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Solid state relay

  1. Oct 30, 2011 #1
    1519-1.jpg

    So I want to use this solid state relay, to fast switch an electromagnet.

    About 2 times a second.

    Weight: 112g
    Size:5.9 x 4.5 x 2.3cm
    Color : Grey
    Fast switching
    Reduced electromagnetic interference
    Output voltage: AC 24-380V
    Output current: 25A
    Input voltage: DC 3-32V
    Control Method: DC to AC
    Mounting Method: Bolts fixed

    These are characteristics of this SS relay.

    From these characteristics, it says output should be AC. But my electromagnet is powered by DC, will it still work if i pass DC current through output of this relay?
     
  2. jcsd
  3. Oct 30, 2011 #2
    What kind of power source are you using for the electromagnet??
     
  4. Oct 30, 2011 #3
    30 v dc, I wrote it.
     
  5. Oct 30, 2011 #4

    jim hardy

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    draw out your circuit


    some solid state relays will switch dc, some will not
    it depends what they used for a switch inside the device

    some use a scr which won't turn off until next sinewave zero crossing, which for dc is never.
    some use a device that can be gated off mid-cycle, like a power transistor inside a bridge rectifier, and that should interrupt DC.

    so check the datasheet carefully perhaps there's a clue there.
    i wonder what they meant by "fast switching"...

    if you have it already try it out on a big light bulb, use one of those big 300 watt halogens so itll pull some current at 30volts.
    if you're ordering it go back to the catalog and look for one that'll switch DC.

    you may have to switch instead the ac that feeds your power supply
    or resort to a power transistor

    in either case since an electromagnet is inductive you probably ought to include a "flywheel diode" across its coil.
     
  6. Oct 30, 2011 #5

    Well,

    My uncle came today and told me this:

    We need to design a circuit, that turns on the electromagnet, which pulls a piston of sort. When that piston is pulled, a spring gets tightened. After electromagnet goes off, that spring pulls that piston back in.

    This pulling motion is creating pressure somewhere(didn't really get that part) that sprays a tint or something.

    My uncles is a mechanic at a newspaper printshop, their engineer has been having problems with this circuit so he told me to try and build it.

    I said well yea, its no big deal.

    I had relay in my head, and some PIC controller. Program it, and set a varicap(or resistor even, possibilities of PIC are many) diode somewhere to control the rate of pulling/sec.

    But then electromagnet self-induction crossed my mind.

    Its all fine and dandy that my circuit will turn on and off as desired, but will my magnet do so too? But then you mentioned this flywheel diode, and immediately i realised what it does.
    I will try some simulations in National Instruments, and see what will happen.

    I understand what you were trying to say with this thyristors in SS relays. If they do switch with thyristors, then they won't go on dc.

    A normal relay has slow response, yes?
     
  7. Oct 30, 2011 #6

    jim hardy

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    Thank you - thyristor is the word i was searching for.

    Industrial electromechanical relay like Westinghouse BF is surprising - it'll operate within one line cycle. But there is a finite life on the contacts and DC shortens it.

    a 555 timer could generate the timing for you and its output could drive the gate of a power mosfet directly.

    "Its all fine and dandy that my circuit will turn on and off as desired, but will my magnet do so too?"
    Two effects are at play here. But they're related.
    first - induction in its coil - the energy stored in the coil will maintain current for a short while. A parallel diode allows that current to flow, dissipating the energy as heat in the coil windings and in the diode. Some resistance in series with the diode willl make that energy dissipate faster, driving current to zero more quickly, so you have some control over how fast your electromagnet will lose its mmf.
    Probably it's fast compared to the process of your Uncle's paint sprayer but i dont really know that's a guess....
    that'd be found by experimenting with resistor values. Some folks shorten release time by using a zener diode for flywheel.
    twice per second sounds like reasonable cycle rate for a medium sized solenoid.

    second effect - iron itself is a conductor and in presence of a changing magnetic field Lenz tells us currents will flow that oppose the change in magnetization.
    So when current is switched on the magnetization proceeds from surface toward center like painting the layers of an onion.
    That's why they laminate magnetic cores, of course. Is your uncle's electromagnet laminated?

    Anyhow - when you turn off the current the mirror image of that effect happens and there's a delay in de-magnetizing, just because of the iron..
    This effect is called "Retardation of Magnetization".
    Old railway dynamos ca 1900 (which were huge) could take a minute to completely magnetize a two foot diameter pole piece. Made them slow to respond to contol signals.
    [note - i read about that, i'm not THAT old ! see "Dynamo Electric Machinery" by Sylvanus P Thompson, 1901 ]

    You knew all that. So examine the core of a small transformer. the laminations are just a mm or so thick for line frequency - that gives you a rough feel for the rate at which magnetization progresses. Thicker than that and the eddy currents will retard it.

    If your electromagnet is not just huge you'll probably be okay.

    What is the nature of the trouble your uncle encountered?

    old jim
     
    Last edited: Oct 30, 2011
  8. Oct 30, 2011 #7
    He wasn't very specific, he told me what he wanted this circuit to do. Like he cares for hysteresis and all that EE stuff. From what he said, I don't think that the electromagnet is huge, but I will call him for more specifics.

    Anyway, thank you very much for the information. I will google this through, you gave me a very good baseline. This might be my first job as EE ! (2nd year and still learning how to use oscilloscope,theres your engineer :rofl: )
     
  9. Oct 30, 2011 #8

    jim hardy

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    ""2nd year and still learning how to use oscilloscope,.."
    the key is learning to set up your trigger condition.
     
  10. Oct 30, 2011 #9
    A lot of stuff to work with. But in the end I will have the profit of experience and gained knowledge.
     
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