Power questions

velociostrich

Hi,

I'm working on a project involving an LCD display and five buttons, four
of which are normally-open momentary contact SPST pushbuttons; the fifth
being a SPST toggle switch. Ultimately, I'd like to make my own PCB, but
I'm not sure what to do about voltage regulation. Do I just use an LM2940
like on the BOE or will that not be enough? Or is the BS2's on-board
voltage regulator sufficient? The LCD is a 4x20 backlit model -- the one
offered by Parallax.

Using Parallax's BOE schematic as a reference, I drew the attached
schematic -- is this correct?

EDIT: When looking at the schematic for the Serial BOE, Revision C, it seems
as though the positive terminal of the battery is being supplied directly to the
Stamp, instead of letting the LM2940 do all the work; am I correct in this
understanding? So, then, the breadboard offers more Vdd pins than Vin pins
in an effort to prevent the user from overloading the Stamp's less powerful
voltage regulator?

EDIT: Is the heatsink as seen on the BOE (and elsewhere) necessary?

Post Edited (velociostrich) : 8/20/2009 1:03:30 AM GMT

Mike Green

Yes, the BOE uses Vin directly to power the Stamp so the Stamp uses its on-module regulator. The BOE's regulator is just for I/O devices. If you run noisy devices off the BOE's regulator, the noise won't easily feedback to the Stamp's internal supply.

Sometimes the heatsink is necessary. It depends on Vin and on the amount of current drawn. If you run the BOE off a 6V supply and draw 1A, that's 1W of power that has to be dissipated from the regulator. It wouldn't get particularly hot even without a heatsink. If you use a 9V supply and draw 1A, that's 4W of power. If you exceed the recommended voltage range and run the BOE off an automotive supply (13.8+V) at 1A, that's 9W of power. The heatsink supplied is likely to get quite hot and the regulator may well go into thermal shutdown.

velociostrich

Thanks for the reply. Given that, I don't think I'll need a heatsink, so my only remaining question is in regards to the circuit I posted -- is it correct?

I'd like very much not to fry my Stamp, as they are rather expensive.

Mike Green

Regarding your circuit ... It's correct as far as it goes. Read the LM2940 datasheet for specific information on input and output capacitors. Do be sure to check the output voltage before you plug in your Stamp. Even correct circuits sometimes have bad parts put into them.

velociostrich

Thanks for the advice and the info -- I should be able to complete my project now.