Homemade long USB 2.0 cable. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Computers

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The USB 2.0 specification requires that the power wires in the USB cable have a copper cross section of 0,08 to 0,52 mm². However, many manufacturers of USB cables, in order to reduce their cost, prefer to lay wires in them with a minimum gauge. For this reason, devices that do not have their own power source, but consume significant current from the USB connector, have to be connected with short cables, the supply voltage drop on which does not exceed the allowable one. For example, external 2,5" hard drives will not work if the USB cable is longer than 80 cm. For more freedom in placing peripherals, you can make a homemade USB cable with much less power loss.

On fig. 1 shows a diagram of a homemade USB cable. All parts, except for the XP1 and XS1 connectors, are optional - they only provide ease of use and reliability of the cable. Connect the XP1 plug to the USB socket of the host device (for example, a computer). The information flow passes in both directions along the lines D+ and D- without any transformations.

Rice. 1. Scheme of a homemade USB cable

The supply voltage +5 V is supplied from the master to the slaves via the Vbus interface line through the self-resetting fuse FU1. But if the switch SA1 is open, the power supply to the slave is not supplied. In this case, the transistor VT1 is closed, VT2 is open, the LEDs HL2, HL3 of white light are on. The ability to manually turn off the power of the device connected to the XS1 socket greatly increases the convenience of its operation. And the described device with the SA1 switch open can be used, for example, to illuminate the keyboard.

When the SA1 switch is closed, the power supply is supplied to the slave device. Transistor VT1 is open, VT2 is closed, LEDs HL2 and HL3 are off, and HL1 is on, signaling the power of the device connected to the XS1 socket. Zener diode VD1 reduces the likelihood of damage to this device in the event of a power supply failure. It, together with the VD2-VD5 diodes, limits possible voltage surges on the D + and D- lines.

Resistors R2, R3 are needed not only for the operation of the node on the transistor VT1. They prevent the accumulation of charge on the D+, D-, Vbus lines relative to the GND line. Capacitors C1-C3 - blocking. They improve the stability of the connected devices, and further reduce the likelihood of damage to the USB ports of the equipment being used.

The cable itself is three-wire shielded with an outer diameter of 7 mm and with a cross section of each wire of about 0,5 mm² (Fig. 2). Its type is unknown to the author, but usually such a cable is used for connections with sensors and video cameras in security alarm and video surveillance systems. In this case, the braided shield is used as the GND wire, the blue wire is Vbus, the black wire is D+, the white wire is D-. The cable length is 3,5 m, the capacitance of each wire relative to the braid is about 460 pF. The ends of the cable should be sealed with rubber glue or BF glue.

Rice. 2. Cable

At a current of 0,5 A, the supply voltage drop on this cable did not exceed 0,34 V. For comparison, the voltage drop on an industrial USB cable 3 m long at the same current is 0,95 V. None of the two external 2,5-inch hard drives with such an industrial cable did not work. With a homemade cable, supplemented with a short (46 cm) cable with USB and miniUSB connectors, both drives worked flawlessly. When two self-made USB cables with a total length of 7 m were connected in series, both drives remained operational. However, two of the four computers used in the tests changed the USB port to which the drive was connected from USB 2.0 mode to the slower USB 1.1 mode.

Rice. 3. PCB

Most parts of the device are placed on a 30x20 mm printed circuit board, shown in fig. 3. Resistors R5, R6 are soldered directly to the terminals of the corresponding LEDs. All capacitors are multilayer ceramic for a rated voltage of at least 6 V. PMLL4446 diodes can be replaced with any of PMLL4150, PMLL4151, PMLL4153, PMLL4148, PMLL4448, 1 N4148, 1SS244, KD503A. Instead of a zener diode 1SMB5919BT3, you can install 1 N5339. Instead of white LEDs RL30-WH744D, luminous intensity 5 cd, you can install, for example, ARL-5113UWC-17CD, ARL-5213UWC-17CD-NS, ARL-5213UWC-17cd-BS, ARL-5213UWC-20cd-BS, ARL-5213UWC -20cd-NS, ARL-5213UWC-25cd, ARL-5213UWC-35cd. The RL310-DR344S LED can be replaced by any general purpose LED, for example, from the L-63, KIPD66 series.

The BCP54-16 transistor can be replaced by any of the BCP54, BCP55, BCP56, BCP68, SS8050, KT6114, KT698 series. Instead of the BC547 transistor, any of the SS9013, SS9014, 2SC3199, KT3129A, KT3130A, KT6111A will do. The transistors indicated in the options for possible replacements have differences in the types of cases and the purpose of the pins.

Resettable surface mount fuse found on a discarded computer motherboard. Similar fuses MF-R160, MF-S175, LP30-160 can be used. Switch SA1 used rocker KCD-2011. Another similar one will do. The device is assembled in a telephone socket housing with dimensions of 58x42x23 mm (Fig. 4). The USB socket, switch and LEDs are glued to the case with polystyrene dissolved in acetone.

Rice. 4. Complete device

A long USB cable with low resistance power wires allows not only to install peripheral devices in a place convenient and inaccessible to children, but also, for example, to easily place radio communication devices, WEB-cameras outside the room if the signal level inside it is weak or video surveillance is required behind walking children or protected objects.

Author: A. Butov

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