Battery Pack

DNAMusic

Can someone please point me at some information regarding using batteries to power a 5v proto board, such as do I need any additional circuitry or do I just attach batteries to the power jack, and if so what kind and how many? Also regarding a 12v battery supply for the propeller accessory kit's LCD screen. Is there a way to use a single battery pack for both devices? Any help would be greatly appreciated. Thanks very much...

Zoot

You can apply 6-9v (directly from batteries) to the proto board. The 5v regulator on the board is "low dropout" this means that it only needs .5 v or so MORE than it's output to work properly. Theoretically, you could put much much higher voltages into that reg (12v or even higher) BUT IT WILL GET HOT ( wattage, i.e. heat dissipation = volts dropped * current, so if you drop 12v down to 5v and use 500ma of power, that would be 3.5 watts -- ( 12v-5v ) * .5 amps. Not ideal.)

In a few cases where I wanted to use higher voltage batteries/power supplies, I remove the LDO regulator and replace it with a regular regulator (e.g. a 7805) -- such regulators need 2-3v more than the output so you're not wasting as much in heat.

If you want to use a single battery pack for 12v LCD and for input to 5v regulator, I would:

- replace regulator with a non-LDO regulator AND maybe put on a bigger heat sink (if single 12v battery/power source)

or

- if it's an actual battery pack (say 8AA) then I would put two taps into the pack -- one on 4AAs (for 6v out to the regulator) and one on all 8AAs (for 12v to your LCD)


P.S. -- ideally, if you need clean regulated power from a much higher voltage source (say 12v or 24v to 3.3v or 5v) then you are better off using a switching regulator -- these waste almost no power as "heat" and are much more efficient at stepping down higher voltages. This also translates to longer battery discharge times -- the battery power is not being used to heat up the regulator. But that's an added complexity and/or cost. There are integrated switching regulators with the exact same pinout as regular TO-220 regulators like the LM2940 and 7805 -- these are available at houses like Digikey for about $16.

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When the going gets weird, the weird turn pro. -- HST

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Post Edited (Zoot) : 6/4/2008 3:21:03 PM GMT

Mike Green

Zoot,
The extra voltage gets turned into heat regardless of whether you use a LDO or non-LDO regulator. The best thing is to use a switching regulator as you suggested.

I've had success fastening a small heatsink from RadioShack to the tops of the two regulators on the Protoboard using metal-filled epoxy glue. It's not a great solution for a lot of heat since there's a big temperature differential between the chip and the top of the package, but it worked well for me keeping that area of the Protoboard quite warm to the touch, but not really hot.

Whit

Would this same rationale apply to the question I posted here http://forums.parallax.com/showthread.php?p=654636?

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Whit+


"We keep moving forward, opening new doors, and doing new things, because we're curious and curiosity keeps leading us down new paths." - Walt Disney

Zoot

Whit -- yes. Actually it applies in any case where you are regulating down voltage significantly.

A few practical comments --

- a regular 7805/LM2940 can supposedly dissipate nearly 14 watts (that's *hot*) -- I ran two of my larger 'bots from car batteries with a pair of 7805s for 2amps of logic supply and never had a problem -- but even with medium size heatsinks, those suckers got really warm. Remember most modern linear regulators have both over-current and over-temperature protection, so you probably won't fry anything, but the reg. could shut down if over-burdened. Heat sinks of all sizes can usually be scrounged from old, broken equipment if $$ are tight. Or a nice sized piece of angle aluminum, etc. Putting a hefty diode (6A, say) between the battery fuse and Vin for logic power helped a bit in minimizing mess from the motors and to drop the voltage another .6v to ease up on the reg (that's something I picked up from Beau Schwabe).

I recently changed out the pair on one 'bot for a single 2amp switching regulator -- was well worth it in terms of heat/efficiency, and I felt I had pushed my luck long enough -- but it's not the only way.

- if you are working on a big 'bot platform with a single power source (i.e. a big battery), I would say that correctly routed power to the motors, logic, servos is more important than heat from your logic regulator(s) -- you want to avoid ground loops; make sure you don't have serious motor noise affecting your logic supplies; hefty enough wiring the various sub-systems, etc. See this thread w/the as always helpful Mike Green: http://forums.parallax.com/showthread.php?p=610598, and particular this: http://forums.parallax.com/attachment.php?attachmentid=43706

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When the going gets weird, the weird turn pro. -- HST

1uffakind.com/robots/povBitMapBuilder.php
1uffakind.com/robots/resistorLadder.php

Post Edited (Zoot) : 6/4/2008 6:42:24 PM GMT

Whit

Thanks a lot Zoot. Very helpful. A 12 volt system is new territory for me.

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Whit+


"We keep moving forward, opening new doors, and doing new things, because we're curious and curiosity keeps leading us down new paths." - Walt Disney

Post Edited (Whit) : 6/4/2008 8:58:41 PM GMT