solenoid control circuit

Discussion in 'The Projects Forum' started by meeotch, Jun 11, 2010.

  1. meeotch

    Thread Starter New Member

    Feb 26, 2010
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    I've built the following circuit to control a solenoid from an Arduino. (S2 in the attached diagram is currently a button for testing, but the transistor base will eventually be hooked up to an Arduino output pin.)

    L1 is the solenoid, which measures around 8ohms when disconnected. When activated, it draws about 1A steady. No idea what the inrush current is. I have no other info on it (it's an old pinball machine coil.) S1 is a 12VDC relay.

    The issue is that I'm still getting visible sparking across the S1 relay contacts. I picked .47uF and 50ohms for the RC values, based on some guidelines I found online. I then doubled up the capacitor, which seemed to help with the sparking, but didn't eliminate it.

    Are my RC numbers way off? I hate to just keep adding caps - there will eventually be about 15 of these circuits on the board, and the two mylar .47uF caps I've got now are already bulkier than I'd like.

    I know much has been written on RC snubbers - maybe too much, which is why I've been unable to sift through & solve this on my own as yet.

    Context: the project involves 15 or so pinball scoring reels, being controlled by an Arduino. They'll be firing a couple of times a second max, in groups of maybe 6 at the most.
     
  2. Bychon

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    Mar 12, 2010
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    Sorry if this seems dense of me, but can you verify that you are using 24 VAC RMS at 60 cycles per second to energize the inductor, L1?
     
  3. meeotch

    Thread Starter New Member

    Feb 26, 2010
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    Yep. It's a 24V transformer, plugged directly into the wall (US).
     
  4. Bychon

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    Mar 12, 2010
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    Skipping about 20 lines of math, the first answer is: .12 = CV^2

    That means that the voltage developed when the relay opens is inversly proportional to the capacitor size. The inductor will always have to dump energy when the relay opens and it can be mitigated by the size of the capacitor, but you already knew that. The point of this equasion is that the reflex voltage can be quantified.

    If you want to limit the voltage developed to 50 volts, .12/2500 = 48uf
    In addition, the resistance in series with the capacitor reduces its effectiveness.

    This is a partial answer. Please reply with the direction this takes you and how your goal is affected by this knowledge.
     
  5. Bychon

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    Mar 12, 2010
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    PS, inductors don't have inrush current.

    The way this takes me is, if we're going to treat this like a triac, why not use a triac? They have extreme voltage survival and that makes the capacitor a lot smaller.
     
  6. Bychon

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    Mar 12, 2010
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    ps, the circuit is drawn improperly, but the mistake is so simple that I assumed you could fix in in about a second.
     
  7. Bychon

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    Mar 12, 2010
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    Here's an example.
     
  8. Bychon

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    Mar 12, 2010
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    Hello?



    ....
     
  9. meeotch

    Thread Starter New Member

    Feb 26, 2010
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    Thanks for the responses. 48uf does seem much less doable than what I've got on there now.

    Just for context: I chose relays b/c of the sound - the project is an art piece that's supposed to make clicking & clanging noises, in addition to its other functions. Also for "authenticity" reasons - the pinball machine the solenoids came from used big honking relays to control them.

    I guess I was assuming that any amount of visible sparking was too much (and early tests without the snubber did seem to indicate that the relay contacts got dirty pretty quickly)... But perhaps this is not the case?

    Here's a (sort of) sheet on the relays I've got: http://www.cimisf.com/14fk.html I'm not entirely clear how to determine what a "safe" value would be for the back-voltage induced when the contacts open.

    mitch

    p.s. - In reply to post #5 re inrush current. The inductor is actually a solenoid, which would have an inrush current - no? (Not that it's relevant to the snubber discussion, of course.)
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    You could also use a couple of 33v to 36v 1W Zener diodes back to back across the coil. That would kill the reverse-EMF spike.
     
  11. SgtWookie

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    Jul 17, 2007
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    When a solenoid is first energized, it does have an inrush current; much like a motor with it's rotor locked, or starting from a stop.
     
  12. Bychon

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    Mar 12, 2010
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    48uf seems less doable that the (2) 47 uf caps you have there? How is removing one of the caps less doable? Just leave a 47 in there and pretend it's a 48. The guaranteed capacitance variance means a 47 and a 48 are just about the same thing.

    Sparking contacts are not a problem. The air conditioners I repair use 100 amp surges, have nothing to suppress sparking, and last over 100,000 events.

    the resistive contact rating is 240 voltsAc. Since the inductive reflex does not persist, it can be treated as AC in this case. Try to limit the voltage back kick to 240 volts.

    In reply to post #5...inductors do not have inrush current. A solenoid is an inductor.
     
  13. Bychon

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    Mar 12, 2010
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    Darn wookie, I thought the definition of an inductor was about voltage first, then current. or, di/dt = E/L. Whatever voltage is applied, the rate of rise of current is dependent on the inductance.
     
    Last edited: Jun 11, 2010
  14. meeotch

    Thread Starter New Member

    Feb 26, 2010
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    Sounds doable, and potentially saves me board space as well. A quick scan of Jameco turns up three 33V zeners - highest power dissipation about 1.3W. Seems low... How would one guesstimate the proper power value?

    I assume the back-voltage spike is at least 24V, but with very little resistance through the zeners & coil, wouldn't the current be quite high?
     
  15. Bychon

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    Mar 12, 2010
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    Now that you've learned a few things, I can not rely that you are still going to use the same drawing you originally posted. Please provide an update.
     
  16. meeotch

    Thread Starter New Member

    Feb 26, 2010
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    @Bychon - looks like my last post hit somewhere in the middle of your last few. I'll try to straighten out the tangle...

    re 47uF (post #12) - That's poor resolution of the decimal in my diagram, sorry about that. The caps I have in there are 0.47uF - so a little less than 1uF together.

    For a 240V kick, your formula gives roughly 2uF... a lot closer to where I'm at, for sure.

    Re inrush current: Some googling seems to suggest that the movement of the plunger changes the inductance of the solenoid, resulting in a higher "start" (inrush) current than "hold" current. This page states that AC solenoids have inrush currents, but DC ones do not. I'm not sure I understand why, though.

    Attached is circuit redrawn with zeners. I found this part at digikey that has 5W max power dissipation. Still not sure how to determine the correct power value. Would I need to throw a resistor in series to limit current? (Which would presumably change the choice for zener voltage, too, due to the drop across the resistor.)
     
  17. SgtWookie

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    Jul 17, 2007
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    Here, have a look at this TVS diode:
    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=497-6626-1-ND

    Just one per solenoid. Bi-directional, 37V, 600 Watts, about $42 per 100.
     
  18. Bychon

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    Mar 12, 2010
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    When the core is out of the solenoid, it has a lot less inductance, or, why do you think they put cores in inductors if it doesn't help them be better inductors? (You're not supposed to answer that.) The inductance you have is so low that the dominant limit will be the 8 ohms of DC resistance. If the AC voltage happens to arrive at its highest possible voltage, that will cause the highest "start surge".

    Now, here is where my experience cries out, "foul". A 24 VAC transformer will tend to have higher than the rated voltage when it is unloaded. On top of that, most transformers are rated at 115 or 120 VAC input. The power company delivers 125 VAC to my house. So, I do 24 x 110% x 125V/115V x 1.414 to get the likely peak voltage of an unloaded transformer, and that's 40.5 volts peak, normal, not under unlikely conditions. I say your zeners should be AT LEAST 40 volts.

    Please check with Wookie about this.
     
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