[0001] The present invention relates to an arrangement for indication of the need of cleaning
of an air filter for instance in a stove hood. After some time of working the cleaning
ability of the filter is reduced because of pollution of the filter and the arrangement
shall then indicate the need of cleaning of the filter.
[0002] It is priorly known to use a timer for measuring the working time and, after a predetermined
period, to make the timer give a signal. As a ventilator can work with different degrees
of capacity and pollution of the filter is dependent thereon, it is impossible to
determine a proper time of working of the filter between the cleaning procedures.
In order to have an equivalent of the time passing from one cleaning to another it
is necessary to consider the capacity of the ventilator on each occasion. As a measure
of the capacity of the ventilator the current of the motor can be used, the arrangement
then constituting a time circuit which is programmed by the current of the ventilator
motor, in other words, when the ventilator motor is working at the highest capacity
an indication is received after for instance 30 hours, when the ventilator motor is
working at half the capacity the indication occurs after for instance 50 hours, and
when the ventilator is adjusted to the lowest rpm no time counting at all takes place
owing to the fact that in this position no pollution will fasten to the filter as
the flow speed of air is then so low that the pollution will follow the air through
the filter without the time limit for cleaning getting nearer.
[0003] By the present invention it is presented a circuit arrangement with a time circuit
where the main component is constituted of a time summing galvanic cell comprising
an anode and a cathode and an electrolyte therebetween. By charging the cell a certain
current passes during a time period in the direction anode - cathode whereby a certain
quantity of anode material, in this case silver, is transferred to the cathode which
e.g. is made of gold. If the current is reversed through the cell the silver is transferred
back to the anode, but when no more silver remains and gold will not be dissolved,
the cell ceases conducting. The voltage over the cell during the electrolysis is about
1 V but increases when the cell is discharged. The transient voltage then occurring
can in an arrangement with components be used for emitting a signal. A realization
of this principle is featured as the arrangement according to the characterizing clause
of claim 1.
[0004] An embodiment of the invention is described in the following with reference to the
accompanying drawing which shows a wiring diagram of the arrangement of the invention.
[0005] When the arrangement is connected to the mains an AC-voltage is supplied on the input
10, 11 and applied via a series resistor 12, a control 13 and a switch 14 to a ventilator
motor 15. From the voltage input 11 current is supplied to a time meter, the substantial
part of the arrangement. The voltage is rectified by a rectifier 16 and limited by
a series resistor 17 and a Zener diode 18 which via a return wire 19 is connected
to the connector 10 of the input. A capacitor 20 keeps a DC-voltage nearly constant
at 10 V on a wire 21.
[0006] In the circuits shown in the Figure a current passes between the wires 21 and 19
through La. a resistor 22 and a transistor 23 which has a control current via a resistor
24 and a diode 25. A thyristor 26 is then connected between an LED 27 and a connection
28 between the components 22 and 23. The LED lights when the thyristor is switched
on by a current to its control electrode 29.
[0007] When the switch 14 is closed a voltage is supplied to the resistor 12 and applied
via a rectifier 30 and a resistor 31 to the base of a transistor 32 and via a rectifier
33 and a so called E-cell 34 to the connection 28. When the cell is discharged it
has a high resistance which causes a control current to the transistor 32 which is
switched on and causes a voltage drop in a resistor 35. In this way a control voltage
will arise on a transistor 36 which then switches on and makes a voltage drop in a
resistor 37, whereby a control voltage is put on the electrode 29, so that the thyristor
switches on and the LED lights owing to a current in the circuit 21, 27, 26, 28, 23,
19.
[0008] Another current circuit can be established through a resistor 38, a pulse switch
39 and a capacitor 40 when a push button 41 on the pulse switch is actuated. The current
circuit is created from 21 to 38, 39, 40, 25, 23 and 19 making the capacitor 40 be
charged in less than one second. After this the push button is disengaged and the
pulse switch is reset into the shown position. The transistor 23 will have a negative
control voltage and breaks the circuits 22, 28, 23, 19 and 27, 26, 28, 23, 19. The
LED 27 stops lighting. At breaking the first one of said circuits a voltage will arise
in the E-cell from 28 to the base of 32. An engaging current will pass from 21 to
22, 28, 34, 32, 19 during a period when the transistor 23 is off, i.e. while the capacitor
40 is discharged through a circuit comprising the resistor 24, capacitor 40, the pulse
switch 39 and a Zener diode 42. Said period is some minutes, after which the transistor
23 is switched on and the activation (charging) of the E-cell ceases. When the transistor
23 switches on and half-wave pulses corresponding to the current of the motor 15 are
supplied to the E-cell via the rectifier 30, a current passes in the opposite direction
through the cell which starts discharging. A certain base voltage appears on the transistor
32 which still does not conduct at this low base voltage. A condition for making the
cell work is a voltage in it, exceeding a step of some 1 V. This step is not reached
when the ventilator works at its minimum rpm, whereby a cancellation of time summing
at such rpm, as said in the introduction, will automatically occur. The discharging
of the E-cell proceeds at a speed proportional to the rpm, and when the cell is totally
discharged the current through it ceases and the voltage on the base of the transistor
32 increases so that the transistor starts conducting in the circuit 21, 35, 32, 19.
A voltage drop occurs in 35 and makes the transistor 36 switch on and, as described
before, supplies a starting pulse to the thyristor 26, whereby the LED lights. This
is a signal for the need of cleaning the filter, and when this is done, the push button
41 is re-activated, whereby the E-cell is re-charged, the LED put out and another
working period starts.
[0009] In a real embodiment of the invention applied in a stove hood the push button 41
and the LED 27 are positioned in a control panel where also other control members
are gathered, however, the push button is separated so that it is not actuated during
a working period by mistake. In the wiring diagram modifications can, of course, be
made without departing from the inventive idea. The shown circuits and components
are only examples which shall not be considered definite.
1. An indicator in an air filter whith a ventilator driven by an electric motor and
time after time requiring cleaning and during the period between the cleanings working
with a varying capacity which causes a pollution proportional to said capacity, characterized
in that in the circuit of the electric motor a resistor (12) forms a voltage which
is a power input in a circuit of an electrolytic cell (34) having a higher voltage
drop when discharged than what it has when charged and that a detecting circuit sensing
said voltage drop is disposed to supply a signal when the cell is discharged.
2. An indicator according to Claim 1, characterized in that a second circuit is connected
in parallel with the circuit of said cell by a change-over switch (23,32) and constituting
a second power input to the cell with an inversed polarity in relation to the said
voltage drop.
3. An indicator according to Claim 2, characterized in that the detecting circuit
is an amplifying circuit of a thyristor (26) connected in series with a lamp (27)
and with one branch of the change-over switch.
4. An indicator according to Claim 3, characterized in that the electrolytic cell
(34) is connected to the input of the amplifying circuit.
5. An indicator according to Claim 2, characterized in that the said change-over switch
is provided with a holding current circuit controlled by a manually operated pulse
switch (39) and a time circuit (40,24,42).
