Bypassing the voltage regulator??

Jerry Elya

Purchased a Stamps In Class for a now-canceled project a year or two ago, and have now been asked if it can be used for a middle school control project. Unfortunately, rule-writers have manipulated the rules to eliminate most of the Parallax products-- we can use either 4 AA batteries, or a 4.8v battery pack.

Is there any way we can use 4 AA batteries, a low-dropout regulator to get 5V, and then bypass the on-board regulator on the Stamps In Class board? If so, would it be stable enough to not have fluctuations or spikes from a stepper motor powered from the batteries too? In this case, we have a bunch of tiny steppers and an ultra-light balsa "car"-- a frame only, nothing un-necessary. In fact, it looks nothing like a car other than wheels mounted on the steppers.

Jerry

Mike Green

Part of the problem is that 4 x AA Alkaline cells can have a voltage when new of well over 6V. A 4.8V NiMH or NiCd battery pack (4 cells) can also have a voltage of well over 4.8V when freshly charged. When you add a stepper motor running off the batteries, the voltage will sag depending on the drain from the motor.

The Stamps in Class board is probably the Board of Education which already has a low drop-out regulator that works fine down to about 5.5V, then quits regulating. The HomeWork board also uses a low drop-out regulator. A 4.8V NiMH or NiCd battery pack won't work for long ... the Stamp resets around 4.3V.

Your best bet is to use 4 x AA Alkaline cells and the existing regulator on the board. Run the stepper motor off the unregulated input voltage (6V). You'll need diodes across the stepper windings to protect against back-EMF and you'll need switching transistors as drivers. Nuts and Volts Column #6 shows how to drive motors using either switching transistors or the ULN2803A Darlington Array. Go to the main Parallax page and click on the Resources tab. You'll see a link to the Nuts and Volts Columns index.

Ugha

If this is just for a VERY quick class demo... there's one other option.
Recharagable batteries usually are 1.25 volts and normally that'd never work for a BS2 on a BOE.

However, freshly charged (And I do me FRESHLY... as in the past hour) NiMH batteries sometimes have a charge of 1.4 to 1.5ish volts. With four of them, this is enough to run the BOE, BS2 and possibly your motors for all of 30 seconds to 5 minutes before the batteries go back to normal charge (the 1.25v).

Do not depend on this though. Test your batteries after charging and run a demo on it. It may not work with your brand.

Another option would be a DC-to-DC boost circuit. Your best bet is to buy one pre-made as the circuit is somewhat complex. You could get 5v off one or two AAs. There might also be one
to use your 4.8v battery pack.

Remember though, a boost circuit drains the batteries a LOT faster than normal. It can also cause overheating problems if you tax the batteries too horribly much.

I believe www.sparkfun.com has a couple boost DC-to-DC converters.

Good luck!