Question Re: Power Supply for Broad Current Swings

bobledoux

I put this topic here, and not under robotics, because it deals with general current draw issues that can brownout a Stamp:

I'm building a new robot using a Stamp II and Parallax servos. I want to run it using a single 6 volt, 1.2 amp gel cell, both for servo and processor power.

The servos are powered directly by the battery, not through a voltage regulator.

My regulator for the electronics is an LM2940 with low dropout. In order to bridge motor start pulses I've proposed the following measures:

Place a 4700uf cap to the input of the voltage regulator.

Use .200 wide traces on power supply board to ensure adequate current capability.

Add 150uf to the regulator output.

Stage the motor starts by about 20milliseconds to reduce starting pulses.


Are there any other ideas?

My earlier robots used a separate 9 volt battery to power electronics. This unit has too many ancillary controllers and digital chips to do that.

allanlane5

Why put the 4700 uF cap to the INPUT of the regulator? Oh, because that's where the servo's get their juice, BEFORE the regulator. Right. Good idea.

Second, what is this tremendous motor you're concerned about? The one in the servos?

Aren't you more concerned about tracking accuracy, than in reducing the startup current pulse?

I don't think you need to 'stage motor starts', but your other actions seem like good ideas.

bobledoux

My intent, with the 4700uf cap, was to provide some support to the regulator to help prevent regulator input voltage draw down from the motors. But you make a good point. The cap effectively provides support to the motors, not the regulator. It might make more sense to add capacitance on the regulator output, but many low dropout regulators, the LM2940 included, require low ESR caps on the output. These caps tend to be tantalums and they don't come in really big sizes. I see that Xicon makes a reasonably priced low ESR electrolytic cap for about $3.

PJAllen

· Here's a way you can add a large value Cap to the input of your voltage regulator without aiding the motors [noparse][[/noparse]see pic attached.]

· Use a Schottky diode (V_F = 0.3V)