USB connector in the laboratory power supply. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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Laboratory power supplies (PSUs) with adjustable output voltage are usually used only for debugging and repairing electronic equipment; they are rarely used to power devices on an ongoing basis. This is due to the fact that an increased voltage can be accidentally set at the output of such a PSU, which is dangerous for the connected load.

To expand the functionality of the laboratory PSU, I suggest equipping it with a USB socket, to which you can connect various mobile devices to power them and charge the batteries built into them. To avoid damage to such a load, a switch should be built into the power supply unit that automatically connects this outlet to the stabilizer only if a voltage close to 5 V is set at its output. it is undesirable to install an additional linear or switching regulator designed for an output voltage of 5 V at a load current of at least 0,5 A.

The switch diagram is shown in fig. 1, and the diagram of its connection to the PSU - in fig. 2 (here A1 is the electronic voltage regulator of the laboratory PSU, A2 is the described device, C1 and C2 are filter capacitors).

Rice. 1 (click to enlarge)

Fig. 2

On the DD1 chip (see Fig. 1), a generator controlling the signal switch is assembled. Element DD1.1 is used as a two-threshold voltage comparator [1]. If the output voltage of the stabilizer A1 is in the range of 5,2 ... 5,6 V, there is a log at the output of the DD1.1 element. 1. With a smooth change in the voltage at the inputs, the trigger effect when switching voltage levels at the output of DD1.1 is weakly expressed, so the control signal passes through three more logic elements included by the inverters. When the output DD1.1 - log. 1, at the output DD1.2 - log. 0, and at the output of the elements connected in parallel DD1.3 and DD1.4 - log. 1. At the same time, transistors VT2 and VT3 are open, and a voltage of about 1 V is supplied to the load connected to the XS5 socket (its presence is signaled by the HL2 LED).

If the current consumed by the load exceeds 80 mA, which usually corresponds to the charging mode of the battery built into the multimedia device, then the voltage drop across the resistor R7 is enough to open the transistor VT1, it opens and the HL1 LED included in its collector circuit lights up. If the voltage at the output of the stabilizer is less than 5,2 or more than 5,6 V, then a log is set at the output of the DD1.1 element. 0, at the output DD1.2 - log. 1, and at the outputs DD1.3, DD1.4 - log. 0, so the transistors VT2 and VT3 are closed, the load is de-energized and the LEDs go out.

A parallel stabilizer is assembled on the VT4 transistor, the R13 resistor and the VD5 zener diode, which protects the load from overvoltage in the event of a malfunction of the control nodes. Capacitors C3, C4 reduce the sensitivity of the DD1.1 element to interference, and also prevent its self-excitation. The presence of the resistor R4 makes the adjustment of the thresholds DD1.1 trimming resistors R3, R5 smoother. The Schottky diode VD4 reduces the voltage rise across the resistor R7 with increasing load current. The use of a germanium transistor VT1 allows the use of a lower resistance resistor R7.

The elements of the DD1 chip are powered by a voltage of about 6,85 V from a parametric stabilizer assembled on a VD2 zener diode and a resistor R1. Capacitors C1, C2, C5, C6 - blocking in power circuits. Diode VD1 prevents premature discharge of capacitor C2 when the power supply is turned off. The VD3 diode protects the inputs of the DD1.1 element from possible damage if a microcircuit of the first years is used (without built-in protective diodes).

All parts of the device, except for LEDs and resistor R14, are mounted on a fiberglass board with dimensions of 47x28 mm (Fig. 3). Mounting - double-sided hinged with thin colored stranded wires in PVC insulation. The wires through which the load current flows must have a cross section of at least 1 mm².

Fig. 3

The fixed resistors MLT, C1-4, C1-14, C2-23 and for surface mounting can be used in the switch (one of them - R14 - is soldered to pins 2 and 3 of the XS1 socket, the rest are installed on the connection side of the board). Trimmer resistors - any small-sized. Oxide capacitors - K50-68, K53-19 or imported analogues. The remaining capacitors are ceramic for surface mounting. Capacitor C1 is installed in close proximity to the power pins of the DD1 chip.

Diodes KD522B are replaceable by any of KD510A, KD521A-KD521D, KD522A, KD522B, as well as imported 1N4148, 1N914, 1SS244; Schottky diode

MBRS130LT3 - any of MBRS140T3, 1N5817-1N5819, SB120-SB160. Instead of the KS168A zener diode, 1N4736A, TZMC-6V8, KS126I, KS407D, KS468A are suitable, and instead of 1N4734A - KS156G, BZV55C-5V6, TZMC-5V6. LEDs RL30-SR114S (red glow) and RL30-YG414S (green) can be replaced by any similar continuous glow, for example, KIPD66, KIPD21 series.

Possible replacement of the 2SC2458 transistor - any of the BC547, 2SC3199, SS9014, KT6111, KT6114 series, GT115G germanium transistor - any of the 1T321, GT321, MP25, MP26 series. We will replace the SS8550D transistor (collector-emitter saturation voltage - no more than 0,2 V at a collector current of 0,5 A) with any of the SS8550, KT684, KT686 series (the greater the current transfer coefficient of the base of this transistor, the better). If instead of a bipolar one in place of VT3, a field-effect transistor with a p-type channel (for example, IRF4905, as shown in Fig. 4) is used, then at a load current of 0,5 A, no more than a few millivolts will fall on it. The KT815B transistor can be replaced with SS8050, BD139 or any of the KT815, KT817, KT646 series. When choosing transistors for a device, it should be remembered that those recommended for replacement can be made in other cases and have a different pinout than those indicated in the diagram.

Fig. 4

Instead of the K176LP2 chip, you can use the K561LP2; the operation of the device with their imported counterparts has not been tested.

The resistance of the resistor R1 is chosen so that when the load is connected, the current through it does not go beyond 10 ... 20mA. On fig. 1, its resistance is indicated for the voltage across the capacitor C1 (see Fig. 2) of about 25 ... 30 V.

Establishing the device is reduced to setting the thresholds for the operation of the comparator. Temporarily disconnecting the VD5 zener diode and not connecting the load to the XS1 socket, set the trimmer resistor R3 engine to such a position that the HL2 LED lights up when the stabilizer output voltage is more than 5,2 V. Then this operation is repeated with the trimmer resistor R5, but its engine is set to such a position that the HL2 LED lights up when the voltage at the output of the stabilizer is less than 5,6 V.

If a field-effect transistor is installed in place of VT3 (Fig. 4), then the comparator operation thresholds are chosen equal to 5,0 and 5,4 V, respectively.

The described device can work in conjunction with a power supply unit, in which, when the load current changes within acceptable limits, the change in the output voltage is several times less than the specified interval (0,4 V). This can be provided, for example, by laboratory power supplies with linear and switching voltage stabilizers, assembled according to the schemes [2, 3]. The device is connected to the nodes of the voltage stabilizer with the shortest possible wires with a copper cross section of at least 1 mm². The author's copy of the device was tested together with the indicated PSUs at a load current of up to 2 A (for a short time), self-excitation of the elements of the DD1 microcircuit did not occur.

If a multimedia device, for example, an MP1 player or a mobile telephone, is connected to the XS3 socket, and UMZCH is connected to the output of the modified PSU, then it is possible to connect its input to the audio output of the mobile device only if the mobile device has a single common wire - "minus " to both the USB socket and the headphone jack (which is rare), otherwise the unit may be damaged.

By slightly changing the circuit, such a switch can be built into devices powered by external power supplies if they are critical to the appearance of an abnormal supply voltage for them.

Literature

1. Leontiev A. Signaling device on a two-threshold comparator. - Radio, 1992, No. 5, p. 36-38.
2. Butov A. Laboratory power supply with self-resetting fuse protection. - Radio, 2005, No. 10, p. 54-57.
3. Butov A. Laboratory switching power supply on the L4960 chip. - Radio, 2011, No. 11, p. 27, 28.

Author: A. Butov

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