newbie ? - power supply for boe/bs2

OopsOhNo

Just getting started and cautious.· Double checking that the Sceptre 9V wall wart is OK to use.· Mine is 16V+ with no load, and 14V+ with the boe powered up but not running anything.

Thanks,
Doug

PJAllen

You are pushing the envelope, methinks.

LoopyByteloose

We all distrust Wallwarts [noparse][[/noparse]some are good, some are not so good] as they have an output that is often higher than the label. The BOE uses so little current that the voltage creeps up.

This keeps us guessing if something is going to be damaged. LCDs are most often the first to be damaged.

You could put a parallel load on it [noparse][[/noparse]like a 5 watt resistor at 30 ohms for a 300ma load], but you have to be sure that the wallwart can power both [noparse][[/noparse]like 500ma or more]. Re-measure with the parallel load connected. You might find you have 9 volts.

Alternatively, you can just put a 7809 to 'pre-regulate' down to a steady 9 volts. I do this so that I don't have to worry about all that math again and again as I hook up different things.

Parallax is now providing +7.5 wallwarts because this has been a big headache.

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"If you want more fiber, eat the package.· Not enough?· Eat the manual."········

···················· Tropical regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan

OopsOhNo

Kramer - newbie I am - just starting out. Can you tell me how to connect the input power from the wall wart to the 7809 and which pins are the regulated output? Or is there a data sheet for the 7809 that would show me a sample circuit?
Thanks,
Doug

OopsOhNo

BTW the Sceptre wall wart is the one I bought from Parallax. Also, the wall wart wire has a white stripe on one conductor. Would that be the + or - conductor?
Thanks again,
Doug

PJAllen

Here's the link to the recommendation chart -- http://www.parallax.com/html_pages/products/componentshop/power.asp

The recommendation chart·should suffice as the final arbiter.· If it matches up (Stamp and board) then it's OK.

Post Edited (PJ Allen) : 6/24/2006 9:33:28 PM GMT

LoopyByteloose

The 7809 is one of a family of voltage regulators - the 78?? group. It uses the same configuration at the 7805, but just puts out +9volts instead of +5volts DC. Be careful, the 79?? family is the Negative output. You don't want to buy those.

You should use a PDF for the EXACT regulator that you purchase. Google is your friend. Sometimes we go to a store and buy something that is a bit different that the part we ask for. There can be small differences in how they are wired or what values of capacitors are optimal [noparse][[/noparse]I have some strange 3.3volt regulators here that require 100uf, but most 78xx use only 10uf].

Some people think that bigger capacitors on the regulators always improve the output, but there are situations that over 10uf can actually damage the regulator if you don't add protection diodes. It seems that when you shut down, the capacitor might discharge backwards through the regulator!

I GUESS maybe the white wire is negative (or maybe positive) -- BUT don't trust me. Always check your polarity with a meter. Check several times before you power up anything of value.

Both memory and guesses are the source of a lot of damage. I check everytime. This is especially important with all power connections as a reversed polarity is likely to do significant damage. Years ago, electronic standards were gospel, but these days you might get an odd one.
Reversed polarity might not immediately damage a BS because of the quality of the regulator on board, but it does immediate harm to ICs and LCDs. One reversal and a $30 LCD is garbage.

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"If you want more fiber, eat the package.· Not enough?· Eat the manual."········

···················· Tropical regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan

Post Edited (Kramer) : 6/25/2006 11:59:30 AM GMT