Timer for coffee grinder. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Clocks, timers, relays, load switches

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The Swedish-made Zauber coffee grinder used by the author allows continuous operation for only 1 minute with a mandatory long break to cool the electric motor. The attached instructions do not say anything about the duration of the break, but according to rough estimates, it should be at least 30 minutes. In order not to accidentally overheat the coffee grinder, it can be equipped with a timer, the diagram of which is shown in fig. 1.

To the left of the dash-dotted line is a diagram of the coffee grinder itself, the rest is a timer that forcibly limits its operation time. When you press the SA1 button, which turns on the M1 electric motor, the AC voltage is supplied to the diode bridge VD1 -VD4, the diagonal of which includes a powerful trinistor VS2 and all other timer nodes.

Immediately after switching on, the capacitor C4 is discharged, the field effect transistor VT1 is closed. The current flowing through the resistor R4 opens the low-power trinistor VS1, in turn, it closes the control electrode circuit of the trinistor VS2, allowing it to open in each half-cycle of the mains voltage. The coffee grinder is working.

Fig.1 (click to enlarge)

As capacitor C4 charges, the voltage between the gate and source of transistor VT1 increases. When it reaches the threshold, the transistor will open and part of the current that previously flowed through the control transition of the trinistor VS1 will flow through it. As a result, both trinistors in each half-cycle will open later than in the previous one. The charging current of the capacitor C4 will increase, and the process will go like an avalanche. It will end with a complete cessation of the opening of trinistors. The grinder motor will stop. The zener diode VD5 limits the voltage across the capacitor C4 and the gate of the transistor VD1 at an acceptable level for these elements.

With the ratings of the elements R2, R3, C4 indicated in the diagram and the zener diode VD5 of the specified type (stabilization voltage 7,5 V), the coffee grinder's operating time before automatic shutdown is about 1,5 minutes. It is impossible to turn it on again by releasing the SB1 button and pressing it again until the capacitor C4 is discharged through the resistor R2 to a voltage below the threshold when the button is released. This will take approximately 20 minutes. After that, it becomes possible to turn on the grinder for a short time, depending on the degree of discharge of the capacitor C4. And only after 90 minutes the original duration of continuous operation will be fully restored.

The zener diode VD5 is necessary for a small delay in the opening of the trinistors at the beginning of each half-cycle of the mains voltage, which guarantees the charging of the capacitor C4 at the required speed without selecting resistors R2-R4, which compensates for the spread of the parameters of the trinistors.

Fig. 2

Varistor RU1 is installed to protect the grinder motor and timer SCRs from self-induction voltage surges that occur when switching an inductive load - the motor winding. The classification voltage of the varistor can be in the range of 390...470 V.

The timer can be mounted on a 65x42mm printed circuit board shown in fig. 2. This board is placed inside the coffee grinder housing, where there is enough free space for it.

Diodes RL205 with a coffee grinder motor power of up to 400 W can be replaced with RL206, RL207, 1N5406, KD226E, KD257V and other similar ones. Diode replacement 1 N4148 - any silicon low power diode. Instead of zener diodes 1N4737A and 1N4740A, other medium power ones with approximately the same stabilization voltage (7,5 and 10 V, respectively) can be used. The duration of the ban on restarting the coffee grinder depends on the stabilization voltage of the VD5 zener diode. To reduce it, you need to set the zener diode to a lower voltage (but not less than the threshold voltage of the transistor VT1), and to increase it, to a larger one (but not more than 15 V).

The field effect transistor KP501B can be replaced by any of the series KP501, KP502, KP504, as well as ZVN2120, BSS124. When installing, you should take into account the differences in the pinout of these types of transistors.

Trinistors KU221V were used in domestic TV sets UPIMTsT-61/67. These trinistors are distinguished by reliable operation with inductive loads and resistance to significant short-term current overloads, which can occur, for example, when the motor shaft is jammed or the breakdown of its windings. You can install any trinistor of the KU221 series in the timer or try to use an imported one, for example MCR225-10FR

At the time of checking the manufactured timer, it is possible to connect to it as a load not the coffee grinder electric motor, but an incandescent lamp of 100 ... 300 W.

Author: A. Butov

See other articles Section Clocks, timers, relays, load switches.

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