GB2478774A

GB2478774A - Driving an LED with current limiting device and capacitance

Info

Publication number: GB2478774A
Application number: GB1004532A
Authority: GB
Inventors: Mark Jordan
Original assignee: Oxley Developments Co Ltd
Current assignee: Oxley Developments Co Ltd
Priority date: 2010-03-18
Filing date: 2010-03-18
Publication date: 2011-09-21
Also published as: GB201004532D0

Abstract

An LED 10 is connected between a pair of supply terminals via a current limiting device 12 which can simply take the form of a resistor. A capacitance 16 is connected in parallel with the LED. A pulsed voltage supply is applied to the supply terminals so that at the beginning of a voltage pulse illumination of the LED is delayed as the capacitance charges. The duration of the voltage pulses may be modulated to vary the output intensity of the LED, The capacitor may be connected to a discharge resistor 14 through which the capacitor discharges between voltage pulses.

Description

ELECTRIC CIRCUIT FOR AN LED LAMP

DESCRIPTION

The present invention is concerned with control of light emitting diodes.

The abbreviation "LED" will be used throughout to refer to a light emitting diode.

It has become a common practice in recent years to replace incandescent bulbs (filament-type bulbs) with LED lamps, which offer advantages inter alia in longevity and energy efficiency. LED lamps are commonly made with casings and terminals which mimic those of incandescent bulbs, and are adapted to be driven by the same electrical power supply, so that they can directly replace the bulbs without adaptation of the host apparatus.

Warning lights such as those in aircraft cockpit instrumentation panels provide an example. Many existing instrument panels are adapted to mount and drive warning lamps of incandescent type. It is desirable to be able to retro-fit LED based lamps without adaptation of the circuitry used to drive the warning lamps.

Instrument panel warning lights are often switchable or even continuously adjustable between a high (daytime) setting and a low setting suited to night use, in order to avoid dazzling the instrument operator when ambient light is at a low level.

One known way of varying the light intensity of an incandescent bulb is to use a pulsed voltage supply such as a pulse-width modulated (PWM) power supply. A PWM power supply provides a series of voltage pulses. Figure 1 shows a typical PWM power supply voltage trace 8 where the voltage output from the PWM power supply is switched between a low value of zero volts and a high value of 20 volts. The PWM power supply can modulate the width of the pulses in order to vary its duty cycle (the fraction of time for which the voltage is high). Typically a PWM power supply used to power a lamp is operated at frequencies greater than 50Hz so that the flashing of the lamp is not perceived by the eye. If the frequency of voltage pulses is fixed then the duty cycle is proportional to the pulse width. By modulating pulse width, the average power to the lamp is correspondingly modulated. In this way PWM supply provides a convenient and potentially energy efficient means of dimming a lamp. This mode of control is currently used in relation to instrument panel warning lights.

A problem arises in this context when attempting to replace a filament bulb with an LED based lamp because the dimming characteristics of the two, when driven by the PWM supply, are typically different. An LED based bulb designed to have the required light output at the high (daylight) supply setting would not be expected to provide the correct light level when driven at the low (night) setting.

1) There are examples in the prior art of LED lamps incorporating circuitry intended to enable the LEDs to mimic the dimming characteristics of incandescent bulbs. US 6683419 discloses one such lamp which uses a microcontroller or reduced instruction set controller to control the LED light output. Such a solution is potentially undesirably complex and/or expensive.

EP 0890894 discloses an incandescent bulb luminance matching LED circuit" which uses a Zener diode in series with the LED in order to enable the luminance of the LED to mimic that of an incandescent bulb over a range of supply voltages or currents. The challenges presented by a switched mode dimming circuit are not addressed.

In accordance with a first aspect of the present invention there is a method of driving an LED comprising connecting the LED between a pair of supply terminals via a current limiting device, connecting a capacitance in parallel with the LED, and applying to the supply terminals a pulsed voltage supply, so that at the beginning of a voltage pulse illumination of the LED is delayed as the capacitance charges.

The LED is illuminated only once the capacitance has charged to a voltage equal to the forward voltage of the LED. Also the LED's level of illumination increases progressively over a period determined by the rate at which the capacitance charges. By suitable choice of the relevant aspects of the circuit, including the capacitance and the current that charges it, desired high and low levels of LED illumination can be achieved with an existing power supply.

Preferably, in order to provide dimmed operation of the LED, duration of the voltage pulses is set to be shorter than the time taken for the capacitor to charge.

A capacitance approaches its maximum level of charge asymptotically, of course. For present purposes the time taken for the capacitor to charge can be taken to be the time taken for it to reach 95% of the voltage it would reach if charged from the supply voltage indefinitely.

Preferably the method further comprises connecting to the capacitor a discharge resistance through which the capacitor discharges between voltage pulses.

The discharge resistance could in some embodiments be connect to earth, but in the preferred method the discharge resistance is connected in parallel with the capacitor.

In accordance with a second aspect of the present invention, there is an electronic circuit for a Light Emitting Diode (LED) lamp, the circuit comprising an LED connected between a pair of supply terminals via a current limiting device, a capacitance in parallel with the LED, and a discharge resistor connected to the capacitance such that the capacitor is able to discharge through it.

The discharge resistance is preferably connected in parallel with the LED.

The current limiting device is preferably a series resistance.

The circuit is preferably incorporated in a lighting system comprising an LED lamp connected to a pulsed voltage supply.

Preferably the pulsed voltage supply is able to modulate duration of the voltage pulses in order to vary LED output intensity.

A specific embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a graph of LED current and supply voltage for an LED arrangement (known fromihe prior art) having a current limiting series resistor; Figure 2 is a circuit diagram of an LED lamp according to the present invention; and Figures 3 to 6 are graphs of supply voltage and LED current in the Figure 2 circuit.

When driving an LED it is necessary to limit current through it. A very simple and well known means of limiting current comprises a resistance connected in series with the LED. The behaviour of this type of known LED circuit is ) represented (in order that it can be compared with the behaviour of the circuit embodying the invention, which will be considered below) in Figure 1. As noted above, PWM supply voltage 10 is periodically switched between low and high values. In this example switching is not instantaneous -the voltage pulses are seen to have sloped edges typical of the rise and fall times of dimmer unit outputs. LED current 8 is seen to closely reflect changes in supply voltage.

Turning now to Figure 2, the lamp circuit embodying the invention has positive and negative terminals +Ve and -Ve for connection to a PWM power supply.

While the packaging of the lamp could take many different forms, the terminals may for example mimic those of an incandescent bulb, such as a bayonet or screw fitting, to allow the lamp to be directly substituted for an existing bulb. An LED 10 is connected between the positive and negative terminals via a series resistance 12. Note that the invention could be implemented using an array of LEDs in place of the single LED 10, e.g. to increase light output. The series resistance 12 may be formed by a parallel combination of a plurality of resistors. This may be preferable where the circuit is to be implemented in a sub-miniature package and a plurality of resistors are together able to handle the requisite current/power whilst being sufficiently compact. In other embodiments a single series resistance suffices. The series resistance serves to limit current through the LED 10. A capacitance 16 is connected in parallel with the LED 10. A discharge resistance 14 is likewise connected in parallel with the LED 10.

* ) Figure 3 shows how current through the LED varies when the lamp is driven by a PWM supply. In this example the capacitance 16 is at least substantially discharged when the supply voltage goes from low to high. Application of the high voltage initially causes current to flow to the capacitance, to charge it.

Only once the voltage across the capacitance (which is of course the same as the voltage across the LED) reaches the forward voltage of the LED is the LED illuminated. Hence there is a delay 50 between the voltage going high and the commencement of current flow through the LED. The value of the LED current then increases progressively toward its maximum over a period determined by the rate at which the capacitance charges. When the supply voltage returns to its low value, the capacitance 16 discharges through both resistance 16 and LED 10, which is why current flow through the LED 10 continues for a period 52 after the end of the voltage pulse.

The rate at which the capacitance 16 charges is determined, for a given value of the supply's high voltage level, by the values of the capacitance 16 and resistances 12, 14. Figures 3 -6 illustrate how the LED current trace varies, given the same PWM supply signal, as the value chosen for the capacitance 16 is changed from a low value (Figure 3) to a high value (Figure 6). In Figure 3 there is a protracted period during each voltage pulse when the current is at its maximum. In Figure 6 the rise time of the voltage across the capacitance is longer than the voltage pulse, so that the LED is only illuminated for a relatively brief period, and LED current never reaches the maximum. A high value of the capacitance 16 thus produces a low level of illumination at the "dim' setting of the PWM supply. )

To a rough approximation, in the circuit considered in Figure 1 the LED's output intensity can be considered to vary in proportion to pulse width, since the LED (being able to react quickly to changes in applied voltage) is illuminated at the maximum intensity for the entire duration of the voltage pulse. In the Figure 2 circuit, variation of the LED's output intensity with pulse width can be non-linear because of (a) the delay prior to illumination of the LED at the start of the pulse; (b) the rise and fall times of the LED current, determined by the charge rate of the capacitance, and (c) in some cases (Figure 6) the fact that the current does not reach its maximum level, because the current rise time is longer than the pulse width.

The effect of the capacitance 16 on the LED's output intensity varies depending on the pulse width and the frequency of the PWM supply. If the pulse width is long enough at the supply's "bright" sefting, the effect of the capacitance at that setting on the LED's output intensity will be small. By appropriate selection of the component values, one can design a circuit which mimics the intensity of an incandescent bulb at both settings.

Claims

CLPJMS1.Amethod of drMng an LED comprising connecting the LED betweenapair of supply temilnals via a current lhnlting device, connecting a capacitance in parallel with the LED, and applying to the supply terminals a pulsed voltage supply so that at the beginning of a voltage pulse Uluminatlon of the LED Is delayed as the capacitance charges.
2. A method of driving an LED as claimed In claim I further comprising modulating the duration of the voltage pulses to valy the output Intensity of the LED.
3.Amethod as claimed In claim 2inwhich the duration ofthevoltage pulses is switchable between a high setting, at whIch the said duration is longer, and a low setting, at which the said duration is shorter.
4. A method as clained in any preceding claim in which, in order to provIde dimmed operation of the LED, duration of the voltage pulses Is set to be shorter than the time taken for the capacitor to charge.
5. A method as claimed In any preceding claim in whIch madmum current flowing through the LED is controlled by modulation of the pulse duration.
6. A method as claimed in any preceding claim, further comprising connecting to the capacitor a discharge resistance through which the capacitor discharges between voltage pulses.
7. A method as claimed in claim 6 in which the discharge resistance is connected in parallel with the capacitor.
8. A method as claimed in any preceding claim in which the current limiting device is a series resistance.
9. An electronic circuit for a Light Emitting Diode (LED) lamp, the circuit comprising an LED connected between a pair of supply terminals via a current limiting device, a capacitance in parallel with the LED, and a discharge resistor connected to the capacitance such that the capacitor is able to discharge through it.
10. An electronic circuit as claimed in claim 9 in which the discharge resistance is connected in parallel with the LED.
11. An electronic circuit as claimed in any preceding claim in which the current limiting device is a series resistance.
12. A lighting system comprising an LED lamp incorporating the electronic circuit of any of claims 9 to 11 connected to a pulsed voltage supply.
13. A lighting system as claimed in claim 12 in which the pulsed voltage supply is able to modulate duration of the voltage pulses in order to vary LED output intensity.
14. A lighting system as claimed in claim 13 in which the pulsed voltage supply is settable to provide voltage pulses of a duration shorter than a time taken for the capacitance to be charged by the voltage pulse.
15. A lighting system as claimed in claim 13 or claim 14 in which maximum current flowing through the LED is controlled by modulation of the pulse duration.
16. An electronic circuit substantially as herein described with reference to, and as illustrated in, accompanying Figures 2 to 6.
17. A method of driving an LED substantially as herein described with reference to, and as illustrated in, accompanying Figures 2 to 6.