Power off timer for Electronics ММЦ-01
Encyclopedia of radio electronics and electrical engineering / Clocks, timers, relays, load switches
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In order to save the battery, and also because of the unreliability of the power switch, I decided to make a power off timer for my domestic multimeter "Electronics MMTs-01" according to one of the schemes described in [1].
Each of them has both advantages and disadvantages. The first circuit (author I. Nechaev) provides a clear power off, but a large voltage drop across the key inverters does not allow full use of the battery life. The second circuit (author S. Petrov), on the contrary, provides a minimum voltage drop across the key, but is distinguished by a long smooth shutdown time (about 30 s). Therefore, it was decided to combine the advantages of these devices, getting rid of the disadvantages. The timer circuit and its connection to the multimeter are shown in the figure.
The principle of operation of the device is similar to the prototypes: when the button SB1 is pressed, the capacitor C1 is charged, which gradually discharges until the logical inverter DD1 is switched and, accordingly, the electronic key DA1 is turned off. Given that the inverters DD1 in the circuit in Fig. The 1 in [1] are no longer key to supply current, there is no need to connect them in parallel and only one of them can be used. The inputs of free elements should be connected to one of the outputs of the power source.
The device is assembled on a printed circuit board made of double-sided foil fiberglass with dimensions of 24x26 mm using the volume-hinged mounting method. On one side of the board are all the elements of the timer and fuse FU1 from the multimeter. On the other - a contact block from the Krona battery. At the corners of the board, 11 mm high supports are soldered, which at the same time are conductors connecting both sides of the board. The device is connected to the multimeter board with a harness made of a flexible wire in fluoroplastic insulation, and is installed in the power compartment instead of the standard terminal block.
The device uses a Philips HEF4093BP chip, analogous to K561TL1. As C1, a capacitor from the K53-19 series with a capacity of 100 μF for a voltage of 6,3 V was used. As tests have shown, it reliably operates from a freshly charged battery with a voltage of about 10 V. With a real capacitor capacitance of 130 μF, the delay time is about 3 minutes when consuming current multimeter is about 7 mA. The voltage drop across the key does not exceed 40 mV.
All resistors - MLT-0,125. The MP-1 microswitch installed instead of PD7-9 was used as the SB1 button. The numbers of the contact pads on the board of the multimeter A1 (see Fig.), as well as the contacts X1.3 - X1.6 correspond to the designations on the multimeter factory circuit [2].
Literature
- Power off timers in a digital multimeter. - Radio, 2001, No. 9, p. 28.
- Multimeter MMTs-01 instruction manual. (Appendix 2).
Author: S. Rebrushkin, Saransk, Mordovia
See other articles Section Clocks, timers, relays, load switches.
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