BobG wrote... >>>Hope this helps... > >Wow, it sure does. Thanks for detailed explanation. You're welcome; glad I could be of some help. >Here is another, related question: I have two enclosures making up my >device - one powering the other through a cable that also carries serial >Tx/Rx. The first is powered from a wall transformer. Can I supply >regulated 5V to the remote device (6-12 ft), or am I better off sending = the >higher, unregulated voltage to the remote device and give it its own, = second >5v reg? The remote unit draws 50ma typical, 120ma in surges. Hmmm... Depends on the size of the wire, and how tightly you need the 5 volt supply to be regulated at the end of the 6-12 feet of cable. =20 If you have any analog stuff out there in the remote unit that needs an accurate supply voltage, sending 12 volts to the remote and then regulating it down to 5 volts locally might be the best choice. Otherwise, if there's only digital stuff out there, I think I might be tempted to just ship the 5V down the cable (with a generous bypass capacitor at the receiving end). >Details of what I am doing if you are curious: > >Wall Transformer: Main Enclosure: LCD Enclosure: >12V unregulated: -> --------12V-----> > 5V@250ma 5V@100ma > 3V@40ma -12V@2ma > >My breadboarded prototype (which works) uses a 12v unregulated wall >transformer and I have lots of regulators around. The 12v wall wart >connects to the main enclosure where a 5V linear reg supplies about = 250ma. >This enclosure also has a 3V linear reg that drops the 5V down for about >50ma at 3v. >A twisted pair cable takes the +12V unregulated and serial signals to = the >second enclosure that is basically a serial LCD. In fact the prototype = uses >a serial LCD I bought from www.earchLCD.com. It's a nice product, but = too >expensive for this project and with a lot of extra features I am not = using. >This serial LCD board has another 5v linear reg to supply the logic = (100ma) >and an LTC1044A converts the +12 to -12V for the 2ma Vee for the LCD. = Since >I am making a new, leaner serial LCD board from scratch, I figured might= be >able to reduce the components. > >Also, the more important thing I am trying to solve with changing the = power >supply method is the heat generated by the 5V linear reg in the main >circuit. It has well over 7 volts of head voltage @ 1/4 amp, so it tends= to >get hot. I want to put it in an enclosure without a fan. Now, that might be a candidate for a switching regulator: you've got a modest amount of power there, and a heat problem you want to solve. Check out National Semiconductor's LM2671, which is rated at a half-ampere. That might do the trick; it will certainly run cool, as they project about 92% efficiency for operation at 12V in/5V out at 500 mA. >So, I figured that I could use a 5V transformer (over-rated so the reg = sees >at at least 5+(its drop out) at full load) and use only one 5v reg in = the >main enclosure. I could send the regulated 5V over the serial = connection to >power so that the display enclosure's would not need another. Then at = the >display enclosure boost and invert the +5v to -12v. The -12V really = needs >to be set between -10V and -8V to adjust the contrast for a particular = LCD. >The data sheet for the LCD suggests a simple voltage divider with a pot >since the current is so small. I am thinking of putting in a variable = reg >so that the adustment will be more stable for the end user. You might not need a regulator; but you could always design one in, and bypass it if it proves unnecessary. Regards, Dave -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics