several different configurations may be stored within the unit's memory. The user may then switch between these different configurations either by applying appropriate signals to one or more input channels, or by means of a dedicated configuration selector switch.

Configuration Protection

Once the unit's behaviour and functionality have been configured by means of the configuration program, there may be certain instances where it is necessary or desirable to lock' the configuration such that it cannot be altered, copied, or tampered with by unauthorised personnel. In this respect, the unit provides the facility to protect the configuration against unauthorised access by means of a password. Thus, once a password-protected configuration has been created, it is effectively 'locked' and cannot be changed or revised in any way until it is 'unlocked' by means of the password.

The unit can also provide different 'levels' of access, each of which is protected by a unique password. For example, a top level 'administrator' password which is known only to a small number of users would allow access to all elements of the configuration (this represents the highest level of security). On the other hand, a lower level 'general' password would allow access to relatively few elements of the configuration by a larger number of people (the lowest level of security).

Supply Voltage Monitoring

The unit features internal circuitry that constantly measures the vehicle's battery voltage connected to its supply terminals. By means of the configuration program, the user may set one or more outputs to switch on or off in response to changes in the battery voltage level. For example, the unit could be configured to switch off a sensitive load connected to a given output whenever the battery voltage exceeded a pre-set threshold.

Output Logic and Timing Functions

The unit features a range of logic and timing functions that may be used to configure the behaviour of one or more outputs.

The logic functions may be used to establish a degree of 'interaction' between two or more outputs. For instance, the unit could be configured to turn off a load connected to channel 10 output whenever channel 6 output was turned on. Similarly, the unit could be programmed to activate channel 2 output whenever channel 7 output or channel 9 output was turned off.

Closely allied to the general logic functions is a 'fault logic' function. This function allows the presence of a fault condition on one or more outputs to affect the state of one or more other outputs. Thus, for example, an overload condition detected on channel 15 output could be configured to turn on channel 4 output and disable channel 9 output. Similarly, an abnormally low current level can also be considered a fault ■condition and used to affect one or more other outputs. For instance, a low current level detected on channel 7 output could be used to switch on channel 3 output, and so on.

The output timing functions allow the user to configure an output to turn on and off repeatedly at a predetermined rate ('pulsed' operation), or to remain active only for a certain length of time ('one-shot' operation). All of the timing parameters are fully variable, such that the user may select any desired frequency and duty cycle for pulsed operation, and any suitable 'on' time for one-shot operation. As an example of these timing functions, the user could arrange for all of the vehicle's indicator lights to flash on and off at a predetermined rate whenever a 'hazard' switch was activated. Similarly, the unit could be configured to activate a cooling fan only for a certain length of time, such that the fan would turn off without requiring to be switched off by the vehicle ' s driver.

Current Limit ^c Auto-Set' Feature

As mentioned earlier, a primary function of the unit is to provide overcurrent protection for every output channel. This allows the user to select the threshold (i.e., the current limit) at which a given output shuts down its load(s) in the event of a fault. For example, the user could configure channel 7 output to switch off if the current taken by the load exceeded 7.5A.

However, in certain cases, it may be more convenient to allow the unit to select, automatically, an appropriate current limit for a given channel based upon load data supplied by the user. Ia this way, the user would input certain characteristics about the load (such as type of load, power rating, wire specifications, etc.) via the configuration program and the unit would then 'auto set' the corresponding current limit to an appropriate level. For example, if the user specified that the load connected to channel 2 was a 6OW bulb, the unit could auto-set the corresponding current limit to 5 A. Note that the auto-set function may also be used in conjunction with the variable current limit delay function (see below) in order to select, automatically, a suitable delay period for a given load.

Variable Current Limit Delay Function

In certain instances, it may be desirable to delay activation of the current limit function for certain types of load. This may be achieved by means of the Current Limit Delay function, which allows the user to set a delay period, via the configuration program, for a particular output or outputs. This feature is mainly intended (but not restricted to) loads such as incandescent lamps and motors which exhibit a phenomenon usually termed 'inrush current' .

When such a load is first activated ('turn on' see figure 7), the current taken by the load instantaneously rises to a peak value (I _P ) and then gradually settles to a steady- state value (Is). Since it is common for I _P to be five or ten times the value of I _s , it is

necessary to provide a delay period (t _D ) during which, the current limit function is disabled, otherwise the current limit would be activated by the relatively large 'inrush' current. The user can select any suitable value of t _D , typically ranging from around 100ms to several seconds, thereby ensuring that the current limit is not falsely triggered by the inrush current.

If required, the unit may be configured to ^c auto set' a suitable delay period based upon load data provided by the user (see figure 7)

Interconnected Units

The unit 10 is essentially a 'stand-alone' unit which can function as an autonomous load control element in a power distribution system. However, it is possible to interconnect two or more units to form part of a larger system in which the units interact with each other (i.e., share commands, data and other information) via a suitable bus.

In this way, each unit continues to function as an intelligent power control node within the overall system, whilst at the same time generating and responding to data and stimuli to/from other units on the bus. As well as allowing a much larger number of loads to be controlled, the advantages of such an interconnected system are that units can interact with each other using the logic functions described above, and can also share common data generated either by the units themselves, or by sensors and other devices that have access to the bus. The interconnected system can also provide other benefits, such as redundancy protection.

Single-Wire Indicator

It is often necessary to provide visual indication about the state of a load, either to confirm that the load is switched on, or to signal a fault condition. Typically, a Light

Emitting Diode (LED), usually mounted in the coclφit of Hie racing car or other

vehicle, is the preferred way of providing the visual indication. However, in conventional systems, it is necessary to provide extra wiring to illuminate the LED(s), and this has obvious disadvantages in terms of cost, weight and system complexity.

The unit offers a way of illuminating the indicator LEDs without requiring extra wiring. A simplified view of the general scheme is shown in the example in Fig. 8, where input channels 1 and 2 are activated by momentary-action, normally-open switches connected to the high-side and low-side, respectively.

It can be seen that an LED is connected in parallel with each switch. By means of circuitry within the unit, it is possible to use the single input wire both to sense the switch position (open or closed) and also to illuminate the associated LED.

The LED itself may be mounted inside the switch housing (using a transparent cover), ^• or could be located next to the switch. In this way, the scheme lends itself to commercially-available illuminated switches, or to conventional (non-illuminated) switches with a separate, shunt-connected LED.

By using Pulse Width Modulation (PWM), or a similar technique, the circuitry that contols the LEDs can be made to vary their brightness so as to cater for different levels of ambient lighting. The user sets the desired LED brightness using the configuration program.