Electric continuous-flow water heater with controllable outlet temperature and electronic power output stage therefor

An electric continuous-flow water heater generates a basic power with heating resistors in heating passages and a controllable power with three heating resistors in heating passages. The controllable power is controlled by a control system through a triac. At low flow rates two phases and a low controllable basic power is connected to the heating resistors. At high flow rates the heating resistors are star-connected and the controllable heating-resistors are delta-connected. An electric power output stage comprises a mains rectifier bridge, a Schmitt-trigger connected to the output thereof and a frequency divider with two outputs. The signals at the outputs are set by an edge of each third pulse and are reset by the corresponding edge of the following or next but one pulse. A semiconductor relay is optionally by the signals at the outputs through a selector switch or a logic circuit and is rendered conductive for one, two or three half waves.

The invention relates to an electric continuous-flow water heater with 
controllable outlet temperature, comprising 
(a) a through-flow path with a plurality of heating passages connected in 
series in the flow path, 
(b) three-pole switching means arranged to be connected to three phases of 
a three phase current and responding to flow rate in the through-flow 
path, 
(c) a temperature sensor arranged on the outlet side in the through-flow 
path, 
(d) first heating resistors, which are arranged in some of these heating 
passages and are arranged to be switched on by the switching means when a 
predetermined flow rate has been reached, 
(e) second heating resistors, which are arranged in other ones of these 
heating passages, and 
(f) a control system, which is connected to the temperature sensor and by 
which the second heating resistors are controllable through electronic 
power output stages in order to maintain a desired value. 
The invention relates also to an electronic power output stage, 
particularly for controlling the outlet temperature of electric 
continuous-flow water heaters, comprising 
(a) a rectifier bridge connected to a mains-frequency a.c. voltage and 
supplying a d.c. voltage pulsating with twice the mains-frequency, 
(b) a Schmitt-trigger, to which the pulsating d.c. voltage is applied for 
generating a square wave pulse sequence with twice the mains frequency, 
(c) a frequency divider, to which the square wave pulse sequence is 
applied, 
(d) a semiconductor relay with zero voltage switch performance connected to 
the mains a.c. voltage and arranged to switch the power, and 
(e) means for controlling the semiconductor relay as a function of output 
signals of the frequency divider. 
The control of the outlet temperature with electric continuous-flow water 
heaters presents special problems. Electric continuous-flow water heaters 
have a very high installed heating capacity of, for example, 22 kW. The 
outlet temperature must be maintained very exactly and is only allowed to 
hunt slightly around the desired temperature. It is difficult to control 
the heating capacity with corresponding sensitivity. 
The electric heating capacity can be controlled continuously by a phase 
control. But a phase control of the high power present here is not 
permissible because of the higher harmonic waves generated and the 
disturbances caused thereby. It is only permissible to switch the 
alternating current fully on or off, whereby a zero voltage switch takes 
care that the switching on and the switching off occur always at moments 
at which the a.c. voltage passes through zero anyway, such that no phase 
control occurs. 
But a control of the heating power by switching on or off requires a 
relatively high switching frequency, if the requirement is to be met, that 
a desired outlet temperature is maintained with high accuracy. If the 
heating capacity would be switched on and off with low switching 
frequency, too cold or too hot water would alternately flow out of a 
continuous-flow water heater, even if the average heating power 
corresponds to the desired temperature. But a high switching frequency of 
the switching on and off of a high heating power results again in other 
problems: The high heating power of the electric continuous-flow water 
heater affects the mains-frequency due to the mains impedance. Therefore a 
high switching frequency would be accompanied by an unpleasant flicker of 
electric filament lamps. Therefore the switched heating power permitted by 
the public utility company is the smaller the higher the switching 
frequency is. These requirements counteract the efforts to control the 
outlet temperature of a continuous-flow water heater of high power with 
high accuracy. 
For this reason it is known to subdivide the installed heating power in an 
electric continuous-flow water heater into at least two stages. Thereby a 
control system with a temperature sensor arranged in the outlet switches 
only a respective one of the stages for the temperature control (DE-OS No. 
28 37 934). 
From DE-OS No. 28 37 934 a device is known for controlling the outlet 
temperature of electric continuous-flow water heaters, which device 
permits an accurate control of the outlet temperature also with 
continuous-flow water heaters of high power of, for example, 22 kW. 
According to DE-OS No. 28 37 934 this is achieved in that the installed 
heating power of the continuous-flow water heater is subdivided into at 
least two stages and only one stage respectively is switched for the 
temperature control. Thus a fixed heating power is applied to the 
continuous-flow water heater, said fixed heating power being not affected 
by the control action of the temperature control system. The temperature 
control occurs by a stage of the heating power which can be substantially 
smaller than the total heating power of the continuous-flow water heater. 
The smaller heating power of this stage can then be switched with a 
correspondingly high frequency during control action, such that accurate 
maintaining of a desired outlet temperature is ensured. 
A through-flow path with a plurality of heating passages connected in 
series in the flow path is provided in the continuous-flow water heater 
known from DE-OS No. 28 37 934. Furthermore the electrical continuous-flow 
water heater comprises switching means responding to flow rate in the 
through-flow path, said switching means preferably being of the three-pole 
type and being arranged to be connected to the three phases of a three 
phase current. A temperature sensor is arranged on the outlet side in the 
through-flow path. First heating resistors are arranged in some of these 
heating passages and are arranged to be switched on by the switching means 
when a predetermined flow rate has been reached. Second heating resistors 
are arranged in other ones of these heating passages. 
A control system is provided in the continuous-flow water heater according 
to DE-OS No. 28 37 934, said control system being connected to the 
temperature sensor and controlling second heating resistors through a 
electronic power output stages in order to maintain a desired value. A 
rectifier and control stage connected to the mains frequency generates a 
square wave voltage syncronous with the mains frequency and of twice the 
mains frequency. The output signal of the control system, which is 
provided by a temperature sensor arranged in the outlet, is applied to 
comparators with stepped reference signals. The output signals of the 
comparators and the outputs of the binary counter are applied to a logic 
circuit, the electronic switching means being controlled by the output 
signal, such that an increasing number of half-waves of the mains a.c. 
voltage is periodically permitted to pass when an increasing number of 
comparators responds. In the known arrangement a binary counter with a 
single counter stage is provided for switching the heating power in two 
stages by two comparators. The logic circuit comprises an AND-gate, to the 
inputs of which the counter stage and the output of the comparator with 
the lower reference signal are applied. The logic circuit comprises also 
an OR-gate, to the inputs of which the output of the AND-gate and the 
output of the comparator with the higher reference signal are applied. The 
output of the OR-gate represents the output of the logic circuit. 
Furthermore it is known from DE-OS 28 37 934 to control part of the heating 
power continuously by means of a pulse width modulator with constant clock 
frequency. The pulse width modulator is controlled by the output signal of 
the control system. The pulse width modulator has a comparator, to which 
the output signal of the control system and a saw-tooth signal of constant 
clock frequency as reference signal are applied, such that the comparator 
supplies an output signal during the respective intervals, during which 
the output signal of the control system is higher than the reference 
signal. When a stage of the heating power is connected into circuit 
through a comparator, a signal appears at the output of this comparator 
and is applied to the comparator of the pulse width modulator as an 
additional signal superimposed to the reference signal, such that the 
heating power controlled by the pulse width modulator is reduced by the 
heating capacity of the stage connected into circuit. 
Preferably the electric continuous-flow water heater has heating coils as 
heating resistors which are arranged uninsulated in the heating passages, 
and water resistance passage arranged upstream and downstream and 
permitting the inlet and outlet of the electric continuous-flow water 
heater to be grounded. 
Further it is known from DE-OS No. 28 37 934, to provide several groups of 
heating resistors, which are arranged to be connected into circuit in 
addition to the heating resistors with a heating power continuously 
controllable as a function of the output signal of the control system and 
thus of the outlet temperature. In prior art devices constructed in 
accordance with DE-OS No. 28 37 934 the heating passages for the heating 
coils with continuously controllable heating power and the heating 
resistors arranged to be permanently switched on are provided in separate 
passage blocks, which are interconnected by connectors in the flow path. 
It is the object of the invention to simplify the design and construction 
of an electrical continuous-flow water heater of the above defined type. 
It is a more specific object of the invention to provide an electric 
continuous-flow water heater of the above defined type such that with low 
water flow rate in the through-flow path a correspondingly lower heating 
power is switched on permanently and a heating power also correspondingly 
reduced as compared with full load operation is switched on or off for 
control purposes. 
Furthermore it is the object of the invention to provide an electronic 
power output stage of the above mentioned type, in particular for 
controlling the outlet temperature of electric continuous-flow water 
heaters such that, on one hand, signals are generated, by which one or two 
respectively, of three half waves of the mains a.c. voltage are passed 
and, on the other hand, a respective one of the signals is applied through 
switching means independent of these signals, to the semiconductor relay 
for the control thereof. 
The direct current average of the current flowing through the load is to be 
zero. 
According to the invention the objects of the invention with respect of the 
continuous-flow water heater are achieved in that 
(g) the through-flow path comprises at least six geometrically parallel 
straight heating passages, which are formed in a common passage block, 
(h) the heating resistors at the ends of the heating passages are passed 
out of the passage block with first and second heating resistor ends 
respectively, 
(i) the first heating resistor ends are directly connected to terminals of 
the switching means, 
(j) the second heating resistor ends of the first heating resistors are 
interconnected and 
(k) the second heating resistor ends of the second heating resistors are 
connected to terminals of the switching means through the electronic 
output power stages. 
Modifications of the invention, which also serve to achieve this above 
mentioned more specific object, are subject matter of the corresponding 
sub-claims. 
According to the invention, the above mentioned object in respect of the 
power output stage is achieved in that 
(f) the frequency divider has a first output, at which an output signal is 
set by each third pulse and is reset by the following pulse, 
(g) the frequency divider has a second output, at which an output signal is 
set by an edge of each third pulse and is reset by the corresponding edge 
of the respective next but one pulse, 
(h) the semiconductor relay is arranged to be rendered conductive for one 
half cycle, if a signal is applied to a control input at the time of the 
cross-over of the mains voltage, and 
(i) the first or the second output of the frequency divider is applied to 
the control input by said means for controlling the semiconductor relay 
depending on a further switching state. 
The Schmitt-trigger supplies square wave pulses in the region of the maxima 
and minima respectively of the mains a.c. voltage, the edges of these 
square wave pulses being sufficiently spaced from the cross-over points of 
the mains a.c. voltage. Such a cross-over is always located between two 
adjacent pulses of the Schmitt-trigger. If an output signal is set at the 
first output of the frequency divider by such a pulse and this output 
signal is reset by the next but one pulse, the output signal is applied to 
the a.c. voltage during the intermediate cross-over. Consequently the 
semiconductor relay is rendered conductive for the half cycle following 
the cross-over. As only each third pulse of the pulse sequence sets such 
an output signal the semiconductor relay is also set only for each third 
half wave of the mains a.c. voltage. This has the consequence, that 
successive half waves connected through have opposite polarity such that 
the direct current average of the flowing current becomes zero. 
At the second output of the frequency divider a signal is set by an edge of 
a pulse and is reset by the corresponding edge of the next but one pulse. 
Consequently a signal appears at the second output of the frequency 
divider, which is applied to the mains a.c. voltage during two successive 
cross-over points. If the semiconductor relay is controlled by the signal 
at the second output of the frequency divider, a full wave of the mains 
a.c. voltage is connected through by the semiconductor relay while the 
following half wave is not connected through. Also with this mode of 
operation successive waves connected through of the mains a.c. voltage are 
in antiphase such that the direct current average of the current flowing 
through the load becomes zero. 
The signals thus made available at the frequency divider determine already 
unambigously the respective mode of operation of the semiconductor ralay. 
They can be applied to the semiconductor relay in different ways. In the 
most simple case the said means for controlling the semiconductor relay 
are formed by a selector switch, the position of which represents the said 
"further switching state". But the invention also provides the possibility 
to apply the output signals of the frequency divider to semiconductor 
relay in three phases of a three phase current through an appropriate 
logic circuit as a function of a control system output signal such that 
the total power can be switched in relatively small steps. 
Modifications of the invention are subject matter of the corresponding 
sub-claims.

The electric continuous-flow water heater as shown in FIGS. 1 to 3 has a 
through-flow path 10 from a cold water inlet 12 to a hot water outlet 14 
with a plurality of, namely six, heating pasages 16, 18, 20 and 22, 24, 26 
connected in series in the flow path. Furthermore the continuous-flow 
water heater comprises three poles switching means 28 arranged to be 
connected to three phases of a three phase current and responding to water 
flow rate in the through-flow path 10. A temperature sensor 30 is arranged 
on the outlet side in the through-flow path. 
First heating resistors 32,34 and 36 are arranged in the heating passages 
16, 18 and 20, respectively, and are arranged to be switched on by the 
switching means 28 when a predetermined flow rate is reached. 
Second heating resistors 38,40 and 42 are arranged in the heating passages 
22,24 and 26, respectively. A control system 44, which can be of the type 
disclosed in DE-OS No. 28 37 934, is connected to the temperature sensor 
30. The heating resistors 38,40 and 42 are controllable by the control 
system through electric power output stages with triacs 46 and 48 and 50, 
respectively. 
As can be seen from FIG. 3, the through-flow path 10 has six geometrically 
parallel straight heating passages 16,18,20,22,24 and 26, which are formed 
in a common passage block 52. The heating resistors 32,34,36,38,40 and 42 
are uninsulated heating coils arranged in the heating passages. The 
heating resistors extend out of the passage block 52 at the ends if the 
heating passages 16,18,20,22,24 and 26 with first heating resistor ends 
54,56,58 and 60,62, 64, and second heating resistor ends 66,68,70 and 
72,74,76. The first heating resistor ends 54 to 64 are directly connected 
to terminals 78,80,82 of the switching means 28. The second heating 
resistor ends 66,68,70 of the first heating resistors 32,34,36 are 
interconnected, the second heating resistor ends 72,74,76 are connected to 
terminals 80,82 of the switching means 28 through the triacs 46,48,50 of 
the electronic power output stages. 
As can be seen from FIG. 1, the first heating resistor ends 54,56,58 of the 
first heating resistors 32,34,36 are connected to one terminal each 80 and 
82 and 78, respectively, of the three-pole switching means 28. Because the 
second heating resistor ends 66,68 and 70 of the first heating resistors 
32, 34,36 are intersconnected, the first heating resistors 32,34 and 36 
are arranged in a star circuit, when one phase of the three-phase current 
is applied to each of the three terminals 78,70 and 82 of the switching 
means 28. Of the second heating resistors 38,40, 42 the first heating 
resistor end 64 of a heating reistor 42 is connected to a first terminal 
78 of the switching means 28 and the first heating resistor ends 60 and 62 
of the two other heating resistors 38 and 40 are connected to a second 
terminal 82 of the switching means 28. Furthermore, of the second heating 
resistors 38,40,42, the second heating resistor ends 76 and 74 of the said 
one heating resistor 42 and one of the two other heating resistors, namely 
of the heating resistor 40 are connected to a third terminal 80 of the 
switching means 28 through one triac each 50 and 48, respectively, which 
forms part of the electronic power output stage. The second heating 
resistor end 72 of the heating resistor 38 is also connected to the said 
first terminal 82 through a triac 46. The switching means 28 comprise flow 
rate-controlled switch contacts 84 and 86 and close at a first lower flow 
rate in the through-flow path. The first and the third terminal 82 and 80, 
respectively, of the switching means 28 are arranged to be connected to 
respective phases of the three-phase current through the switch contacts 
84 and 86. The switching means 28 comprise a further flow rate-controlled 
switch contact 88 and closes at a second, higher flow rate in the 
through-flow path 10. The second terminal 78 of the switching means 28 is 
arranged to be connected to the remaining phase of the three-phase current 
through the switch contact 88. 
Furthermore a three-pole pressure controlled switch 90 is provided, through 
which all three phases of the three-phase current are switched off, when 
water pressure breaks down. In FIG. 1 the switch contacts 84 and 86, on 
one hand, and the switch contact 88, on the other hand, are controlled by 
one sensor each 92 and 94, respectively, responding to the flow rate. In 
practice the sensors 92 and 94 and the switch contacts 84,86 and 88 form a 
unit. First the switch contacts 84 and 86 and then the switch contact 88 
are successively closed by a biased diaphragm exposed to the differential 
pressure in the through-flow path. 
Closing of the switching contact 88 is signalled to the control system 44 
through a line 96. 
The arrangement described operates as follows: 
In each case, a fixed heating power is switched on through the first 
heating resistors, and a controllable heating power is switched on, at the 
same time, through the second heating resistors. However two modes of 
operation result depending on the flow rate in the through-flow passage 
10: Only the switch contacts 84 and 86 of the switching means 28 are 
closed when the water flow rate is low. In this case the heating resistors 
32 and 34 are connected in series between two phases of the three-phase 
current. Thereby a relatively low fixed heating power results. At the same 
time, a controllable heating power is generated in the heating passage 26 
by the heating resistor 42. The circuit is closed through the switching 
contact 84 to the terminal 80 through a line 98 and the triac 50 to the 
second heating resistor end 76 of the heating resistor 42, further through 
the heating resistor 42, the heating resistor end 64 and a line 100 to a 
terminal 82 and the switching contact 86 of the switching means 28. Thus, 
with low flow rates, heating with a low fixed heating power through 
heating resistors 32 and 34 and with a relatively low variable heating 
power through the heating resistor 42 will result. The circuit of the 
heating resistors 36,38 and 40 are broken by the switching contact 88. 
The switching contact 88 closes also when the flow rate exceeds a certain 
higher threshold value in the through-flow path 10. Now a circuit as 
illustrated in FIG. 2 is established. The three first heating resistors 
32,34 and 36 are arranged in a star connection, as has already been 
mentioned, while the heating resistors 38,40,42 form a delta circuit with 
the triacs 46,48 and 50, respectively connected in series. In a 
continuous-flow water heater of 18 kw total power, the basic power, for 
example, which is provided by the heating resistors 32,34 and 36 can 
amount to 6 kW while the controllable power amounts to 12 kW in delta 
curcuit. 
The power output stage of FIG. 4 is particularly adapted for an electric 
continuous-flow water heater of the above described type, for example, 
with heating resistors being arranged in the heating passages, and has a 
rectifier bridge 110, to which mains-frequency alternating voltage 
in-phase with the mains-frequency is applied through a transformer 112. 
The rectifier bride 110 supplies a d.c. voltage pulsating with twice the 
mains frequency at an output 114. This pulsating d.c. voltage is applied 
to a Schmitt-trigger 116. The Schmitt-trigger 116 generates a square wave 
pulse sequence, which is illustrated in the second line of FIG. 5. The 
Schmitt-trigger is triggered as long as the signal applied thereto exceeds 
a predetermined threshold value. That means, that the square wave pulses 
118 are essentially symetrical to the maxima and minima of the mains a.c. 
voltage and spaced from the cross-over points of the mains a.c. voltage. 
The square wave pulse sequence is applied to a frequency divider 121. 122 
designates a semiconductor relay with zero voltage switch performance. The 
semiconductor relay 122 comprises a triac 124, which is applied to the 
mains a.c. voltage in series with a load. Means 128 are provided for 
controlling the semiconductor relay 122 as a function of output signals 
from the frequency divider 121. In the described arrangement the frequency 
divider 120 consists of two JK flip-flops. Only two outputs of the four 
outputs of the JK flip flops are used. An output signal 132 appears at a 
first output 130 (FIG. 5), which is set by an edge, here the front edge, 
of each third pulse 118 of the square wave pulse sequence and is reset by 
the corresponding edge of the following pulse 118. Thus the output signal 
at the output 130 is a pulse sequence as illustrated in the third line of 
FIG. 5. An output signal 136 appears at a second output 134 of the 
frequency divider 120. This output signal is set by an edge of each third 
pulse 118 of the square wave pulse sequence and is reset by the 
corresponding edge of the next but one pulse, respectively. The output 
signal 136 supplies a pulse sequence as illustrated in the fourth line of 
FIG. 5. The semiconductor relay 122 is arranged to be rendered conductive 
for one semiconductor cycle when a signal is applied to a control input 
138 at the time of the cross-over of the mains voltage. Such semiconductor 
relays are known per se and commercially available. Therefore the 
construction of the semiconductor relay 122 is indicated only 
schematically. 
As can be seen from FIG. 5 a cross-over 120 of the mains a.c. voltage is 
located between two adjacent pulses 118 of the square wave pulse sequence. 
Each pulse of the output signal 132 coincides with such a cross-over. 
Between successive pulses of the output signal 132, there are always two 
cross-over points of the mains a.c. voltage in which no output signal 132 
appears at the output 130. Each pulse of the output signal 136 at the 
output 134 of the frequency divider 120 extends over two successive 
cross-over points of the mains a.c. voltage. A cross-over of the mains 
a.c. voltage during which no signal appears at the output 134, is located 
between successive pulses of the output signal 136. If the semiconductor 
relay 122 is controlled by the first output signal 132, the semiconductor 
relay becomes conductive during one half cycle of the mains a.c. voltage, 
which is then followed by two half cycles during which the semiconductor 
relay 122 does not pass current. If the semiconductor relay 122 is 
controlled by the second output signal 136 at the output 134, it becomes 
conductive during two successive half cycles, which are followed by a half 
cycle during which the semiconductor relay 122 is not conductive. Then the 
current through the load 126 has the wavw form illustrated in the last but 
one line of FIG. 5. In the embodiment of FIG. 4. The means 128 for 
controlling the semiconductor relay are formed by a selector switch, the 
switch arm 140 of which is optionally connected to ground (signal "L"), to 
the first output 130, to the second output 134 of the frequency divider 
120 or to a continuous signal "H". The switch arm 140 is connected to the 
input 138 through a resistor 142. 
Different wave forms can be generated through the load 126 with such an 
arrangement, as illustrated in FIG. 6. If the switch arm 140 is 
continuously connected to the signal "L", the triac 124 is continuously 
rendered non-conductive. No current flows. It the switch arm 140 is 
connected to the output 130 of the frequency divider 120, each third half 
wave of the mains a.c. voltage is passed to the load 126, as illustrated 
in the first line of FIG. 6. If the switch arm 140 is connected to the 
third output 134 of the frequency divider 120, a wave form an illustrated 
in the second line of FIG. 6 result in the load 126, as has been already 
explained with reference to FIG. 5. If the switch arm 140 is connected to 
the continuous signal "H", the triac 124 continuously remains conductive. 
All of the mains a.c. voltage is passed to the load 126, the associated 
heating resistor in the embodiment. Accordingly the load 126 is switched 
off, energized with 1/3 of the power, with 2/3 of the power or with the 
full power depending on the position of the switch arm 140. As can be seen 
from FIG. 6, the direct current average of the current flowing through the 
load 126 is zero in each case. 
In the embodiment of FIG. 7 the "means for controlling the semiconductor 
relay" for one semiconductor relay 144 comprise a first AND-gate 146, one 
input 148 of which is connected to the first output 130 of the frequency 
divider, and a second AND-gate 150, one input 152 of which is connected to 
the second output 134 of the frequency divider 120, and an OR-gate 153 
with three inputs 154,156 and 158. Furthermore, a comparator 160 is 
provided, which supplies output signals to a first, a second and a third 
output 164 and 166, and 168 respectively, as a function of an input signal 
at the input 162, when stepped threshold values are exceeded. The first 
output 164 of the comparator 160 is connected to the other input 170 of 
the first AND-gate 146. The second output 166 of the comparator 160 is 
connected to the other input 172 of the second AND-gate 150. The outputs 
of the AND-gates 146 and 150 are connected to the first input 154 and the 
second input 156, respectively, of the OR-gate 153. The third output 168 
of the comparator 160 is connected to the third input 158 of the OR-gate 
153. The output 174 of the OR-gate 153 controls the semiconductor relay 
144. 
A signal "H" appears successively at the outputs 164,166 and 168 when the 
input signal rises. An output signal appears at the first output 164 of 
the comparator 160 with a relatively small input signal corresponding to a 
relatively small error, for example. Therefore the signal 132 (FIG. 5) of 
the output 130 of the frequency divider 120 becomes effective at the input 
154 of the OR-gate 153 through the AND-gate 146, while no signal is 
applied to the inputs 156 and 158 of the OR-gate 153. The signal (132), 
which thus appears at the output 174 of the OR-gate 153, renders the 
semiconductor relay 144 conductive during each third halve wave so that a 
current corresponding to the first line of FIG. 6 flows through the load. 
A signal "H" appears also at the output 166 of the comparator 160 when the 
signal at the input 162 rises further. Now the signal 136 of the output 
132 of the frequency divider 120 becomes effective through the AND-gate 
150 at the input 156 of the OR-gate 153 and thus also at the output 174 of 
the OR-gate 153. The semiconductor relay 144 is rendered conductive during 
two out of three half cycles of the a.c. voltage such that a current 
corresponding to the second line of FIG. 6 flows through the load. Finally 
when the signal at the input 132 rises further, a signal "H" also becomes 
effective at the third output 168 of the comparator 160, which is 
connected directly to the third input 158 of the OR-gate 153. Therefore a 
continuous signal "H" appears at the output 174 of the OR-gate 153. The 
semiconductor relay 144 is rendered conductive continuously, such that the 
full mains a.c. voltage becomes effective at the load 126. 
In the preferred embodiment illustrated in FIG. 7, three semiconductor 
relays 176, 178 and 144 are provided for one phase of a three-phase 
current. The comparator 160 has three groups of outputs 180, 182,184 and 
186,188,190 and 164,166,168. Each of these groups of outputs is applied to 
a logic circuit respectively 192,194,196 of two AND-gates 198,200 and 
202,204 and 146,150 and one OR-gate 206,208,153, said logic circuit being 
wired in a way described with reference to group 196 and having also 
applied thereto the outputs 130,134 of the frequency divider 120. One of 
the three semiconductor relays to controllable by each of the logic 
curcuits 192,194 and 196, respectively. In the described arrangement the 
sign "H" appears successively at the outputs 180, 182,184,186,188,164,166 
and 168, when the signal at the input 162 rises. Therefore the 
semiconductor relay 176 is controlled, at first, in three steps through 
the logic circuit 192 in the way described until the phase of the 
three-phase current controlled by the semiconductor relay 176 is fully 
pased, when a signal appears at the output 184. When the input signal 
rises further the second phase of the three-phase current is controlled in 
three steps through the semiconductor relay 178 in the way described, 
while the first phase continues to be fully connected. Finally the third 
phase of the three-phase current is controlled trough the outputs 164, 166 
and 168 by means of the semiconductor relay 144. Thus the installed power 
can be switched in nine steps altogether. In the illustrated embodiment 
the three phases of the three-phase current are successively connected 
into circuit step-by-step. Instead of this it is also possible to switch 
on each of the three phases successively with one third of the power, then 
each of the phases successively with two thirds of the power and finally 
each of the phases with full power. 
Another possibility (when a comparator with three outputs is used) consists 
in controlling all three semiconductor relays 176,178 and 144 at the same 
time and switching them in three steps. 
The first variant is obtained in that the connection of the AND-gates with 
the outputs 182 and 186, with the outputs 184 and 164 and with the outputs 
190 and 166 are interchanged. This variant offers the advantage that the 
three-phase current is more uniformly loaded. 
In the illustrated embodiment the input signal at the input 162 of the 
comparator 160 is formed by a continuously variable control voltage at an 
input 210 and a saw-tooth voltage from a saw-tooth generator 214 
superimposed to the control voltage in a summing point 212. Such an 
arrangement effects a pulse width modulation the power being switched back 
and forth between the two highest controlled power stages. A signal "H" 
appears at the output 190 as continuous signal, for example, while a 
signal appears at the output 164 with a pulse width, which changes 
continuously with the signal at the input 210. In this way the power 
supplied can be changed continuously. The pulsed power corresponds to the 
power difference between two successive stages. The frequency, the power 
is pulsed with, is determined by the saw-tooth generator 214. 
A temperature control for an electric continuous-flow water heater can be 
provided with an arrangement of the type described, with which the 
requirement of DIN EN 50006/VDE 0838 are met.