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SX Tech Board and 12 Volt Power Supply... — Parallax Forums

SX Tech Board and 12 Volt Power Supply...

BamseBamse Posts: 561
edited 2006-04-07 00:35 in General Discussion
I have a bunch of 12 Volt Power supplies and I wonder if anyone have tried using 12 V with the SX Tech board ???
Looks like Parallax don't recomends more than 9v for this board...

Anyone know if this works ???

I just don't want to release the magic blue smoke from either the SX board or my SX-Key...

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/Bamse

Comments

  • jb1311jb1311 Posts: 20
    edited 2006-04-03 17:54
    The onboard regulator will get very hot if you use a 12V power supply. It's possible you will be ok for a short time if the load driven by the SX is light. It gets pretty hot with a 9 volt, I now use a 7.5 transformer.

    If I were you I would try to drop the voltage down with a power resistor or get a supply with a lower output voltage.
  • BamseBamse Posts: 561
    edited 2006-04-03 18:58
    I noticed that too, with 9V the Tech board gets pretty hot but my BS2 BOE's seemt to be fine...

    I have 7 of these 12 v supplies lying around, maybe I could open up a few and see what's inside, maybe add a resistor or something...

    Thanks JB1311...

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Living on the planet Earth might be expensive but it includes a free trip around the sun every year...

    /Bamse
  • John R.John R. Posts: 1,376
    edited 2006-04-03 20:47
    Just a resistor probably won't do the trick. Volts = Amps / Resistance means that if you just add a resistor, as your current load changes, so will your output. You'd either need to find that there is a "multi-tap" transformer inside, or possibly add a voltage regulator. By the time all is said and done, given the price of a wall wart... On the other hand, keeping stuff out of the landfill isn't such a bad thing.

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    John R.

    8 + 8 = 10
  • ForrestForrest Posts: 1,341
    edited 2006-04-03 23:30
    I'd check the voltage on your 12V power supply first. I've seen some inexpensive 12V unregulated power supplies hit 18V with no load. If that's the case, I'd invest in a 7.5 to 9V power supply.
  • David BDavid B Posts: 592
    edited 2006-04-03 23:41
    What you could do is lookup the schematic of your Tech board, and lookup the specs of the regulator itself, and check the voltage and current range specified by the regulator data sheet.

    For example, on the SX48/52 board, even though the Parallax SX4852 pdf document specifies 6-9 VDC, the LM2940 voltage regulator is specified for up to 26 volts. So you can take that as a maximum voltage, as long as no other components are using the unregulated input.

    Then look at how much current you're pulling through the regulator. First try to make sure you're not exceeding the specs of the regulator, 1 amp in the case of the LM2940. Then figure out whether the current will vary much. If your current load is steady, then using the series power resistor would be fine, as long as the voltage at the regulator input is high enough.

    But if the current varies, and you're using a series power resistor, then you run the risk of occasionally pulling the voltage too low, which can lead to some really nasty program behavior where code seems to work, then suddenly it doesn't, then it does again. My hard-learned advice - avoid this situation at all costs, if you value your sanity.

    If your total current needs are high, but within the specs of the regulator, and you really want to use 12V, then you might want to think about increasing the size of the heat sink. The regulator specs go in some detail about allowable temperature rise and heat flow to the sink, which may be more information than the average user really wants to know, but does make the point that a decent heat sink is needed to reach the 1 amp spec of the part.

    I usually just use the finger test - if the regulator is too hot to touch then I figure that something ought to be adjusted.

    But I've been running my SX52 tech board from 12V (actually a 13.8 volt gelcell) for a long time with no problems.

    David
  • BamseBamse Posts: 561
    edited 2006-04-04 02:07
    I'm more worried about my SX-Key than the board...

    I can get an SX48 Protoboard for $10, but the SX-Key is $80...

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    Living on the planet Earth might be expensive but it includes a free trip around the sun every year...

    /Bamse
  • John R.John R. Posts: 1,376
    edited 2006-04-04 02:52
    And a 7.5 Volt, 1 Amp power supply is $10.00 ($8.00 for 9 volt 300 mAmp).

    You aren't worried about blowing a 10.00 proto board, but are worried about salvaging a 12 volt power supply(ies) to save $10.00?????

    Not trying to be critical (really), but other than "because you can", are you sure it's worth the trouble?

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    John R.

    8 + 8 = 10
  • BamseBamse Posts: 561
    edited 2006-04-04 15:01
    Well, I had a bunch of the 12 V ones (leftover cell phone chargers) and rather than throwing them away I thought I could put them to use...
    I just realized that 5 of these are only 200mA and the other two are 300mA...

    Sooner or later I'm going to need more than that so I'll probalby get a new one anyway...

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    Living on the planet Earth might be expensive but it includes a free trip around the sun every year...

    /Bamse
  • Brian RileyBrian Riley Posts: 626
    edited 2006-04-06 17:44
    jb1311 said...
    The onboard regulator will get very hot if you use a 12V power supply. It's possible you will be ok for a short time if the load driven by the SX is light. It gets pretty hot with a 9 volt, I now use a 7.5 transformer.

    If I were you I would try to drop the voltage down with a power resistor or get a supply with a lower output voltage.

    One solution, actually better than a dropping resistor because the voltage drop is mor epredictable is to put 4-5 1N4001 (or the like) in series with the supply, this will drop roughly .7 volts each and bring you down. Utlimately I started with a 9 volt Rat Shack and went down to the 7.5 volt wall wart that Parallax offers, when I found even the 9v running kind of hot.

    My observation is that while I find a few of Parallax's items a bit pricey (though undertsandable they don't do huge volume on many of these items) their price on wallwarts is by far the best you will find save for the surplus houses. That said, the quality of their wallwarts is far superior to most of what I have found elsewhere. A while back I bought me a 'set' (7.5, 9, and 12) and consider it money well spent ... just how do you calculate the value of "chips not smoked"????????

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    cheers ... brian riley, n1bq, underhill center, vermont
  • jb1311jb1311 Posts: 20
    edited 2006-04-06 21:28
    You are right, using diodes to drop the voltage will keep the voltage regulated.
  • JamesxJamesx Posts: 132
    edited 2006-04-07 00:35
    I use a 12 volt lead acid battery for working with an SX based system. The actual voltage is at least .5 volts higher most of the time. No problems with either the SX or SX-key. The LM 7805 voltage regulator chips seem to handle it.
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