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Is it safe? — Parallax Forums

Is it safe?

kenwtnkenwtn Posts: 250
edited 2008-08-21 01:54 in BASIC Stamp
Hi All,
···· I have converted a old power supply from a desktop computer·I had laying around to be used for powering my projects. My question is... Is it safe to connect a Board of Education to 12 Volts on the power supply. My thinking for this power supply is it has 3.3V, 5V and 12V basicly all the voltage I should need.

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When you spend 10 times what it cost to build it yourself - That's when it becomes a HOBBY !

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Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2008-08-21 00:06
    I don't think anyone is going to tell you whether an unknown power supply is "safe" to use. They're not made for this sort of use in that the main supply is usually the +5V one. There's often a minimum current drain on one supply and that's the primary regulated supply. The others may not have their own regulators or they may be regulated loosely and tied to the primary supply. It might work out just fine or it might either provide a higher voltage than +12V or may shut down without the proper load on +5V. Without a schematic or at least a specification, it's impossible to tell.

    The regulator on the Board of Education and the one on the Stamp module itself all have datasheets that you can easily download and these give the upper limit of the input voltage for the regulator. You'll find that both of them can easily tolerate +12V and quite a bit higher. They do have to dissipate any excess voltage as heat proportional to the current drain. The regulator on the Stamp module is intended to supply up to 50mA and the one on the BOE is intended to supply up to 1A. The higher the input voltage, the more heat has to be dissipated. With a 12V input and a 1A current drain, you're dissipating (12V - 5V = 7V) x 1A = 7W which would make the BOE regulator quite warm. It has a small heatsink, so that'll help. Most regulators will shut down if they get too hot, so you wouldn't damage them, but the BOE might stop working until things cool off.
  • kenwtnkenwtn Posts: 250
    edited 2008-08-21 00:11
    ·

    ·· This is a link that is used: http://web2.murraystate.edu/andy.batts/ps/powersupply.htm I just thought from what I saw this would make a idel power supply for robotic projects.

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    When you spend 10 times what it cost to build it yourself - That's when it becomes a HOBBY !

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  • kenwtnkenwtn Posts: 250
    edited 2008-08-21 00:18
    I guess I am just becoming FRUSTRATED I have asked for help but so far have not received much. I need to power 3 stepping motors that require 1.2 amp per coil. Since this power supply can produce 6amps I thought it would work.

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    When you spend 10 times what it cost to build it yourself - That's when it becomes a HOBBY !

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  • Mike GreenMike Green Posts: 23,101
    edited 2008-08-21 00:48
    You've gotten help. You just haven't gotten the answer you wanted. That happens.

    I didn't say it wouldn't work. I just don't know whether it will or whether it will blow something. If you read the article you referenced in the link, there are some caveats. There's a minimum load provided. You have to check the result to make sure it works as expected before you hook up something expensive or hard to replace to it. You probably should fuse everything since there's often not much difference between a PC power supply and a small arc welder. Other people have done successfully what you would like to do, but some people I'm sure have ended up with a small boat anchor.
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2008-08-21 00:54
    kenwtn said...
    I have converted a old power supply from a desktop computer·I had laying around to be used for powering my projects.
    Does that mean that you added the pre-load resistor mentioned in the murraystate.edu link you provided?· That "pre-load resistor" is important.
    Carry out the mods in the article and then do a load (regulation) test.··From that point you could decide to use it just for your motors.·

    12V may be pushing the envelope for use as BoE/HWB "Vin".· There may be a trim-pot in the 'ATX' that you could tweak to adjust that somewhat lower.
    ·
  • kenwtnkenwtn Posts: 250
    edited 2008-08-21 01:10
    Much better thanks for all the information and things to consider.

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    When you spend 10 times what it cost to build it yourself - That's when it becomes a HOBBY !

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  • kenwtnkenwtn Posts: 250
    edited 2008-08-21 01:54
    How about if I use the PC power supply just to drive the load and use a 9 volt battery to drive the Board of Education. If I tie the grounds together will that work and protect the BS2? I am sorry I just don't understand.

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    When you spend 10 times what it cost to build it yourself - That's when it becomes a HOBBY !

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