USB Power Supplies used for Tablet Enclosures

By: Paul Vail

3 Min Read

22 Feb, 2017

A typical USB power supply is rated at 5 volts nominal and a capacity of 1 to 2 amperes. The supply voltage will not be exactly 5 volts, but will vary depending on load (hence the amount of current being drawn). The voltage must never exceed 5.25 volts. A typical USB Power supply is rated at 10 watts (i.e., 2 amperes at 5 volts DC). When plugged directly into a generic USB supply, a tablet (or mobile phone) might appear to charge very slowly. This problem is common even when the supply has an adequate current rating. This is generally because the power supply is not a DCP power supply.

Many mobile telephones are sold with a basic charger and provide the option to upgrade to a “high capacity” charger. The “high capacity” charger will be a DCP power supply. You can use the better grade of charger provided with a Samsung tablet to charge a mobile telephone faster. A USB Power Supply with a DCP (Direct Charge Port) supply has the two USB data lines connected to indicate it is a high capacity power supply, and that more than 500 mA (milli-amperes) can be drawn.

Samsung Wall/USB Charger

The Samsung Wall/USB Charger provided with most Samsung tablets is equivalent to a USB Power Supply with a DCP. This supply is typically rated at 5.3 volts with a 2-ampere capacity. It provides 5.0 to 5.1 volts to a tablet for the range of expected currents.

Do not worry about the 5.3-volt rating, as the voltage delivered to the tablet is lower. Due to manufacturing variations, some power supplies may have a lower voltage and some a higher voltage.

The power supply current rating must be adequate for the sum of all devices to be powered. The current rating identifies the maximum capacity of the power supply. Electrical current is measure in amperes, which sometimes referred to simply as “amps”. A larger current rating does not force the attached devices to use more current. Also, the Micro USB-B connector is not intended for loads above 2 amperes.Low-cost supplies can have the voltage drop well below 4.75 volts under full load, which is ok for some types of USB devices, but not for a tablet. The voltage should not drop much below 5 volts at the input to an STS** product or TL002.

All USB power supplies are “regulated”, hence the output voltage is kept within a tightly control range for a specified range of loads. Many low-cost USB supplies do not compensate for voltage drop along the cable that attaches it to a device. Compensation for voltage drop requires using a cable with known characteristics. The Samsung Tablet Power Supply compensates for voltage drop, provided the supplied cable is not extended or replaced. There are other methods of compensating for voltage drop; however, they are not practical when using standard USB cables due to the lack of spare wires.

Be wary of cheap USB supplies with long built-in cables, as the voltage delivered under load is too low for reliable tablet charging. Supply might be rated at 5 volts and 3 amperes, however, under full load the voltage could drop to 4.4 volts or lower. Always verify the voltage at the STS**/TL002 input for the minimum and maximum currents your system will use. Be wary of the USB “cigarette lighter” power supplies. They have the same issue as other cheap USB supplies already noted. Do not rely on the device specifications. Always verify the voltage at the STS**/TL002 input for the minimum and maximum currents your system will use.

Using a generic regulated power supply requires heavy gauge wires to minimize the voltage drop between the supply and the STS**/TL002. If the voltage reaching the STS**/TL002 is too low, charging the tablet battery may fail. The wire “gauge” identifies the wire thickness. A larger “gauge” refers to a thicker wire. If there is any doubt about the voltage level reaching the STS**/TL002, it is best to measure it. The 5V Power Input on the STS** products can be used with external USB Battery Packs. A drawback to USB Battery Packs is the voltage tends to be 5.0 volts, which requires careful attention to the battery cable and the STS** to Tablet cable to avoid voltage drop problems. Recharging a USB Battery Pack requires the pack be removed from the system.

When testing with different USB power supplies, always double check the current rating stamped on the charger. Some mobile devices (small tablets and mobile telephones) are shipped with 0.7 ampere and 1.0-ampere capacity power supplies. The can look just like the 2-ampere versions. Using the smaller supply will not harm your system, but is could be why the system does not work all the time.

The STS** PoE devices use electrical energy transferred along the Ethernet cable to generate 5.1 to 5.25 volts within the STS** device. The STS** PoE devices eliminate the voltage drop issues associated with using an external power supply. Running the CAT-5 cable used for an Ethernet connection is significantly cheaper than hiring an electrician to install a new electrical outlet. Electrical outlets in the customer area of a store are a safety hazard.