Flame-monitoring device
The invention relates to a flame-monitoring device in which an a.c. input voltage (U 1 ) is limited to a voltage limit (U 2 ) by means of a voltage limiter ( 4 ). Said voltage limit (U 2 ) is applied to a flame sensing device ( 7 ) which operates by means of the rectifying effect of a flame, and through which a current (i) flows, especially when a flame ( 6 ) is present. An asymmetric voltage limit (U 2 ) can be defined by said voltage limiter ( 4 ), said limit being then applied to the sensing device ( 7 ).
1. A flame monitoring apparatus in which an input ac voltage is limited to a limit voltage with a voltage limiter, wherein the limit voltage acts on a flame sensor through which a current flows when a flame is present,
characterized in that
the voltage limiter is operable to produce the limit voltage such that the limit voltage comprises an asymmetric limit voltage which acts on the sensor.
2. A flame monitoring apparatus as set forth in claim 1 characterized in that the voltage limiter comprises a plurality of limiter elements which symmetrically limit the voltage, and that in addition, to produce the asymmetry, at least one more limiter element is arranged, so that when a flame is present a higher current can be achieved than without asymmetry.
3. A flame monitoring apparatus as set forth in claim 1 characterized in that the voltage limiter comprises series-connected diodes which also conduct in the reverse direction from a reverse voltage on, that about half of the series-connected diodes are connected in the forward direction and the other half in the reverse direction, and in addition, for producing the asymmetry, in one direction there is at least one more diode, so that in the forward direction when a flame is present a higher current can be achieved than without asymmetry.
4. A flame monitoring apparatus, as set forth in claim 3 characterized in that the diodes are of the same type.
5. A flame monitoring apparatus as set forth in claim 1 characterized in that the sensor is an ionization electrode or a UV-sensor.
6. A flame monitoring apparatus wherein an input ac voltage is limited to a limit voltage, the flame monitoring apparatus comprising:
a flame sensor through which a current flows when a flame is present, wherein the limit voltage acts on the flame sensor; and
a voltage limiter operable to produce a limit voltage that comprises an asymmetric limit voltage which acts on the flame sensor.
7. The flame monitoring apparatus of claim 6 wherein the voltage limiter comprises a first plurality of limiter elements which symmetrically limit the voltage and wherein at least one additional limiter element is arranged to produce the asymmetry, such that a higher current can be achieved when a flame is present than would be achieved without asymmetry.
8. The flame monitoring apparatus of claim 6 wherein the voltage limiter comprises an even number of series-connected diodes which conduct in the reverse direction from a reverse voltage, wherein a first half of the series-connected diodes are connected in the forward direction and a second half are connected in the reverse direction, and wherein in one direction there is at least one additional diode for producing asymmetry, such that in the forward direction when a flame is present a higher current can be achieved than without the asymmetry.
9. The flame monitoring apparatus of claim 8 wherein the series-connected diodes are of the same type.
10. The flame monitoring apparatus of claim 6 wherein the flame sensor is an ionization electrode or a UV-sensor.