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5VDC/3.3VDC Methods — Parallax Forums

5VDC/3.3VDC Methods

gibbmangibbman Posts: 98
edited 2006-02-20 06:30 in General Discussion
Can't decide on the appropriate forum, so I'll try it here:
I run across situations where I want to use a 3.3VDC device, but have the usual 5VDC available. This seems to come up a lot lately. In this particulary instance, the 3.3 is mandatory, as that is the only option for the essential device. It is on an SX project, so I guess I could try to find the other required devices in 3.3v; but sometimes it's seeminly not possible. So:
-with a 5VDC system, that probably should have ~.5A on the 3.3 side, what's a reasonable means to get the 3.3 from the 5? I'm leaning toward using an LM3940 or LM2937 for the job, but I'm wondering if something as simple as a zener would also work? I'm a little tight for board space on one project. Or is it too inefficient and prone to error? I mean, the first two items are real regulators...any thoughts from you project hounds?
Thanks,
Jim

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In the end, it seems that it's all about getting the LEDs to blink....

Comments

  • NewzedNewzed Posts: 2,503
    edited 2006-02-19 00:18
    I use a Digikey IRU1206-33CY· 3.3 volt regulator.

    It is an SMD chip but not very difficult to solder.· Only takes up about a 1/4" square.

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    Sid Weaver
    Do you have a Stamp Tester yet?
    http://hometown.aol.com/newzed/index.html

    ·
  • rockin_rickrockin_rick Posts: 32
    edited 2006-02-19 01:18
    I'm not certain, but I don't think that a zener for regulation can provide big amps (like 1/2A)...

    Rick
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2006-02-19 07:09
    Recently someone mentioned a DIY conversion of a generic AT power supply to a Bench supply.

    I took a look and notices that some of these provide a separate 3.3DC !

    The conversion is quite simple really -- all the wires of same voltage are bundled and connects to a Banana Plug for that level.

    And there is 10watt 10 ohm ballast resistor connected to the +5volt output to stabilize the switcher's output.
    Other features include a DC on/off and a LED.

    You may get a hefty 3.3V. Try 'Wiki EHow' for ATX Power supply converted to Bench Supply
    http://wiki.ehow.com/Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-Supply
    Is that worth the trouble?

    ~~~~~~~~~~~
    The SX works fine with 3.3volts if you properly set the brownout fuse.
    If you have a SX-48 protoboard, you can replace the voltage regulator. Additionally, you will need to get a new Parallax device to provide the SX-Key with +5 with the board remains independently at +3.3volts.

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    "When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)

    ······································································ Warm regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan
  • gibbmangibbman Posts: 98
    edited 2006-02-19 08:08
    Thanks, Kramer, Rick, and Sid,

    Yes, the PC power supply has a hefty supply of power on 5, 12, and 3.3. (It also has a -12 and -5 for those so inclined). Have read those articles and got myself some binding posts and that big wirewound, but still in the project inbox.

    Eventually my current project will be running on batteries, as it will be remotely sensing a few basic environmental variables. There will be need for power efficiency. I think the best way out will be to only use the 3.3v components (I will have to replace the DS1307 with something). The DS1631 temp sense and the 24C64A EEPROM are OK. The Maxstream Xbee module is 3.3 only. The device can sleep a lot; only needs readings every hour or so and the radio only needs to be awake for about 10 hours a day, with only very intermittent use).

    I do have the SK-KeyRing for the programming, so I guess it's just the reluctance to give up that DS1307 that is killing me.

    Jim

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    In the end, it seems that it's all about getting the LEDs to blink....
  • gibbmangibbman Posts: 98
    edited 2006-02-19 08:29
    Re: the above post, it looks like the DS1337 will work out fine. 3.3VDC, I2C. 8pinDIP.
    Jim

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    In the end, it seems that it's all about getting the LEDs to blink....
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2006-02-19 10:29
    gibbman,
    I am very interested in that AT [noparse][[/noparse]actually ATX] power supply as it is a very cost effective way to go. But I keep wondering if the 10 ohm resistor is enough to stabilized the output. So if you become disappointed, I'd like to know. I can't find much more about it on the web.

    BTW, it appears that a lot of the components actually overlap the 3.3 to 5.0 volt range. So double check [noparse][[/noparse]both when buying and designing] as it really can reduce the trips back to the store [noparse][[/noparse]or the drawing board]. Generally the microprocessors and the memories are this way.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    "When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)

    ······································································ Warm regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan
  • gibbmangibbman Posts: 98
    edited 2006-02-19 11:08
    Kramer,

    See the link below. It has a good write up with pics of a PC PS project, and a number of links at the end to other projects.

    Jim

    http://www.nfdc.net/home/cbdb/Teardrop%2012%20V%20Power%20Supply.htm



    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    In the end, it seems that it's all about getting the LEDs to blink....
  • John R.John R. Posts: 1,376
    edited 2006-02-19 14:05
    The two 10 Ohm resistors (in parallel) seem a bit overkill to kee the thing running. I believe this works out to 1 Amp. of load on the 5 volt line. If disk drive kept the thing on, I would thing you only need a fraction of that, and could use a higher value, hence reducing the amount of heat and wasted energy (along with resistor size). My experience also says that not all power supplies need a load to at least fire up and have the fan turn. I can't say they will stay on, but I have powered up more than one power supply and had the fan turn without anything connected.

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    John R.

    8 + 8 = 10
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2006-02-20 06:30
    I just wonder what is expected as·a quiesent ATX supply's current levels.

    The problem with switching power supplies is that they often have an operating range. Both a max and a min. When you go below the minimum, it is possible for very strange things to happen. Sometimes, they create noise at a frequency that happens to interfer with the microcontoller.

    Much of the reason we use linear regulators for most hobby work is that we don't have to worry about this.
    They are both simpler and cleaner [noparse][[/noparse]the have less ability to create noise interferance].

    Try my bible, The Art of Electronics for reading a good explanation.

    Having said all that, switchers have gotten a lot better over the years because SPICE software can actually create situational test on a trial and error basis before you go into production.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    "When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)

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

    Post Edited (Kramer) : 2/20/2006 6:42:38 AM GMT
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