9.0 VS, Power Supply [T-Series Datasheet]

VS and Power Supply Specifications By Device

VS Terminals

The supply voltage (see below) goes through some protection circuitry and then is presented on the VS terminals.  The VS terminals are designed as outputs for the supply voltage.  The supply voltage is nominally 5 volts and typically provided through the USB connector.

All VS terminals are identical and connect to the same supply.

The VS terminals are outputs, not inputs. Do not connect a power source to VS in normal situations.

The max total current that can be drawn from VS is:

MaxVScurrent = ratedSupplyCurrent - DeviceSupplyCurrent

Example: If you have a typical USB 2.0 supply that is within spec (up to 500mA supply current), the T4/T7 requires up to 300mA to run, so up to 200mA would be available from the VS terminals of the T4/T7.

The voltage on VS can be noisy and can change unexpectedly. Circuits that are sensitive to changing or noisy supply voltage, such as bridge circuits, should not be supplied from VS.  The voltage on VS will also drop as higher amounts of current are drawn by peripherals powered by the T4/T7. This is due to the inline 0.1 ohm resistors that allows for the device's current draw to be measured.

Measuring Current Draw

T4/T7

One way to measure how much current the T4/T7 is drawing is by measuring the voltage across R15.  R15 is a 0.1 ohm resistor, so if you measure 0.025 volts, that means the current through the resistor is 250 mA.  R15 is a large resistor located on the top of the PCB just behind the green LED.  To measure the voltage across R15, connect the positive lead of your meter to the test point "Vhost" and connect the negative lead of your meter to the test point "Vs".

R15 is in series with the 5 volt supply from the USB connector.  If powering from J5 (see "Alternate Power Supply" in the OEM section) use R21 instead.  The "Vs" test point is also the negative for R21, but there is no positive test point so you just have to touch the upstream side of R21.

Note that the "Vs" test point is actually the Vsupply bus described under "J5 - Alternate Power Supply" in the OEM section, and technically not exactly the same as the "VS" bus documented in this section.

T8

The T8 can measure it's own supply voltage and current draw. Use the register below to get the internal measurements.

To measure current manually, measure the voltage across R22 and divide that value by 0.02.

Power Supply

T4/T7

Power supply is typically provided through the USB connector. 

• USB host or hub.

• Wall-wart power supply with USB connection (included with normal retail units—not OEM).

• Power-over-Ethernet splitter (e.g. TP-Link TL-POE10R with Tensility 10-00240 with Tensility 10-00648).

• Car charger (12V supply) with USB ports (e.g. Anker 71AN2452C-WA).

• Rechargeable battery with USB ports (e.g. Anker Astro E5 79AN15K-BA perhaps with Belkin F3U133-06INCH).

• Battery with car charger (e.g. Anker 79AN15K-BA with 71AN2452C-02WA).

• Battery with solar panel (e.g. Anker 79AN15K-BA with 71ANSCP-B145A).

• Pigtail a cable with a USB-B connector to get at the red and black wires, and make some sort of custom cable for your 5V power supply.

The supply range for specified operation is 4.75 to 5.25 volts, which is the same as the USB specification for voltage provided to a device.  Nonetheless, we have seen some USB host ports providing a lower voltage.  If your USB host port has this problem, add a USB hub with a strong power supply.

Typical power draw for a T7-Pro with everything on is 280 mA.  Running at 5 volts that is 1.4 watts.  See related data in the General section of Specifications.

USB batteries are typically specified with a mAh rating at the internal battery voltage.  A large USB battery such as the Anker PowerCore+ has a battery that provides 26800 mAh at 3.7 volts, which is 99.2 watt-hours.  If we guess at 80% efficiency for extracting this energy and converting it to 5 volts, this battery should be able to power aT7-Pro for roughly 0.8*(99.2/1.4) = 57 hours.

See information about Power over Ethernet (PoE) see the PoE App Note.

T8

The T8 has two power supply options, USB and Power over Ethernet (PoE). Both power supplies are fed into a power switch. The switch selects which supply will be used to power the T8. Selection is based on the voltage of the PoE input. When V_PoE is below the threshold voltage, the USB supply will be used. When V_PoE exceeds the threshold voltage, the PoE supply will be used. See Section A-5 for specific values.

OEM Versions

For board-level connection options see "Alternate Power Supply" in the OEM section.  Typical power supply sources include:

Normal retail units (not OEM) include a 5V, 2A wall-wart style power supply:

Note that the JX-B0520 supply is rated for 0 to 40 deg C operation.

Some VS Terminals Not Working?

See the Screw Terminals App Note for assistance.