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Solid State Relays and EMI

Discussion in 'General Electronics' started by David Harper, Jul 24, 2004.

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  1. David Harper

    David Harper Guest

    I'm using some solid state relays, and having some problems with (I
    believe) EMI.

    I'm using this part in the B and C configurations.

    These items are located near some RF sources operating at about
    148MHz. During transmit (.5W power), I'm getting some false "on"
    states. I'm pretty sure this isn't due to the equipment on the load
    side, as removal of the SSR's eliminates the effect.

    1. Anyone know if these type of SSRs are subject to EMI that can
    cause false on-states?
    2. Would induced AC voltages (from the RF source) in the load lines
    (around 148MHZ) decrease the R-off resistance?
    3. If the problem is in the SSR's, anyone have any suggestions
    regarding how to lessen/prevent this? (EMI shielding, other PN's that
    are immune to this, inductors/caps that could be placed in the load
    lines, etc).

    Thanks for your time!
    Dave Harper
  2. Robert Baer

    Robert Baer Guest

    Be advised that relays made with copper (a *solid*), iron (a *solid*),
    sometimes plastic (a *solid*) and sometimes glass (a *solid*) ARE SOLID
    STATE (certainly *not* gas state or liquid state)...
    Also there are a number of variations that handle RF very well with
    low VSWR, and some have response times under a millisecond.
  3. "David Harper" ...
    What type _exactly_? URL?
    Not if it's a mains load switching type with a triac or thyristor output,
    they have no "R-off", they are either ON or OFF.
    SSR's with an optocoupler inside are easily triggered by RF voltages
    occurring between in-and output. You might try adding a ceramic Y-rated
    capacitor (470pF ... 1nF should do), between the ground on the input
    (optocoupler) side and the supply pin on the output (contact) side, as short
    a connection as possible. Also one over the output pins.
    **** Warning **** this may solve the problem but will add leakage current
    between controller and mains. Check you stay on the safe side!

    Another test could be just the 1nF Y-rated cap over the output (less likely
    to help).This does not introduce leakage to the controller ground, just some
    extra OFF-state current in the load.

    Arie de Muynck
  4. David Harper

    David Harper Guest

    Well, obviously. Solid state generally means made of semi-conducting
    material and containing no moving parts. Are you thinking of magnetic
    (coil) relays? The ones I'm using involve a LED and a MOSFET, not a
    These have an average response time of about .35ms. Also, I'm not
    running an RF signal through them. They are being affected by a
    nearby RF source.

  5. Chris S.

    Chris S. Guest


    Main Entry: solid-state
    Function: adjective
    1 : relating to the properties, structure, or reactivity of solid
    material; especially : relating to the arrangement or behavior of ions,
    molecules, nucleons, electrons, and holes in the crystals of a substance
    (as a semiconductor) or to the effect of crystal imperfections on the
    properties of a solid substance <solid-state physics>
    2 a : utilizing the electric, magnetic, or optical properties of solid
    materials <solid-state circuitry> b : using semiconductor devices rather
    than electron tubes <a solid-state stereo system>

    So no, the molecular make-up of a device does not necessarily categorize
    it as solid-state. It's solid state if every atom in the device is
    relatively stationary, meaning no moving parts (e.g semiconductors).
  6. Robert Baer

    Robert Baer Guest

    I *do* know the difference, and that the terminology "solid state
    relay" is used for relays using some kind of semiconductor technology -
    starting from incandescent lamp used to illuminate a CdSe or similar
    With sufficent RF field, all "solid state relays" will be affected -
    ie: act as if some non-zero input had been applied.
    Sometimes, the RF field can even make the "solid state relay" act as
    if it were linear and/or erratic.
    In the case of LED/FET combo, the FET is the first to be affected.
    Proper shielding and RF bypassing will help reduce the effect.
    If the RF field is very strong, then the FET and perhaps the LED would
    be destroyed.
    Depending on the severity of the problem, one could go back to the
    older "solid state relay" technology and use the lamp/CdSe scheme OR use
    "iron state" relays (to coin a term), which can be rather sensitive.
  7. Robert Baer

    Robert Baer Guest

    Let us see - - the definition you quoted maid no mention, even
    indirectly concerning *moving* parts.
    And the last time i looked, each (individual) piece of copper , iron,
    plastic, glass, whatever does not move...
    And the combination of the copper and iron uses the magnetic
    properties of (solid) iron; which fits the quote exactly...
    BTW, iron has a rather definitive crystalline structure, and since
    annealed OHFC is not used for the coil, then the copper also has a
    crystalline structure.

    Mind you, i am not arguing.
    Just point out a few iregularities...
  8. Chris S.

    Chris S. Guest

    True, although it does mention semiconductors, and I can count the
    number of semiconductors composed of moving parts on zero hands.
    When used in solid-state electronics, correct. move mechanical components in an electro-mechanical device.
    Could have fooled me ;)
    Well sure, if you want to get technical, vacuum tubes and semiconductors
    are both "solid" objects in the sense that they're not made out of gas,
    water, or plasma. But if you're going to nitpick subtle nuances of the
    English language there are far easier targets (why do we drive on a
    parkway and park on a driveway?).

    I think the confusion concerns the use of "solid", which is used in the
    sense of the device's physical configuration, not so much its molecular
    state. We could use the term "finite-state", but that's generally used
    to describe electric circuits.

    I'm just pointing out convention. In all fairness, the term is still
    jargon, but referring to any electro-mechanical device as "solid-state"
    will confuse most people.
  9. R.Legg

    R.Legg Guest

    With the turn-on time being ten times the turn-off time, I don't
    expect that false turn-on will be a result of LED emitter modulation
    at the interfering frequency.

    Fet structures usually have a hefty input capacitance; the coupling
    capacitance to the gate is piddling by comparison.

    Biggest coupling capacitance that could produce false gate turn-on is
    from the two drains. Is there a fairly large amplitude of RF voltage
    present there? These thinga are really only isolators at low frequency
    or DC, unless a voltage close to at least 1/10 their rating exists
    when 'off'.

  10. I read in that Chris S. <>
    Not any more. Consider the TI micro-mirror-matrix chips, for example,
    and solid-state microphones are appearing on the market, too.
  11. Solid-state accelerometers with on-chip bending arms have been
    available for about a decade- and have sold in large quantity.

    Best regards,
    Spehro Pefhany
  12. Chris S.

    Chris S. Guest

    I wouldn't necessarily classify MEMS devices as solid-state. While
    certain types make use of "bending" to move small actuators, they still
    rely on this movement to perform their function. The characteristic I
    typically associate with solid-state is a lack of dependence on physical
    motion to function.
  13. IMHO, the key characteristic is that the function depends on
    semiconductors. The branch of science involved is solid-state physics.

    Best regards,
    Spehro Pefhany
  14. David Harper

    David Harper Guest

    That's more of a technicality. Those microphones use piezo-electric
    effects, which is a function of [micro]strain.

    Yes, technically the elements are moving a few microns (or less).
    There's several "solid state" temperature sensors that experience far
    more "movement" from thermal expansion than those microphones do.

    They are not moving fractions of an inch (or more), like they are in
    electro-mechanical devices.

  15. David Harper

    David Harper Guest

    I think you're missing the original point. In the field of
    computer/electrical engineers, "solid state" refers to a branch of
    electronics based primarily on semi-conducting (ex. silicon)

    The whole "movement" part is really a minor footnote.

    Your original statement about them being "solid" and not "gas" or
    "liquids" was obvious, and came from left field. I don't think anyone
    reading this thread would go into Radio Shack and ask for "liquid

  16. Jim Thompson

    Jim Thompson Guest

    Burridge might ;-)

    ...Jim Thompson
  17. OK, but I've seen SSRs used at broadcast transmitter sites running
    many kilowatts of RF with no problems. SSRs are like Rank telecines.
    When you see it 'misbehave' don't assume it isn't being 'told' to do
  18. The ones I'm using involve a LED and a MOSFET, not a Thanks for answering my question - although on another part of the thread...

    You may not know it, but the light from the LED does not directly turn-on
    the MOSFET. It shines on a string of small photovoltaic cells that charge
    the gates. When the light is off, the photocell leakage current (sometimes
    with an extra resistor across them) discharges the gate again.

    Any idea what happens if an RF voltage is capacitively coupled from LED pins
    to the _string_ of photodiodes? It's called rectification. The resulting
    voltage turns on the gate.

    Did you alrady do the test with the 1nF between driver size and output side?

    Arie de Muynck
  19. NeoRenegade

    NeoRenegade Guest

    1. It's very likely. In my classes, I've been taught that EMI can bugger
    up the proper working of just about any circuit, so it should always be
    taken into consideration.
    2. Er, I think it's a simple matter of induced current in the line
    giving false hi signals.
    3. First thing I'd do is make absolutely sure your relay coil is
    grounded rather than floating when in the "off" state. Being assured of
    that, if the problem persists, you might want to try a simple RC
    band-elimination filter. If you do not have it handy, I can give you the
    formula to determine the R and C values.

    I apologize in advance if your electronics knowledge is more advanced
    than mine and I've just wasted your time.

    - NR
  20. Robert Baer

    Robert Baer Guest

    I was having a little fun...
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