12V Power Supply Feeds My Robot

[Al Skierkiewicz]

OK Guys,
There was a lot posted since my last post. So one at a time...
There was a team at Midwest that had just such a cart, Mike. it looked like it had four, six amp chargers on a top shelf and room for 6 batteries on the bottom. The cart followed the robot onto the playing field.
Building your own (or rolling your own in experimenter's terms) does save money but you must know what you are doing. 35 amp, 12 volt power supplies are after all, 500 watt devices. The right size wiring and layout are extremely important. In these designs, even the resistance of the transformer comes into play.
Corey, the power supply you reference looks really wrong to me. The output regulation is dependent on the current through R7 being constant and for our purposes it never will be. The base voltage on the pass transistors will vary with voltage developed across R7 and therefore the output will vary proportionally. The output of a 24 Volt transformer will be more than 33 volts peak. Throw away a few volts for the drop of the internal resistance of the transformer, diode drops and R7 and you have the 7812 trying to drop almost 20 volts. At 871ma that is close to 20 watts so I disagree with the designer that the 7812 will run cold. I do agree that the currents and voltages as shown, would be the expected values with the stated load, but regulation would be comprimised with a varying load. Of course a hard regualted supply is not needed for our purposes as long as the stated maximums for the RC are not exceeded.