Load shedding device

Quite often, not very pleasant things happen in the home workshop: the home ones distract you from exciting activities, considering them a waste of time. Therefore, you have to drop everything in a half-step and run to do emergency housework.

And everything would be fine if you use only screwdrivers, wrenches or chisels and a plane. But, if in the process of work is used soldering iron and devices powered by the network, then in the process of such shoots the doubt often creeps in: “Did I turn off the soldering iron or some heating element with which I was debugging the thermostat?”. Indeed, such forgetfulness often leads to burns, electrical injuries, and even to a fire.

So that such doubts do not arise and a peculiar time relay. It can be used with other equipment, for example, with a TV. True, other developments are known for the TV, but this one is quite suitable.

Device Operation Algorithm load shedding quite simple. After the set time, about one and a half to two hours, the device starts to give an annoying sound signal, which is very difficult not to notice. If within a certain time, about five minutes, press the button, the sound signal will stop and the device will remain on for another two hours. Otherwise, the device will disconnect from the network itself and de-energize the load.

A schematic diagram of the device is shown in Figure 1.

Figure 1. Load shedding device

In fact, the device is a regular timer. The main node of the timer is a counter on the chip D1, which counts the pulses generated by the generator, executed on the elements D2.1 D2.2. But first things first.

When you press the button S1, the mains voltage is supplied to the primary winding of the transformer T1. The voltage of the secondary winding rectified by the diode bridge VD2 is smoothed by capacitor C4 and stabilized by a parametric stabilizer on resistor R3, capacitor C3 and Zener diode VD1. This voltage is used to power the chips.

A positive voltage drop across the differentiating circuit R1 C1 goes to the reset input of the counter R (pin 11), which brings the counter D1 to zero - the logic zero voltage is at all outputs of the counter.

Logical zero at the input 12 of the element D2.4 leads to the appearance of a logical unit at its output 11, which opens the transistor VT1. Through an open transistor, relay P1 is turned on, which, with its contact, turns on the load, and in addition, keeps the device itself in the on state. Logical units and zeros for refreshing information can be found in the series of articles “Logic Chips”.

It would seem that the inclusion of a load using a relay is not entirely modern. Now more common triacs, thyristors and solid state relays. But the whole point is that the load connected to the described device can be 100 or more watts, and only 1 ... 2 watts.

In addition, the load can be purely inductive (primary transformer winding, magnetic starter coil). Therefore, with a powerful load, a low-power thyristor switch will heat up, and a low-power load can consume a current lower than the holding current of the thyristor - the load simply will not turn on.

With inductive load, you have to install additional RC - chains, otherwise the load will simply rattle. This is most noticeable when you turn on the magnetic starter - it works like an electric bell. With such versatility of loads, “contact” switching is the most simple and fully justified.

After all the described events, the generator starts operating on the elements D2.1 D2.2.With the values ​​of resistor R2 and capacitor C2 indicated on the diagram, the pulse frequency is about 1.5 Hz. If necessary, a more accurate selection of the frequency is made by changing the value of the resistor R2.

These pulses are fed to the counting input C (pin 11) of the counter D1. When the 8192nd impulse arrives at the counter input, its logical unit level is set at pin 3. It is easy to calculate that at the indicated pulse repetition rate this will happen approximately one and a half hours after the entire device is connected to the network.

This logical unit will go to input 9 of the element D2.3. it will allow the passage to the output of the element D2.3 pulses from the output 9 of the counter D1, which with a frequency of 0.75 Hz through the element D3.1 allow and prohibit the operation of the generator on the elements D3.2 D3.3. As a result, the piezo emitter F1 emits packets of pulses with a frequency of about 1000 Hz. This is the very annoying sound that was mentioned above.

If during this sound you press the S2 button, the supply voltage will be applied to the reset input of counter D1, which is equivalent to supplying a logical unit, the counter will reset and everything will start to work as if the power was turned on. The load will remain on.

But what happens if the S1 button is not pressed in time? In this case, the counter will continue to count further. At the same time, pay attention that the logical unit will remain at output 3, because 8192 pulses have already been counted! When another 512 pulses are counted, a logical unit will appear at the output of counter 14. At the indicated pulse frequency of the generator, this will take another 5 minutes. This will be the beep time.

Now at the inputs 12 and 13 of the element D2.4 there will be two logical units, which will lead to the appearance on its output of 11 a level of logical zero. Therefore, the transistor VT1 will close and disconnect the relay P1, which with its contact will disconnect the load and the device itself.

Details and design. It is best to place all parts except the transformer on a printed circuit board. You can also make the entire installation mounted. To do this, you can glue the microcircuit upside down on a piece of plastic, and then unsolder everything, using the findings as reference points for installation.

The transformer is suitable for anyone with a power of at least 5 watts, for example, from Chinese network adapters. The voltage of the secondary winding should be within 15 ... 17 V. As a rectifier bridge, anyone with a load current of 0.5 ... 1 A is suitable. It is also possible to simply use diodes, for example, widely used imported 1N4007. Now it’s much easier to buy such than domestic KD209.

As a relay, a relay from the remote control systems of 3УСЦТ TVs is used, which can also be replaced by an imported one, for example TIANBO. Buying such a relay now is also not difficult.

The entire structure can be placed in a plastic box of suitable sizes, which are sold in electrical goods stores. On the wall of the housing, place the outlet block and buttons S1 and S2. With serviceable parts and the absence of errors in the installation, the circuit does not require adjustment, it starts working immediately.

Boris Aladyshkin