Power Expander
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General Description
The VEX Power Expander is used for adding a secondary power source to power up to 4 VEX Continuous Rotation Motors or VEX Servos. The Power Expander enables the user to spread a VEX robot's motor load across two batteries, extending run time and increasing performance.
- see the Product Page for more information
Parts Required
- 1 Power Expander
- Mounting Hardware
- PWM Extension Cables (one per motor, optional one for Status)
- Additional VEX 7.2 Battery
Installation Instructions
- Mount the VEX Power Expander securely to the robot using standard VEX hardware (not included).
- Provide power to the unit by connecting a 7.2v VEX Battery Pack, or a 7.2v VEX Battery Holder to the power connector. The Power Expander LED should flash green briefly signaling power is being supplied to it.
- Connect VEX PWM Cables from the Microcontroller Motor Ports to the “In” port of the Power Expander. The Input Ports (A-D) connect to any four of the Motor Ports (#1-8) of the VEX Microcontroller. In the picture, Microcontroller Motor Port 5 is connected to VEX Power Expander Input Port A. The Transmitter channel that controls the Microcontroller Motor Port 5 will now control the Power Expander Motor Output A.
- Note: A "Keyed" connector and will only insert in one (correct) orientation. When using Non-Keyed PWM Cables on the VEX Power Expander, the Gnd/Blk wire should be closest to the key slot on the STATUS and IN Ports, and it should be the furthest from the key slot on the OUT ports.
- Connect VEX Continuous Rotation Motors or VEX Servos to the corresponding “Out” ports of the Power Expander. Slide the Power Expander PWM Lock to secure cables.
- Note: VEX Motors & Servos have a “keyed” connector and will only insert in one (correct) orientation
- Note: VEX Motors & Servos have a “keyed” connector and will only insert in one (correct) orientation
- Turn on your Microcontroller. The feedback LED should now show the status of the Power Expander and motors/servos should respond normally based on the secondary battery power.
- The VEX Power Expander includes a Status port that can be connected to an Analog input on the VEX Microcontroller. This can be used to determine the approximate remaining voltage of the battery connected to it. To determine this value, divide the Power Expander read-out by 70.8.
- Example: Using a debugging program while connected to the Microcontroller returns a value of 460.
- 460 / 70.8 = 6.5.
- 6.5V is the current voltage of your Power Expander battery.
- Example: Using a debugging program while connected to the Microcontroller returns a value of 460.
Circuit Break - LED Status Indicator
The VEX Power Expander also incorporates an internal circuit breaker to prevent damage to the unit or connected devices. The onboard LED provides its current status. Refer to the following LED status chart for more information.
| Green | Ready |
| Yellow | Battery Low |
| Red | Battery Critical |
| Slow Red Blink | Circuit Breaker Tripped |
| Fast Green Blink | Circuit breaker was tripped - recovered |
| Fast Yellow Blink | Circuit breaker was tripped - battery low |
| Fast Red Blink | Circuit breaker was tripped - battery critical |
Background
Disclaimer: The author of this section (Eric Sklar) has neither examined the Power Expander nor reviewed its design. This material was prepared from general knowledge of battery-based power supplies and PWM-controlled model motors and servos.
What problem(s) does this address?
The Power Expander addresses limitations in both energy and power.
How does the Power Expander increase the available energy?
Nominally, the energy available from a battery is the product of the voltage, current, and operating time. Batteries are usually labelled with the voltage and "capacity". Capacity is the product of current and operating time and is usually stated in ampere-hours (Ah). For small batteries, the unit is usually milliampere-hours (mAh). If you have two batteries of the same voltage and capacity, then the total energy is twice the energy of each battery. If you arrange the two batteries in series, you get twice the voltage, but the same capacity (current multiplied by operating time). If you arrange the batteries in parallel, you get the same voltage, but twice the capacity.
How does the Power Expander increase the available power?
If a battery were an ideal voltage source, then the voltage it provides would be constant. However, batteries are far from ideal, and the voltage decreases not only as the battery discharges, but also as the current increases. This is because, in effect, a battery is an ideal voltage source in series with a resistor. (For more on this, see a reference on Thevenin's Law (for example, in Wikipedia).) Therefore, the more current being drawn from the battery, the greater the voltage drop across the "internal resistor" and the lower the output voltage. If one arranges two matched batteries in parallel, the current being drawn from each is only half as much as for a single battery, so the voltage drop (relative to the voltage at zero current). is only half as much and the output voltage is higher. Higher voltage at the same total current equates to higher power.
It sounds, from the discussion so far, that one could place a second battery pack in parallel supplying the controller and get this benefit. In part, that should work. However, the controller has both an effective resistance (the voltage drop increases with increasing current) and some limitation on the amount of power it can handle. Therefore, it is more effective to avoid running the current from the second battery through the controller.
There is also a benefit related to battery discharge. As a battery is discharged, the available voltage decreases. If you are using two batteries, then the amount discharged from each, after a fixed amount of operation, is half what it would be from a single battery. Therefore the available voltage (and, resultingly the available power) is greater. This effect would be the same whether or not the current from the second battery were to pass through the controller.
So, how can this work?
Small DC pulse width modulated (PWM) control systems use three wires: Common (sometimes called "ground"), power, and signal. These three wires form two circuits. The control circuit consists of the signal and common wires; the power circuit consists of the power and common wires.
The easy way to transfer the control signal from the controller to the motor through a power addition unit is to connect the input and output signal wires directly to each other. As the control signal requires very little power, there's no substantial benefit (in this application) to boosting it with the second battery. For the circuit to be complete, the input and output common wires must also be connected to each other.
To provide the drive power for the motor from the second battery, the output power wire is connected to one terminal of the second battery. (The input power wire does not need to be connected.) To keep all parts of the circuit operating at the same absolute voltage, the other terminal of the second battery is connected to the common wire.
By this arrangement, the control signal to a motor comes from the controller, but power to the motor comes from the second battery. This reduces the current from the first battery, increasing the voltage under load to the controller (and the loads that draw power from it), and makes the voltage under load to the motors connected to the second battery higher than it would be if only one battery were used.
The system is not, however, perfect. Because the two batteries are connected to each other on only one side (the common wires), the voltage supplied to loads that draw power from the controller is not exactly the same as the voltage supplied to loads that draw power from the Power Expander. If the system works properly, the available voltages will both be in the acceptable range, so there's no reason to expect the voltage difference to cause damage. There may, however, be enough voltage difference to cause a difference in operating speeds. Therefore, when using such a system, consider this possible voltage difference when selecting which motor gets connected to which supply. For example, it might be troublesome to connect the left tread motor(s) of a tank drive to one battery and the right tread motor(s) to the other. However, connecting both front motors to one battery and both rear motors to the other should not produce a steering bias if the voltages differ.
Note: There are several other components of the Power Expander (overcurrent protection, voltage monitoring, etc.) that have not been discussed in this Section.
- see the Product Page for more information
