Solenoid with Momentary Switch

Discussion in 'The Projects Forum' started by OldSkoolEffects, Jan 1, 2010.

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  1. OldSkoolEffects

    Thread Starter Member

    Nov 18, 2009
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    This is my first post here, so please bear with me.

    I am trying to reproduce a movie prop, and need to have a small electric solenoid activate when a momentary button is pressed. I was originally going to simply wire the power, switch, and solenoid together, but then started looking around for circuit protection.

    I found this thread: http://forum.highlyliquid.com/showthread.php?t=126 and obtained:

    TIP122 NPN Transistor
    1N4004 Rect Diode
    12V 6W Push Solenoid
    1k Resistor
    Momentary Switch

    When I wired up the circuit as shown in that thread, and applied my 12V power, the switch was doing nothing (the solenoid would activate). Every reference I can find using this circuit design has something like a "chip output" and not a simple switch. I am wondering if I have the switch connected to the correct places, or what could be going wrong. I've re-analyzed my circuit several times, ad putzed around with moving components to no avail. Any help, especially a working diagram would be greatly appreciated.

    One other note; I'm assuming that since the solenoid is 12V at 6W, I only need to push 500mA into the circuit?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Connect them like this:

    [​IMG]
     
  3. OldSkoolEffects

    Thread Starter Member

    Nov 18, 2009
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    Well, I got it to work. I had a bad transistor. Now my question is, if I want my solenoid to drive more weight, do I increase voltage or current? I'm thinking the voltage.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    The coil has a fixed resistance. The only way you can increase the current is by increasing the voltage.

    Be careful though; if you drive a solenoid with more voltage than it's designed for, you can burn it up pretty quickly.
     
  5. OldSkoolEffects

    Thread Starter Member

    Nov 18, 2009
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    Well, looking at the datasheet for this solenoid (from Ledex) there's a pretty wide range of voltages you can use. I'm using 12V right now, and I'm thinking of going to 16V.

    So essentially, the input current from my wall adapter really has no bearing on the current going through the solenoid?
     
    Last edited: Jan 2, 2010
  6. SgtWookie

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    Jul 17, 2007
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    Yes, it does.

    "Wall warts" (plug supplies) are typically unregulated. They will generally produce within 10% of their rated voltage at the rated load current. If the load current is less, the output voltage will be higher.

    A wall wart rated for 12v @ 1A will supply more current than one rated for 12v @ 100mA.

    It would help a great deal if you would supply the complete part number of the solenoid, and the V@I ratings for your wall wart. I can't find a direct correlation to your 12v@6W rating in the datasheet for the solenoid.
     
  7. OldSkoolEffects

    Thread Starter Member

    Nov 18, 2009
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    http://www.jameco.com/webapp/wcs/st...oreId=10001&catalogId=10001&productId=1919203

    is the link to this solenoid (191172-001) on Jameco. I think Ledex has moved to a new numbering system. Strangely enough, the overview states that this is a 12V solenoid, but the datasheet lists it as 27V at 25% duty cycle.

    In my application, the solenoid is going to be at most 10% duty cycle. From the sheets I've read, it seems that the lower the duty cycle, the higher the voltage, and the more force behind the stroke. I am trying to get as much force output as I can. The current wall wart I am using is a Hobbico, 12V@600mA Class 2 Transformer, and it seems a bit weak.
     
  8. SgtWookie

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    Jul 17, 2007
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    Well, if you use a 27v source, you'll have a maximum force of 7 oz. at 0.1" extension - and it decreases as the solenoid extends.

    I don't know how much force you need.
     
  9. OldSkoolEffects

    Thread Starter Member

    Nov 18, 2009
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    I was hoping to get 8 oz at .1 " or a little higher. That datasheet is at 25% duty cycle, so if I'm closer to 10%, shouldn't the force be greater with the same voltage? I'm worried that if my voltage is too high, I'm going to pop the resistor on the switch; or am I forgetting a characteristic of transistors on the Base input?
     
  10. SgtWookie

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    Jul 17, 2007
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    You'd need to increase the value of the resistor. 2K would be fine.

    If you really want to give that solenoid a whack, you could pick up a 24vac transformer (like used for heating/A.C. controls), add a bridge rectifier and a capacitor, and you'd have about 34vdc output. Just tap the button to blast off. ;)

    Don't hold the button down though.

    [eta]
    Keep in mind that if you're going that high in voltage, you'll need to increase the resistor to 3k.
     
    Last edited: Jan 2, 2010
  11. OldSkoolEffects

    Thread Starter Member

    Nov 18, 2009
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    O may take you up on that if I'm not getting enough "oomph." Thanks for all your help, and I'll be sure to post more questions after I screw something up!
     
  12. OldSkoolEffects

    Thread Starter Member

    Nov 18, 2009
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  13. OldSkoolEffects

    Thread Starter Member

    Nov 18, 2009
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    Well, I tested the solenoid with my 24V@500ma wall wart, and I should have more than enough "oomph" to do what I want to do.
     
  14. SgtWookie

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    OK, cool. :)
     
  15. BillB3857

    Senior Member

    Feb 28, 2009
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    It typically takes more current to initially energize a solenoid than to maintain position. A trick we used on tape reader pinch rollers was to put a resistor in series with the coil and place a fairly large capacitor across the resistor. When energized, the cap would allow a strong current pulse to the coil, but then as it charged, the resistor controlled the holding current. If you find your coils getting hot due to a long period of being energized, you might try it.
     
  16. OldSkoolEffects

    Thread Starter Member

    Nov 18, 2009
    68
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    Well, once installed, it seems I'm lacking the initial push I'd like. In researching the nuts and bolts of how electromagnetic solenoids work, I noticed that Voltage doesn't seem to be a function of field strength, only current. Would I be correct in the assumption that I would see no real difference between 24V 1A and 12V 1A? I'm thinking of going to a higher current supply, but 12V are much more easily found than 24V. Thanks!
     
  17. SgtWookie

    Expert

    Jul 17, 2007
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    Well, current through the solenoid (which is a coil) will be a function of voltage ;)

    The higher the initial voltage, the more quickly the current will start flowing through the coil. Coils have a DC resistance, but also have an inductance value. Roughly, the more turns of wire, the greater the inductance, and the longer it takes for the current flow to "get up to speed" at a given voltage.

    One way to do that is to charge a capacitor, but if you start getting over 50v charge on even a moderate sized capacitor, you are getting into dangerous territory. Wearing jewelry (rings, watches, chains, etc) when working with such stored power can result in the jewelry getting melted and you suffering serious burns. Even without wearing jewelry, the risk of shock hazard increases as voltage goes up.

    I am not a medically trained person, but generally, if you stay below 50v and are wary of wearing conductive objects, you should be OK.
     
  18. OldSkoolEffects

    Thread Starter Member

    Nov 18, 2009
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    Gah, you're right; I keep forgetting about the fixed resistance. As far as using a capacitor, I'm not too well versed on them. I'm guessing I place the cap in series with the Voltage into the solenoid? That way, I have 24V from the Vin plus the 24V (or whatever) from the cap?
     
    Last edited: Jan 21, 2010
  19. SgtWookie

    Expert

    Jul 17, 2007
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    What does your 24v wall wart output voltage measure with no load on it?

    You might try placing a 1000uF 60v+ capacitor in parallel across the output. It may take several moments to charge it. I wouldn't be surprised if you measured around 30v across the charged capacitor.

    1000uF is just a ballpark guess.
     
  20. OldSkoolEffects

    Thread Starter Member

    Nov 18, 2009
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    How does the capacitance relate to charge/discharge time?
     
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