12VDC, 0.65A from wall power??

Discussion in 'The Projects Forum' started by EEDude, Nov 5, 2009.

  1. EEDude

    Thread Starter Active Member

    Nov 18, 2008
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    I need to create a 12V DC, 0.65A constant source from wall power to initialize a solenoid valve. If I get a 12V DC wall converter does anyone have a basic circuit example that I could use with that to get the 0.65A current while keeping the 12VDC? I know you can accomplish this with a BJT ckt, if anyone has any idea that would be great. Thanks!!
     
  2. someonesdad

    Senior Member

    Jul 7, 2009
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    You can go to Radio Shack and buy a wall wart that outputs 1.5 A at 12 V for $15 or so.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    You can regulate voltage and (optionally) limit current, or you can regulate current.

    You can't do both.

    Marlin P. Jones & Associates has a 12v 1A wall wart supply for $3.95 that should work just fine.
    http://www.mpja.com/products.asp?dept=37&main=1
     
  4. EEDude

    Thread Starter Active Member

    Nov 18, 2008
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    1 amp is too much current. The solenoid valve is 8Watt, and the coil already gets really hot to the touch using the specified 12Volts and .65A of current. Why couldn't you use a BJT with a sensing resistor at the emmiter that will force whatever current you want through the collector and emmiter, and have the load resistance tied to the collector? Or can you just put a resistor in parrelel to the load resistance to reduce the current the load sees?? Thanks for your help!
     
  5. CDRIVE

    Senior Member

    Jul 1, 2008
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    I don't think you understand Ohms Law. The fact that a voltage source is rated at 1.5 Amps does not mean that your solenoid is going to pull 1.5 Amps. If your solenoid is rated at 12V @ .65 Amps a voltage source of 12V @ 1000 Amps would still work fine.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    OK, 0.65A @ 12V works out to 7.8 Watts. That's about the same amount of power as the old-fashioned X-mas tree bulbs. BTW, P=EI, or Power in Watts = Voltage x Current.

    If you want to try less current, MPJA also has a 12v 500mA (0.5A) wall-wart plug supply for a couple bucks. The solenoid may not pull in with the current that low.

    If you wanted to, I suppose you could put a resistor in series with the solenoid. The resistor will then dissipate power in the form of heat.

    Since R=E/I (Resistance in Ohms = Voltage / Current), your solenoid's resistance works out to be about 18.5 Ohms. If you really want to reduce the current through it, you might use a power resistor. Don't think I'd go above 5 Ohms, or your solenoid may not pull in properly.

    Is your solenoid rated for 12VAC or 12VDC? There is a difference. DC solenoids have about 1.4 times the resistance that AC solenoids do. If it's rated for 12VAC and you want to run it on DC, you should be using 8.5VDC.
     
  7. spacewrench

    Member

    Oct 5, 2009
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    You're going to a lot of trouble to solve a problem that could be more easily solved elsewhere. It's true that solenoids often need a healthy slug of current to get them to move, and will hold with less current (so you could design a circuit to deliver controlled current like that) but it might be easier to change the mechanics so that your solenoid doesn't have to be energized all the time. (Think of car door locks -- the solenoid doesn't hold the lock open or closed; it just changes the position when necessary.)

    If your solenoid is getting hot, it's because it's turned on too much of the time. (If it's rated for continuous duty, then it shouldn't be getting too hot, although it's certainly possible that it's hotter than you'd like.)

    The other comments are correct: if your solenoid is a 12V unit, and you're applying 12V, then it won't (or at least shouldn't) draw more than its rated current, even if the power supply is rated for much more current. If it's getting too hot, then either you're exceeding the rated duty cycle or the ratings are bogus.
     
  8. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Think of it this way: suppose you connect your solenoid across your car's 12V battery. Now, most batteries are capable of supplying up to 1000A of current for short bursts during cranking and hundreds of A for several minutes at a time. How much current will your solenoid draw from the car battery? Answer, the same amount it will draw from a 0.6A 12V supply.:D
     
  9. EEDude

    Thread Starter Active Member

    Nov 18, 2008
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    So when you say its 12V at 1A that means the max current it can supply at 12V is 1A and that it will suck any current under that at 12V? If that is the case sorry I didn't understand right away what you were saying, I deal with source measure supplies mainly where you have to specify voltage/current and compliant limits and I write the software to control them. I just want to free up a few power supplies in the mean time while we fix a problem we are having with some other ones. Thanks
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    If a solenoid that measures 18.5 Ohms resistance is supplied with 12v, it will have 0.65 Amperes current flowing through it.

    That's Ohm's Law; I=E/R, or Current = Voltage / Resistance.
     
  11. CDRIVE

    Senior Member

    Jul 1, 2008
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    Yes, you got it now. Even your home's electric service is rated at a max current. Most homes in the US are rated at 240VAC/200A max but each appliance that you plug in is pulling only a fraction of that max limit.;)
     
  12. EEDude

    Thread Starter Active Member

    Nov 18, 2008
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    Thanks for answering my question I wondered how many more people here would step round it and try and make me look dumb. Oh I don't understand ohms law......cmon bro do you know how to communicate between 26 instruments through GPIB????? V=IR, sorry if I though your 1A meant 1A, well it turns out that was the max current. Just so you know other people call that a compliance limit
     
  13. EEDude

    Thread Starter Active Member

    Nov 18, 2008
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    so please give me your code you r using to communicate between your devices because i must be stupid
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    EEDude,
    I think we're talking apples and oranges here.
    Nobody is trying to "make you look stupid", or denigrate you in any way.

    Really.

    If they were, I would report them, and the Moderators would take it from there.

    There are rules against such behavior, and more than one member has been suspended/banned due to it.

    With all due respect, Ohm's Law is Ohm's Law. If a load measures 18.5 Ohms, and you only have 12v available, you're only going to get 0.65A current flowing through it, no matter how hard you might wish for more or less current flow.

    It really is that basic.

    Now if you insist on providing that solenoid a regulated voltage, OR a regulated current, we could proceed with another plan - but it will require a different approach.

    BTW, did you know that in most PC power supplies that +12v is available at generally 8A or more? You could power quite a few of those solenoids (at least a dozen, or as few as one) using a converted ATX-form-factor computer power supply. I have an old Compaq 250W unit sitting next to me that I converted to a bench supply years ago - still does it's thing.
     
  15. EEDude

    Thread Starter Active Member

    Nov 18, 2008
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    yeah i wish it were that easy
     
  16. CDRIVE

    Senior Member

    Jul 1, 2008
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    You've made two assumptions here and you're dead wrong on both of them. Your first assumption is that some of these replies were meant to demean and embarrass you. This is an educational forum and that's what we are devoted to. We would be remiss in our goal if we ignored a misconception by a member. Secondly, many of us spend an equal amount of time programming, as electronics and programming make a useful marriage. Challenging the people that are trying to help you would probably result in even more unwarranted embarrassment.
     
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