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Possible power supplies for Homework board? — Parallax Forums

Possible power supplies for Homework board?

willembadwillembad Posts: 47
edited 2012-06-06 10:35 in BASIC Stamp
I have decided to built my project using a homework board. I'm using it for the stamp and the usb basically since all other circuitry will be on another board. What would the upper and lower limit of acceptable power to the homework board be? It has 9V battery clip only but I'll be supplying regulated dc. I have 12V and 6V available already to power other parts of my circuit and wasn't sure if the 5V regulator on the board would work if I supplied the 6VDC or if I need another 9V regulator?

Comments

  • RDL2004RDL2004 Posts: 2,554
    edited 2012-05-31 18:33
    If all you're going to power is the Stamp and USB, 12 volts should be okay. It won't be dissipating much power. If it's an LDO type, you could use 6 volts. I couldn't find a schematic on the Parallax site, can you read the number on the regulator chip?
  • willembadwillembad Posts: 47
    edited 2012-05-31 19:14
    It is an LM2940LD. Datasheet here: http://www.ti.com/lit/ds/symlink/lm2940-n.pdf
    The MC7806 I'm using to provide power for the servo specs Vout+2V as min. input voltage but it doesn't seem necessary with this one? Think I'll try the 6V and see what happens.

    I couldn't find specs on Parallax site either. Never even dawned on me to read the chip - sorry.
  • davejamesdavejames Posts: 4,047
    edited 2012-05-31 20:04
    Willem,

    If you power the Stamp from +12V, its on-board regulator will be required to drop 7 volts...which in turn causes it to become rather warm (ask me how I know). It'll work, but that Vreg gets hot. Best to power it with a lower voltage.
  • RDL2004RDL2004 Posts: 2,554
    edited 2012-05-31 20:30
    Power dissipation at 12 volts in wouldn't be too bad with just the Stamp and the FTDI chip being powered, less than a watt? (not sure what the FTDI chip draws). But luckily the 2940 is a low drop out type regulator and only needs half a volt "headroom", so you should be fine using the 6 volt supply.
  • willembadwillembad Posts: 47
    edited 2012-06-01 05:29
    Thanks again guys. I wasn't sure what the dropout voltage meant but was hoping that the 6V would be ok. I'm in the first stages of testing the new circuit - looking for all the right supply voltages in various locations. Hopefully the hardware part will be done by the end of the weekend and I can begin coding.
  • willembadwillembad Posts: 47
    edited 2012-06-01 12:03
    6V was a no-go. The 12V worked but I was worried over the heat-sink on the 5V regulator. The homework board does not have the same heat-sink as the board of education. Since I couldn't find a 9V regulator or even a zener in this hole I live in, I improvised and used a trimmer set as a voltage divider and that worked fine. My 12V supply has plenty of headroom so I'm not worried with the current setup.
  • RDL2004RDL2004 Posts: 2,554
    edited 2012-06-01 12:58
    That's kind of odd, at the low current you're running through it the 2940 should work fine with 6 volts input. It should still be okay with 12 volts in though, as long as all it's powering is the board itself. If you can measure the current (I), the power dissipation is (Vin - Vout) x I, usually those TO-220 type packages can handle up to about one watt with no heatsink at all.
  • willembadwillembad Posts: 47
    edited 2012-06-01 14:15
    Not sure why the 6V supply didn't work but the stamp (homework board) would not power up. The regulator on the homework board is not a TO-220 but a TO-263 and it's soldered to the board to provide the heatsink. I wasn't sure if the 12V would be an issue so I set the voltage divider to provide 7.5V or so. I'm using the 5V stamp power to drive a led and a lcd display as well as the dual supply for the amp. I had so much trouble getting the amp to work that I didn't want to try the high rail from the dual supply for anything else even though it should work fine. The 12V switches a solenoid valve through a mosfet and the 6V regulator now only powers a servo.
  • Mike GreenMike Green Posts: 23,101
    edited 2012-06-01 16:09
    At this point, you really need to make a schematic and post it. You've got a more complex setup than first described and, most importantly, it's not working. The Homework Board's built-in regulator should work fine at 6V and up to 12V with just an LED and non-backlit LCD as extra loads. The opamp should add only a couple of mA which should be fine.
  • willembadwillembad Posts: 47
    edited 2012-06-01 16:46
    I will post the final schematic for sure Mike. It all works now :) Can someone suggest an easy piece of software to accomplish this instead of me drawing it out by hand? I didn't add all the bits in the original posts since I had most of them working right off the bat and wanted to keep the questions more specific.
  • willembadwillembad Posts: 47
    edited 2012-06-06 10:35
    Posted schematic and code here: http://forums.parallax.com/showthread.php?140526-Nitrox-Stick-Controller
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