GB2417788A

GB2417788A - Emulation circuit for a low energy lamp

Info

Publication number: GB2417788A
Application number: GB0419696A
Authority: GB
Inventors: David Vernon Housman
Original assignee: Lights and Signals Ltd
Current assignee: Lights and Signals Ltd
Priority date: 2004-09-06
Filing date: 2004-09-06
Publication date: 2006-03-08
Also published as: GB0419696D0

Abstract

An emulation circuit is disclosed for connection to a low energy lamp energised by an AC supply to emulate the current that would be drawn by a filament bulb. The circuit comprises means for sensing correct operation of the lamp, and means operative when the lamp is sensed to be operating correctly to activate an auxiliary load intermittently to draw an additional current through an auxiliary load from the AC supply only between preset phase angles of selected power supply cycles of the AC supply.

Description

24 1 7788 - 1

EMULATION CIRCUIT

Field of the invention

s The present invention relates to an emulation circuit for connection to a load that draws current from an alternating current (AC) supply.

Background of the invention

Traffic signals are conventionally designed to operate with filament bulbs. As compared with light emitting diodes (Low energy lamps), cold cathode fluorescent lamps and RF energised fluorescent lamps, such bulbs draw more current for the same light output and for reasons of energy saving there is considerable interest in replacing existing light bulbs with such low energy lamps. Another reason for converting to such lamps is that incandescent light bulbs have a much shorter life.

Because filament bulbs are not durable, and because failure of a red light at traffic signals can have very serious consequences, the control circuitry used to operate traffic signals conventionally monitors the current drawn by the filament bulbs to ensure that the lights are functioning correctly.

Because of this monitoring of the current drawn by the filament bulbs, their replacement with low energy lamps presents a problem because the current drawn by the Low energy lamps IS not sufficient to indicate to the control circuitry that the lights are functioning correctly.

A possible solution to this problem would be to modify the control circuitry. While it would be possible to redesign the circuitry of new traffic signals to allow for the fact that low energy lamps are being used in place of filament bulbs, it is not economically and commercially viable to modify existing traffic light installations in this manner.

It has therefore previously been proposed to connect a resistance in parallel with the low energy lamps Lo Increase the total current consumption to a level comparable with that of a filament bulb but this clearly is not a sensible solution as it defeats one of the main objects of changing lo to low energy lamps, namely to improve energy efficiency.

Summarv of the invention

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According to the present invention, there is provided an emulation circuit for connection to a low energy lamp energized by an AC supply, which comprises means for sensing correct operation of the lamp, and means operative when the lamp is sensed to be operating correctly to activate an auxiliary load intermittently to draw an additional current through an auxiliary load from the AC supply only between preset phase angles of selected power supply cycles of the AC supply.

In the preferred embodiment of the invention, the low energy lamp is an LED light source connected in place of a filament bulb in traffic signals.

The means for sensing correct operation of the lamp may monitor the current drawn by the lamp in the case of an LED or a cold cathode fluorescent. In the case of an RF energized fluorescent, the RF energy may be monitored.

However, because the current and the RF energy may not be a reliable indication of light output, it is preferred to provide means to monitor the intensity of the light emitted by the lamp and to indicate correct operation only when the light intensity exceeds a preset threshold.

The invention takes advantage of the fact that in conventional traffic signals, the operation of a light is not monitored continuously but by sampling the current drawn by the light at preset phase angles of the AC supply cycle.

Because of this, it is not necessary for the auxiliary load to draw a high current continuously, as was previously carried out in the prior art, and it suffices to increase the current drawn from the AC supply by activating the auxiliary load only during the sampling intervals of the lo monitoring circuit.

By connecting the circuit of the invention to a low energy lamp, one can create a light unit that emulates the current drawn by a filament bulb at the sampling instants that it is monitored by the monitoring circuit, thereby allowing the light unit to be substituted for a filament bulb without making any other modifications to the traffic signals installation.

Because the sampling phase angle may vary from one traffic signals installation to another, it is preferable for the emulation circuit to have means for enabling adjustment of the phase angles between which the auxiliary load is activated. Preferably, both the phase angle at which the auxiliary load is switched on and the phase angle at which the auxiliary load is switched off are adjustable independently to enable both the timing and the duration of activation window of the auxiliary load to be adjusted to optimise emulation and minimize the current consumption of the auxiliary load.

Brief description of the drawing

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The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: 4 - Figure 1 is a circuit diagram of a first embodiment of the invention, and Figure 2 is a circuit diagram of a second embodiment of the invention.

Detailed description of the drawing

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The circuits in the drawings are intended to replace a conventional filament bulb in traffic signals. The circuit of Figure 1 has three connectors, a first to receive the AC supply that would normally be connected to the filament bulb, a second leading to the low energy lamps that are used to replace the filament bulb and a third for connection to an auxiliary external load. If desired, the auxiliary load may be built into the circuit to avoid the need for the third connector.

The low energy lamps are permanently connected to the AC supply by way of the primary of a transformer L1, so that for as long as the AC supply is ON, the low energy lamps will be illuminated. The transformer L1 acts as a current sensor to determine when current is being drawn from the AC supply by the low energy lamps.

The AC power is applied by way of a fuse F1 to a full wave rectifier comprising an optional transient suppressor R1 and a diode bridge D1. The fluctuating DC output voltage of the diode bridge D1, which is developed across a resistor R8, is applied to the auxiliary load. The auxiliary load is switched on and off by means of an electronic switch comprising a MOSFET transistor Q1, a capacitor C1 and resistors R2 and R3. The switch is controlled by means of a signal received over a line GP5 from a pulse generator, which is shown as being formed by a suitably programmed PIC12F675 micro-controller. - 5

A 5V DC voltage is produced by a low voltage supply circuit comprised of a diode D3, resistors R7 and R9, a zener diode D4 and two capacitors C2 and C3. This 5V signal is used internally to power the various integrated circuits to be described, including the pulse generator.

The pulse generator receives an input signal over a line GP3 from a phase detection circuit which comprises three resistors R4, R5 and R6 connected as a voltage divider between the output of the bridge rectifier D1 and ground and a diode D2 connected between the 5V line and the junction of resistors R5 and R6, which is further connected via the signal line GP3 to the pulse generator. The purpose of the diode D2 is to limit the voltage at the CP3 input terminal of the micro-controller for its protection.

The pulse generator receives a further, analogue, input signal over a signal line GP4 which indicates the magnitude of the current flowing through the secondary of the transformer L1, which is proportional to the current flowing through the low energy lamps. The signal on the line GP4 is produced by a first operational amplifier OAT, having a feedback resistor R18 and having its inputs connected by way of resistors R15 and R16 to the secondary winding of the transformer L1. The non-inverting input of the first operational amplifier OA1 receives a reference signal by way of a resistor R17 from the output of a second operational amplifier OA2. The non-inverting input of the operational amplifier is connected to a voltage divider comprising two resistors R13 and R14 connected between the 5V line and ground, wnile its inverting input is connected directly to its output. The two operational amplifier may be formed as parts of the same integrated circuit such as an MCP602.

In operation, after the AC voltage is first applied, a period of inactivity is programmed into the micro processor to allow all the circuit voltages to become predictable - 6 - before operation of the micro processor commences.

Thereafter, the current drawn by the low energy lamps is monitored and is applied as an analogue signal over the line GP4 to the pulse generator.

When the magnitude of the sensed current falls within a preset range that indicates that the low energy lamps are functioning correctly, the pulse generator produces a signal on the line GP5 to activate the auxiliary load during only lO part of selected voltage cycles of the AC supply, corresponding to the sampling times of the traffic signals controller. The phase of the AC supply is indicated to the pulse generator by the signal on line GP3. The output pulse of the generator, which is referenced to the signal on input line GP3, commences and terminates at phase angles that can be adjusted by potentiometers Rll and R12 connected to two further input lines GPO and GP1 of the pulse generator.

As a result, in addition to the steady current drawn from the AC supply by the low energy lamps, current pulses will be drawn by the auxiliary load to simulate a load having a higher rating, but only at such times as the monitoring circuit of the traffic signals samples the total current to determine if the light is functioning correctly.

Ideally, the width and the amplitude of the current pulses should be just sufficient to simulate a higher load during the instants that the current is sampled by the monitoring circuit. The timing of the leading and trailing edges of the output pulses of the pulse generator can be determined empirically by adjustment of the potentiometers R11 and R12 and, once the sampling phase angle has been determined, it can be used to set up all the other low energy light units in the same traffic signals installation.

As described above, the pulse repetition rate will be twice that of the AC supply frequency because of the full wave rectification. If sampling by the monitoring circuit - 7 - is performed only once during each cycle of the AC supply, hen the auxiliary load need only be activated once in every wo cycles of the fluctuating DC output of the rectifier D1.

It has also been assumed above that the monitoring circuit samples during each cycle of the AC supply and while this may be the case for a red traffic light, sampling may be less frequent for green and amber lights. It is therefore possible for the pulse generator to be programmed to produce lo one pulse after a predefined number of cycles the AC supply.

Similarly, if sampling is only carried out for a few seconds after a lamp has been energised, the auxiliary load need only be energised during the same period.

The embodiment shown in Figure 2 is generally similar to that of Figure 1 and most of it will be clear to the person skilled in the art without the need for detailed explanation. For this reason, only the parts that have been modified will now be described.

The use in Figure 2 of a more sensitive transformer L1 dispenses with need to amplify the sensed current. The phase sensing voltage divided in Figure 2 receives the raw mains signal rather than the rectified signal and it is connected to the GP2 rather than the GP3 loput of the micro controller. The 5V DC power supply circuit has the resistor R7 designed as three separate elements mounted in spaced relation on the circuit board to improve cooling. The electronic switch Q1 is the same as previously described.

Of course, the program in the micro controller is different in the embodiment of Figure 2 but the unit may still be a PIC12F675.

The two important modifications made in the embodiment of Figure 2 are first the provision of a programming port in the form of a connector having six terminals connected 8 - respectively to the inputs GPO to GP3 of the micro controller and to the ground and 5V lines. The purpose of the programming port, as its name implies, is to permit connection of a separate unit to program the micro processor, i.e. to install its firmware. The port can also be used by a dedicated unit to read and modify the settings of the phase angles and the selection of the cycles during which the auxiliary load is to be energized. Such a unit can be used to set all the circuits of a traffic signals lo installation once the correct settings have been established empirically for one of the lamps.

A further modification in Figure 2 the provision of a light sensing element which is shown as being a light dependent resistor LDR but could alternatively be a photodiode or phototransistor. Such an element can supply a voltage indicative of the intensity of the light emitted by the monitor lamp to one of the input lines of the micro controller and the latter may be programmed to rely on this signal either instead of or in addition to the signal from the transformer L1 to determine when the auxiliary load is to be energised. As drawn, the light sensitive element is shown as connected to the input line GP4 and is permanently wired as part of the circuit. However, by programming the 2s micro controller to receive this signal on one of the lines GPO to GP3, it is possible to add this functionality by connecting the light sensitive element using the programming port. 9 -

Claims

1. An emulation circuit for connection to a low energy lamp energized by an AC supply, which comprises means for sensing correct operation of the lamp, and means operative when the lamp is sensed to be operating correctly to activate an auxiliary load intermittently to draw an additional current through an auxiliary load from the AC supply only between preset phase angles of selected power lo supply cycles of the AC supply.

2. An emulation circuit as claimed in Claim 1, wherein the low energy lamp is an LED light source connected in place of a filament bulb in traffic signals.

3. An emulation circuit as claimed in Claim l or 2, wherein the means for sensing correct operation of the lamp is operative to monitor the current drawn by the lamp.

4. An emulation circuit as claimed in any preceding claim, wherein the means for sensing correct operation of the lamp is operative to monitor the intensity of the light emitted by the lamp and to indicate correct operation only when the light intensity exceeds a preset threshold.

5. A circuit as claimed in any preceding claim, wherein means for are provided enabling adjustment of the phase angles between which the auxiliary load is activated.

6. A circuit as claimed in claim 5, wherein both the phase angle at which the auxiliary load is switched on and the phase angle at which the auxiliary load is switched off are adjustable to enable both the timing and the duration of activation window of the auxiliary load to be adjusted to optimise emulation and minimise the current consumption of the auxiliary load. 10

7. An emulation circuit constructed and adapted to operate substantially as herein described with reference to and as illustrated in Figure 1 or Figure 2 of the accompanying drawings.