How to get 5V from a Tab SumoBot powered by (4) AA batteries

Forrest

What's a good circuit to get a stable 5V from a Tab SumoBot powered by (4) AA batteries? I'm currently using a regulator diode to drop the battery voltage from 6V to 5.3V, which is good enough to drive a HD44780 based LCD. But when the servo's kick on, this voltage is reduced from 5.3V to 4.8V. While this is within the specs of the LCD (5V +/- 10%) - I can't believe this is good for long-term use. While the Tab SumoBot is driven by an embedded BS2, Vdd is not available on this robot. The BS2 is driven by Vcc regulated to 3.9V - which isn't high enough for the LCD.

allanlane5

Wow, I hadn't looked at the LM2931 (adjustable) circuit on the Tab Bot before. You are right, the output voltage is set (by two resistors) at 4 volts (approx). This lets you get more juice out of the batteries, as they can drop to 4.3 volts (typical) before you lose regulation. NEAT!

However, to your problem. I don't think the low voltage on the LCD will hurt it. You could add a large-ish capacitor (100 uF?) to get through the power-spikes from the motors. And they are DC motors on the Tab Bot, not Servo's.

Another solution would be to add a 2940-5 regulator to the LCD power circuit, instead of the diode -- but the diode probably wastes less current.

Jonb

You could try a voltage regulator IC, most have a dropout voltage that would not work with your 6v source. IMO you would be better off with a 9v battery and a good voltage regulator circuit such as:
http://www.hut.fi/Misc/Electronics/circuits/vga2tv/psu_5v.html
The 7805 in that circuit can source 1 amp with a heat sink and airflow.

You could also feed it with a wallwart.
You could then simplify the dual power source with a "steering diode" to save on batteries ttp/ourworld.compuserve.com/homepages/g_knott/elect212.htm

Post Edited (Jonb) : 1/27/2005 12:00:37 AM GMT

Ben Wirz

Yet another option would be to put a Buck / Boost switching power supply. Switching power supplies can be difficult to design from scratch but Power Trends makes several different switching power supply modules for less then $20 that would do the trick. The even have a series that are roughly the same size and pinout as the old 7805's which are easy to use. Note that Power Trends was purchased by TI a few years back but they still sell the same modules last I checked. A Buck / Boost power supply will produce a 5V output regardless of whether the input voltage is higher or lower then 5V and would be a cadillac solution to your problem.

-Ben