[0001] The present invention relates to a control device for a gas-discharge lamp, particularly
for motor vehicles.
[0002] More specifically the subject of the invention is a control device for a gas-discharge
lamp comprising
a low-voltage dc source,
dc/dc converter means connectable to the said source and operable to provide at its
output a higher dc voltage than that of the source,
inverter means connected to the said dc/dc converter means and to which the lamp is
connected, and
an electronic control unit arranged to control the said dc/dc converter means and
the said inverter means in a pre-determined manner in such a way as to transfer to
the lamp a high electric power in the striking and heating phase, and a low electric
power in the subsequent normal operation of the lamp.
[0003] Upon striking a gas-discharge lamp must be supplied with a high electric power which
can cause rapid evaporation of the materials (metal halides, rare earths) contained
within its interior. During this striking stage the electric power supplied to the
lamp must also be suitably regulated in order to ensure that the light flux emitted
has the desired characteristics.
[0004] After striking the electric power supplied to the lamp is gradually reduced, and
finally stabilises when the lamp reaches normal operating conditions.
[0005] At each re-striking of the lamp the manner in which the high electric power is applied
must be suitably modified in dependence on the time elapsed since the lamp was last
extinguished, for the purpose of avoiding a dangerous overload of the lamp which could
appreciably compromise its operating life.
[0006] The control unit which regulates the application of electric power to the lamp during
striking must therefore have access to information about the time elapsed since the
lamp was last extinguished.
[0007] In known systems the control unit, which is typically formed using a microprocessor,
remains connected to the dc voltage supply source even when the lamp is extinguished.
This unit is, moreover, arranged to measure (for example by means of a counter) the
time for which the lamp remains extinguished. At each re-striking of the lamp the
control unit regulates the electric power applied to the lamp on the basis of the
time elapsed since it was last extinguished.
[0008] This arrangement has the disadvantage of having to have a specific connection between
the voltage supply source and the control unit.
[0009] The object of the present invention is to provide a control device for a gas-discharge
lamp which allows a differentiated control of such a lamp as a function of the time
elapsed since it was last extinguished without requiring the control unit to be permanently
supplied via a suitable supply line.
[0010] This object is achieved according to the invention with a control device of the previously-specified
type, characterised in that it further includes:
detector means operable to provide to the said control unit electrical signals indicative
of the impedance of the lamp,
and in that the said control unit has associated therewith memory means of non-volatile
type in which are stored data indicative of the values of the impedance of the lamp
upon variation in the time elapsed since it was extinguished;
the said control unit being arranged to:
- acquire, at each striking of the lamp, signals provided by the said detector means
to determine the impedance presented by the lamp,
- compare the thus-determined impedance values with the values stored in the said memory
means and on the basis of this comparison determine the time elapsed since the lamp
was previously extinguished, and
- control the said dc/dc converter means and/or the said inverter means in such a way
as to apply to the lamp, immediately after it has been struck, an electric power which
varies as a function of the elapsed time as determined above.
[0011] The invention is based on the observation that the impedance of the lamp measured
immediately after its striking varies appreciably as a function of the time elapsed
since the lamp was previously extinguished. Upon striking, a gas-discharge lamp presents
for example an impedance of about 200 Ohm if the striking follows immediately after
the lamp has been extinguished, and an impedance of for example about 10 Ohm when
the lamp is re-struck from cold, that is to say after a considerable time has elapsed
since it was last extinguished.
[0012] By experimental measurements it is possible to determine, for a given type of lamp,
the correlation between its impedance upon striking and the time elapsed since it
was last extinguished. This correlation converted, for example, into the form of a
table stored in memory devices of non-volatile type, makes possible an acceptable
determination of the time elapsed since the lamp was extinguished, without the necessity
for the control unit to remain supplied when the lamp is extinguished.
[0013] Further characteristics and advantages of the invention will become apparent from
the following detailed description which is given purely by way of non-limitative
example, with reference to the attached drawings in which:
Figure 1 is a partly block-schematic diagram of a control device for a gas-discharge
lamp, according to the invention, and
Figure 2 is an experimental graph which plots the variation of the ratio between the
impedance which a lamp presents upon striking and the impedance presented during normal
operation, as a function of time elapsed since it was extinguished.
[0014] In Figure 1 the reference letter B indicates a low voltage dc source such as, for
example, the battery on board a motor vehicle.
[0015] To the battery B is connected the input of a dc/dc conversion circuit unit generally
indicated 1. This circuit unit can comprise, for example, a dc/dc converter of the
push-pull type and/or of other known types. This circuit unit typically includes one
or more electronic switches (not illustrated) which can be controlled in a pre-established
manner by means of control signals applied to inputs 1a ... 1N of this unit.
[0016] At the output of the dc/dc conversion circuit unit 1 is connected an inverter generally
indicated 2, which is connected to the gas-discharge lamp L. In the exemplary embodiment
illustrated the inverter 2 is of the totally controlled bridge type and includes four
electronic switches S1-S4. The lamp L is connected across one diagonal of the bridge
circuit. The secondary winding of a striking transformer TT is connected in series
with this lamp.
[0017] An electronic control unit indicated ECU formed, for example, with the use of a microprocessor,
is connected to the control inputs 1a-1N of the dc/dc conversion circuit unit 1 and
to the control inputs of the electronic switches S1-S4 of the inverter 2.
[0018] A measurement resistor R is connected to the output of the circuit unit 1 in series
with the bridge circuit of the inverter 2. The terminals of this resistor are connected
to corresponding inputs of the electronic control unit ECU. In operation the resistor
R allows the unit ECU to acquire information relating to the magnitude of the current
flowing in the lamp L. This information can however be acquired from the unit ECU
via other means or devices known per se.
[0019] In the embodiment illustrated in Figure 1 the unit ECU is further connected to the
upper and lower terminals of the bridge circuit of the inverter 2 by connections indicated
3 and 4, in such a way as to be able to acquire, during operation, information relating
to the voltage applied to the lamp L. Also In this case, this Information may be acquired
from the unit ECU in other known ways.
[0020] Associated with the unit ECU are memory devices M of non-volatile type, preferably
of the re-writable type.
[0021] In these memory devices are stored data indicative of the values of the impedance
Z
L presented by the lamp at striking, upon variation of the time elapsed since the lamp
gas extinguished. This data can be determined by experimental measurements as previously
mentioned.
[0022] The control unit ECU is arranged to acquire, at each striking of the lamp L, the
voltage across the terminals of the resistor R which is indicative of the current
flowing in the lamp, and the voltage across the terminals of the bridge of the inverter
2, which is indicative of the voltage applied to the lamp. On the basis of this information
the unit ECU determines the impedance Z
L of the lamp L at striking.
[0023] The unit ECU is therefore arranged to compare the thus-determined values of the impedance
with the values stored in the memory devices M, and to determine on the basis of this
comparison the time t
off elapsed since the lamp L was last extinguished.
[0024] The control unit ECU therefore acts to control the dc/dc conversion circuit unit
1 and/or the inverter 2 in such a way that, immediately after the lamp L is struck,
a variable electrical power is supplied to the lamp depending on a predetermined law
stored in the said control unit, as a function of the time t
off.
[0025] In the exemplary embodiment illustrated in Figure 1 the control of the striking of
the lamp L is achievable by manual actuation of a control switch indicated S. Closure
of this switch causes coupling of the dc/dc conversion unit 1 and the unit ECU to
the dc supply voltage source B.
[0026] When the user opens the switch S to cause the lamp to be extinguished the unit ECU
likewise no longer receives the supply voltage. However, thanks to the data stored
in the non-volatile memory devices M and to the procedure for detection and comparison
of the impedance presented by the lamp at striking the system is always able to perform
a regulation of the electrical power applied to the lamp at striking as a function
of the time elapsed since it was previously extinguished.
[0027] In a variant of the Invention the memory devices M associated with the control unit
ECU conveniently have stored therein data relating to the ratio between the impedance
Z
L of the lamp L at striking and the impedance Z
LO of the said lamp during normal operation, upon variation of the time t
off elapsed since the lamp was extinguished.
[0028] The control unit ECU is then correspondingly prearranged to acquire and store (in
a non-volatile manner), during each striking period of the lamp L, the impedance presented
by the said lamp during normal operation, and to determine at each striking of the
lamp the ratio between the current impedance of the lamp at striking and the impedance
of the lamp during normal operation in the preceding striking period. The unit ECU
then compares the value of the thus-determined ratio with the values stored in the
memory devices M and on the basis of this comparison determines the time t
off elapsed since the lamp was previously extinguished.
[0029] In Figure 2 is shown, by way of example, a graph based on the experimental detections
made on a lamp. This graph plots the variation of the ratio between the impedance
Z
L of a gas-discharge lamp at striking and the impedance Z
LO of the same lamp during normal operation, plotted as a function of the elapsed time
t
off since it was extinguished. Such a graph is stored in the memory devices M for example
in the form of a table.
[0030] The variant of the invention now described allows a more precise regulation of the
electrical power applied to the lamp L at striking as a function of the time elapsed
since it was last extinguished.
[0031] In fact it is possible to hypothesise that the values of the ratio Z
L/Z
LO remain relatively stable as a function of time t
off even with ageing of the lamp, whilst in general the impedances Z
L and Z
LO can be subject to variations.
[0032] The variant of the invention described above also makes it possible to render the
control system of the lamp L self-adjusting, even in the case of possible replacement
of the lamp L.
[0033] Naturally, the principle of the invention remaining the same, the embodiments and
details of construction can be widely varied with respect to what has been described
and illustrated purely by way of non-limitative example, without by this departing
from the scope of the present invention as defined in the appended claims.
1. A control device for a gas-discharge lamp (L), particularly for motor vehicles, comprising
a low voltage dc source (B),
dc/dc converter means (1) connectable to the said source (B) and operable to provide
at its output a higher dc voltage than that of the source (B),
inverter means (2) connected to the said dc/dc converter means (1) and to which the
lamp (L) is connected, and
a control unit (ECU) arranged to control the said dc/dc converter means (1) and/or
the said inverter means (2) in a predetermined manner in such a way as to apply to
the lamp (L) a high electrical power during striking and heating, and a lower electrical
power during the subsequent operation of the lamp during its normal operating conditions;
the device being characterised in that it further includes:
detector means (R; 3, 4) operable to provide to the said control unit (ECU) electrical
signals indicative of the impedance of the lamp (L),
and in that the said control unit (ECU) has associated therewith memory means (M)
of non-volatile type In which are stored data Indicative of the values of the impedance
(ZL) of the lamp upon variation in the time (toff) elapsed since the lamp (L) was extinguished;
the said control unit (ECU) being arranged to:
- acquire, upon each striking of the lamp (L), signals provided by the said detector
means (R; 3, 4) to determine the impedance (ZL) of the lamp (L),
- compare the thus-determined values of the impedance (ZL) with the values stored in the said memory means (M), and on the basis of this comparison
to determine the time (toff) elapsed since the lamp (L) was previously extinguished, and
- control the said dc/dc converter means (1) and/or the inverter means (2) in such
a way as to apply, immediately after striking of the lamp (L), an electrical power
which varies as a function of the elapsed time (toff) as determined above.
2. A device according to Claim 1, characterised in that the said memory means (M) store
data indicative of the ratio between the impedance (ZL) of the lamp (L) upon striking and the impedance (ZLO) of the lamp during normal operation upon variation of the time (toff) elapsed since the lamp (L) was extinguished;
and in that the said control unit (ECU) is arranged to acquire, and store in a
non-volatile manner in each striking period of the lamp (L) the impendence presented
by the said lamp (L) during normal operation, and to determine at each striking of
the lamp (L) the ratio between the current impedance (ZL) presented by the lamp (L) and the impendence (ZLO) presented by the lamp in normal operation during the preceding period of use, and
to compare the value of the thus-determined ratio with the values stored in the said
memory means (M), and to determine the elapsed time (toff) since the lamp was previously extinguished on the basis of this comparison.