Toaster with mains-power-on safety device

A toaster has electrical parts which are energized from a mains voltage. The bread is conveyed to a toasting chamber (4) by means of a lift (6) and a handle (12). In the end position of the handle (12) a main switch (SW5) is turned on, as a result of which the electrical parts can be powered from the mains voltage. The heating elements receive mains voltage via triacs controlled by a microcontroller. After completion of the toasting cycle the microcontroller turns off the heating elements, which precludes burning of the bread, even if the main switch remains in its on-position as a result of a mechanical defect. Subsequently, the microcontroller checks the presence of the mains voltage. In that case a signalling device is activated to warn the user that the mains voltage supply to the toaster has not yet been turned off.

BACKGROUND OF THE INVENTION 
The invention relates to a toaster having electrical parts which are 
energized from a mains voltage, comprising: a toasting chamber for 
receiving a product to be toasted, at least one heating element for 
heating the product, a transport device for moving the product into the 
toasting chamber, and a main switch, which can be activated by the 
transport device, to connect the electrical parts to the mains voltage. 
Toasters of this type are generally known. The bread to be toasted is 
inserted into the toasting chamber and is exposed to heating by the 
heating element. The number of heating elements depends on the type of 
toaster. A common type has two heating elements, the slice being 
interposed between the heating elements and both sides being toasted at 
the same time. The slice of bread is brought into the toasting chamber 
with the aid of a transport device by pushing down a handle. When the end 
position of the handle is reached the main switch is turned on, as a 
result of which mains voltage is supplied to the toaster and the heating 
elements are energized. A locking device ensures that the end position is 
maintained until toasting is finished. After expiry of the toasting time 
the handle is released, as a result of which the transport device 
automatically lifts the toasted slice of bread. The main switch is then 
also turned off, so that all the electrical parts are disconnected from 
the mains voltage and, as a result, the heating elements are also turned 
off. If the mechanism of the transport device falters or jams, the heating 
elements will remain energized and the bread to be toasted will eventually 
be charred. For the user this is generally a sign that there is something 
wrong with the toaster and the user will pull the mains plug out of the 
wall outlet to preclude further damage and may attempt to repair the 
locking mechanism by hand or by means of a tool. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a toaster with improved safety. 
To this end, according to the invention, the toaster of the type defined 
in the opening paragraph is characterized in that the toaster further 
comprises: a controllable switch for turning on and turning off the at 
least one heating element. 
By turning off the heating elements with a controllable switch, for example 
a relay or a triac, after expiry of the toasting time, charring or burning 
of the product to be toasted is prevented and a higher fire safety is 
obtained. 
After expiry of the toasting time the heating elements are turned off by 
means of a suitable control signal applied to the controllable switches. 
In that case the bread will not be charred if the transport mechanism 
jams. However, the bread remains stuck in the toaster. The user is then 
still tempted to take action in order to mend the locking mechanism. 
However, the user is not aware that mains voltage is applied to the 
electrical parts in the toaster because the main switch is still in the 
on-position. Now there is a high risk that during his repair attempts the 
user comes into contact with the mains voltage, particularly when use is 
made of metal knives and forks which are ready to hand. 
In order to mitigate this hazardous situation an embodiment of the toaster 
in accordance with the invention is characterized in that the toaster 
further comprises: a signalling device, means for detecting the mains 
voltage after turning-off of the heating element, and means for activating 
the signalling device in response to the detection of the mains voltage. 
By checking whether mains voltage is present after completion of the 
toasting process, it is possible to warn the user against the unsafe 
situation which has arisen as a result of failure of the main switch to 
open automatically. The signalling device can be implemented in various 
ways. Existing pilot lamps or a special lamp can light up or blink 
together or in accordance with a given pattern in order to give the user a 
visual indication. Instead of or in combination with this, it is also 
possible to produce an acoustic signal by means of a loudspeaker or buzzer 
.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 is a cross-sectional view showing an embodiment of a toaster in 
accordance with the invention. The toaster comprises a casing 7 which 
encloses a toasting chamber 4, in which two heating elements H1 and H2 are 
disposed at either side of one or more slices of bread 5, which can be 
brought into the toasting chamber 4 by means of a lift 6 via a slot 3 in 
the casing 7. The radiating surfaces of the heating elements H1 and H2 
extend horizontally in the longitudinal direction of the toaster and face 
the slice of bread 5. 
FIG. 2 is a longitudinal section view showing the toaster. The product 5 to 
be toasted, shown as two slices of bread 5 in FIG. 2, is placed on the 
lift 6 and is conveyed into the toasting chamber 4 by means of a handle 
12. When the end position of the lift 6 is reached the contacts L/LS and 
N/NS of a main switch SW5 are closed, as a result of which all the 
electrical parts of the toaster are coupled to the mains voltage and the 
toaster is put into operation. The handle 12 and the lift 6 are held in 
this position by means of a locking mechanism, not shown, which is 
released after expiry of a toasting cycle, as a result of which the lift 6 
goes up automatically under the influence of a spring or another 
mechanism, which are not shown either. The contacts of the main switch SW5 
then open again, as a result of which the electrical parts are no longer 
coupled to the mains voltage. 
FIGS. 3A and 3B show an electrical circuit diagram of the toaster in FIGS. 
1 and 2. When the control handle 12 is pushed down the contacts of a main 
switch SW5 are closed in the end position of the lift 6, as a result of 
which the mains voltage on the terminals L and N is transferred to the 
terminals LS and NS, the terminal NS being connected to signal ground. A 
transformer TR transforms the high mains voltage to a lower voltage, which 
is rectified by means of a rectifier bridge D1-D4, yielding a raw voltage 
Vb which energizes a solenoid SLD via a resistor R28. The raw voltage Vb 
is smoothed by a capacitor C8 via a diode D5, yielding a supply voltage 
Vs. The diode D5 prevents the capacitor C8 from being discharged through 
the solenoid SLD. The supply voltage Vs is further smoothed by a resistor 
R4 and a capacitor C9 and is reduced to a supply voltage Vcc of, for 
example, 5 V. The voltage Vcc is connected to the emitter of a PNP 
transistor Q2, which has its collector connected to ground via a resistor 
R6 and which has its base connected to ground via a zener diode Z1. The 
zener diode Z1 receives a bias current derived from the supply voltage Vs 
via a resistor R5. The sum of the base-emitter voltage of the transistor 
Q2 and the zener voltage of the zener diode Z1 determines the magnitude of 
the supply voltage Vcc. When the mains voltage is turned on by means of 
the main switch SW5 the supply voltage Vs increases. The base of the 
transistor Q2 follows this increase until the zener voltage of the zener 
diode Z1 is reached. The supply voltage Vcc also increases but it 
increases more slowly than the supply voltage Vs owing to the resistor R4 
and the capacitor C9. When a given value of the supply voltage Vcc is 
reached the transistor Q2 is turned on and the voltage across the resistor 
R6 increases rapidly from zero volts to approximately the supply voltage 
Vcc. The voltage variation across the resistor R6 is used to reset a 
microcontroller IC1. A diode D6 between the base and the emitter of the 
transistor Q2 protects the base-emitter junction of the transistor Q2 
against excessive reverse voltages. 
Upon receipt of the supply voltage Vcc and the reset pulse the 
microcontroller IC1 (type COP842CN) is put into operation, the clock 
frequency being determined by a resonator RES (for example a ceramic 
resonator at 5 MHz) with a parallel resistor R32. The microcontroller IC1 
turns on a driver transistor Q4 via a resistor R12, which transistor is 
arranged in series with the solenoid SLD. As a result, the contacts of the 
main switch SW5 remain energized until the microcontroller IC1 turns off 
the driver transistor Q4, or until the solenoid SLD is short-circuited by 
means of a switch SW4 across the solenoid SLD, which switch is actuated by 
the user of the toaster. The lift 6 and the handle 12 are then released 
and the lift 6 moves the toasted bread outward. 
The heating element H1 has one terminal connected to the terminal LS. By 
means of a first electronic switch TRIAC1 the other terminal is connected 
to the terminal NS for receiving the mains voltage. By means of a second 
electronic switch TRIAC2 the other heating element H2 is connected to the 
terminals LS and NS. 
The trigger electrode of the electronic switch TRIAC1 receives control 
pulses from the emitter of an NPN transistor Q5 via a diode D10, which 
transistor has its collector connected to the supply voltage Vs via a 
resistor R30. A resistor R19 between the trigger electrode of the 
electronic switch TRIAC1 and the terminal NS prevents triggering in the 
absence of control pulses. The diode D10 prevents an undesirable turn-on 
of the transistor Q5 if the mains voltage on the terminal LS is negative 
relative to the terminal NS. The base of the transistor Q5 receives 
control pulses from the microcontroller IC1 via a series resistor R16 and 
a coupling capacitor C4. The base of the transistor Q5 is connected to the 
terminal NS via a resistor 18 in parallel with a diode D9. The cathode of 
the diode D9 is connected to the base of the transistor Q5, as a result of 
which the negative base-emitter voltage of the transistor Q5 cannot become 
smaller than one diode voltage. The coupling capacitor C4 prevents the 
electronic switch TRIAC1 from being fired in the event of malfunctioning 
of the microcontroller IC1. 
The other electronic switch TRIAC2 is controlled by means of similar 
electronic control circuitry. The microcontroller IC1 turns the electronic 
switches on and off in accordance with a Multi Cycle Control pattern, in 
which the electronic switches are on or off during full cycles of the 
mains voltage. For this purpose the microcontroller IC1 receives 
information from a zero cross circuit ZCC, which converts the sinusoidal 
mains voltage into a squarewave voltage of suitable amplitude and phase. 
The zero cross circuit ZCC comprises an NPN transistor Q1, which has its 
emitter connected to the terminal NS and which has its base connected to 
the terminal LS via three series-connected resistors R1, R47 and R2. The 
collector of the transistor Q1 is connected to the supply voltage Vcc by a 
resistor R3 and supplies a limited mains voltage to the microcontroller 
IC1. A capacitor C2 parallel to the resistor R2 ensures that the signal 
transients in the limited mains voltage are in phase with the zero 
crossings of the mains voltage. A diode D7 prevents an excessive reverse 
voltage across the base and the emitter of the transistor Q1. Instead of 
by means of triacs and Multi Cycle Control the elements can also be 
activated by means of other electronic switches such as relays. 
The toasting time is adjusted by means of a timer circuit TMR, the 
microcontroller IC1 comparing the resistance value of a control 
potentiometer P1 and a series resistor R10 with that of a reference 
resistor R11 by charging a capacitor C3 and subsequently discharging it 
via the reference resistor R11 and via the potentiometer P1 and the 
resistor RIO, and comparing the discharge times. The microcontroller IC1 
further measures the temperature in the toaster by comparing the 
resistance of a resistor circuit including a temperature-dependent 
resistor NTC with the reference resistor R11. This enables the on-time to 
be corrected for a cold or warm toaster. 
By means of the switches S1, S2 and S3 a number of program options can be 
selected, indicators LED1, LED2 and LED3 indicating which options have 
been selected. By means of the switch S1 one-side toasting can be chosen. 
In that case only one of the heating elements H1 or H2 is active. By means 
of the second switch S2 it is possible to correct the toasting time for 
frozen bread, and by means of the third switch S3 the toasting time is 
limited to a fixed duration. The indicators LED1, LED2 and LED3 can be 
arranged in the casing 7 at a location which is conspicuous to the user. 
After completion of the toasting cycle the microcontroller turns off the 
heating elements H1 and H2 and de-energizes the solenoid SLD, as a result 
of which the lift 6 is released and the main switch SW5 is opened. Turning 
off the heating elements H1 and H2 prevents the product to be toasted from 
being burnt and thereby precludes fire hazard. If the lift 6 and/or the 
handle 12 remain stuck or the main switch is not opened for whatever 
reason, the user will not perceive that the mains voltage supply to the 
toaster has not ceased after completion of the toasting cycle. Indeed, the 
heating elements H1 and H2 no longer radiate. The user may now be tempted 
to repair the failing release mechanism in the toaster, which is still 
energized with the mains voltage. In order to warn the user against the 
hazardous consequences of such an attempt, a signalling device has been 
provided. For this purpose, the microcontroller IC1 is programmed to make 
one or more of the indicators LED1, LED2 and LED3 blink, either at the 
same time, after one another, or in another conspicuous pattern some time 
after completion of a toasting cycle, i.e. after the heating elements have 
been turned off. It is alternatively possible to use a separate indicator 
for this purpose. Instead of or in combination with visual indicators the 
microcontroller IC1 can generate an acoustic signal. For this purpose the 
microcontroller IC1 turns a loudspeaker SP or a buzzer on and off via a 
driver transistor Q3 by means of a suitable squarewave voltage on the base 
of the driver transistor Q3. Thus, the user is warned by means of an 
audible signal. 
Instead of a microcontroller it is also possible to use separate electronic 
circuits, particularly commercially available timers, zero cross detectors 
and triac drivers, in order to implement the described functions of the 
toaster. 
An embodiment is disclosed of a toaster having electrical parts which are 
powered from a mains voltage. The bread is moved into a toasting chamber 
by means of a lift and a handle. In the end position of the handle a main 
switch is turned on, as a result of which the electrical parts can be 
powered from the mains voltage. The heating elements receive mains voltage 
via triacs or other controllable switches which are controlled by a 
microcontroller or a timer. After completion of the toasting cycle the 
microcontroller turns off the heating elements, as a result of which the 
bread cannot be burnt, even if the main switch remains in its on-position 
owing to a mechanical defect. Subsequently, the microcontroller checks 
whether the mains voltage is still present. In that case a signalling 
device is activated, which warns the user against the fact that the mains 
voltage supply to the toaster has not yet been turned off.