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Crouzet 50 amp SSR questions.

Discussion in 'Electronic Design' started by James Lerch, May 29, 2004.

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  1. James Lerch

    James Lerch Guest

    Greetings All,

    Just picked up a lot of 60 Crouzet 50 amp solid state relays on Ebay.

    http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=3818024478&sspagename=STRK:MEWA:IT&rd=1

    While I don't have a use for ALL 60 yet, I plan to use at 6 of them to
    add computer control to a 6 element electric ceramic kiln. The kiln
    draw ~50 amps at 220Vac when all 6 elements are on "high"

    The goal for the project is to be able to perform fine annealing on
    various glass wares, specifically glass telescope mirror blanks. My
    current idea is to install one SSR per heating element. The
    temperature control is based on a K-Type thermocouple monitored by the
    control computer, which will cycle the SSR's on/off with a frequency
    of 5 seconds.

    My first question is "Will I need to include a heat sink?" Each SSR
    will only switch a single element at 9amps @ 220vac.

    Second question, will I need to include any ancillary support
    electronics? I plan to trigger the SSRs off a computer printer port.
    In previous adventures with printer ports, I've used an 8 bit line
    driver between the printer port and the stuff I manipulate. The specs
    for the SSR indicate the DC side is optically isolated and only draws
    10mA. This bodes well for direct control via a printer port. While
    on the topic of ancillary electronics, will I need / want to include a
    pull down resistor across the DC control inputs on the SSR? I ask as
    I found out the hard way that strange things can happen with inputs
    left in a 'floating' state.

    My last question is unrelated to the kiln control project, "Are there
    any loads I should not be switching On/Off with an SSR? "
    Specifically, can I switch large inductive loads?

    Finally, for those that will be concerned with safety oriented ideas,
    here's a short list of steps I plan to include to prevent burning my
    shop to the ground.

    #1 The Kiln already has a "Kiln Sitter" installed, which will remain
    active as part of my 'upgrade'. The kiln sitter consists of two
    devices.
    A) A 20 hour timer, normally set to a value larger than the
    anticipated firing time. Since my firing times will exceed this
    value, I'll have to manually reset it every so often.

    B) A pyrometric cone, usually selected to achieve the firing
    temp desired during normal ceramic work. In my application, I'll
    select a cone that will be slightly higher than my anticipated target
    temperature. In the event of a failure causing the kiln to be left in
    an on state, the pyrometric cone will shut the kiln down.

    #2 I also have several software safe guards included.

    A) The computer controlling the kiln is on a network, and logs
    its data and status to a separate computer.

    B) The monitoring computer is able to alert me via an audible
    alarm and my digital pager, in the event the controlling computer
    doesn't respond, or the kiln temperature exceeds some threshold.

    I guess that's all my questions for now, comments and suggestions are
    greatly appreciated!


    Take Care,
    James Lerch
    http://lerch.no-ip.com/atm (My telescope construction, Testing, and Coating site)

    Press on: nothing in the world can take the place of perseverance.
    Talent will not; nothing is more common than unsuccessful men with talent.
    Genius will not; unrewarded genius is almost a proverb.
    Education will not; the world is full of educated derelicts.
    Persistence and determination alone are omnipotent.
    Calvin Coolidge
     

  2. You deserve an answer yet I note that noone has replied just yet...

    Yes, some heatsinking is in order here. If the datasheet specification for
    on state voltage (~1.6V) or so is in the ballpark, then at 9 Amps you can
    expect something like 14W of heatsinking required. I note that the
    operating temperature specifcations are suprisingly low going up only to 80
    deg. C (with storage temp going to 100 deg. C). These are suprisingly low
    numbers compared to other thyristor devices, but nevertheless you should try
    to adhere to them. I wonder if the 80 deg. C operating temperature is meant
    to be case temperature or junction temperature, the datasheet doesn't seem
    all that explicit.

    What is of particular interest to note here is the maximum current and
    maximum voltage drop ratings for the 90 Amp device for instance. Under
    these conditions the power dissipation would be something like 90*1.6 =
    144W. That is a large number. The Rjb parameter is 1.6 deg. C/W. I'm not
    certain what Rjb stands for, but it sounds like junction to case thermal
    resistance. If that is to be believed, at 90A there would be approximately
    a 144W * 1.6 C/W = 230 deg. C junction temperature rise attributed to this
    thermal resistance alone. Given that no thyristors I'm familiar with are
    rated for anywhere near such high junction temperatures, something is rather
    strange/amiss here...

    Perhaps contacting the manufacturer is in order here to figure out what
    their spec. sheet really means. As it is, it is rather contradictory in
    some regards.


    By the looks of the datasheet the beasties you have bought look pretty self
    contained. On the other hand it appears based on the Ebay listing you have
    the 90-280Vac input devices. So, based on this it looks like you'll need
    some additional hardware to interface with the lowly DC parallel port. It
    is concievable that maybe the SSRs will work with a say 160V DC input
    applied to the AC input, but the datasheet isn't very explicit about this,
    so in the worst case scenario damage to the SSR could result if you try
    this... You might try contacting the manufacturer or looking for more info
    about your devices...


    Inductive loads are probably acceptable, but normally directly capacitive
    loads should be avoided. In your case the devices you have claim to be zero
    volt turn-on devices, which I presume to mean they turn on at zero crossings
    of the mains. For charging capacitors this is good, but I would still be
    careful to insure some resistance in series with the capacitor if you are
    going to be playing with capacitor loads.
     
  3. Hi, James -

    Rjb is junction-to-baseplate thermal resistance, so Fritz is correct. He is
    also correct that the 80C temperature referred to is the baseplate. I picked
    up the following on the Crouzet Web site:



    "Q. How do I select an adequate heat sink for my
    application?



    A. You must first determine the amount of power being
    dissipated by the relay in the application. You can calculate this
    multiplying the load current by the voltage drop (Vf) across the output of
    the SSR. The Vf of a SSR can be found in its specification sheet or can be
    measured individually. Once this is determined, then you must calculate the
    difference between the ambient temperature within the panel during operation
    and 80ºC (recommended max base plate temperature). Then, divide the
    temperature difference by the power being dissipated to determine the
    maximum thermal impedance of a heat sink for the application.



    For example, a relay is carrying 50 amps in an application with a Vf of
    1.1Vpk, resulting in 55 Watts of power being dissipated. The ambient
    temperature inside the panel is 35ºC, leaving a 45ºC difference between
    ambient and the maximum recommended base plate temperature of 80ºC. If you
    divide the temperature differential of 45ºC by the 55 Watts of power being
    dissipated, the result will be 0.818, or a 0.82ºC/W heat sink for the
    application. To be prudent, you should always deduct 0.1ºC/W from the result
    to account for the thermal compound used in the assembly, and then round
    down to the nearest tenth. So, a 0.82ºC/W heat sink less 0.1º is 0.72ºC/W.
    Rounding down, you now know that you need at least a 0.7ºC/W heat sink for
    your application.



    Keep in mind that this is the most conservative way to calculate your heat
    sink requirements. If the duty cycle is not 100%, then you may need less of
    a heat sink. The best way to determine the "head room" in your application
    is to measure the temperature of the base plate of the relay within the
    actual application, keeping in mind the 80ºC recommended maximum base plate
    temperature."





    Here is the link to that FAQ: http://www.crouzet-usa.com/faq/SSR_FAQ.doc
    (there's more good info there)



    If I understand your post, each SSR will be handling about 9 amps. That will
    result in considerably less than 1.6 volts drop, but without the device
    curves to consult, just use the 1.6 volts. That will be 14.4 Watts to
    dissipate. Assuming your ambient never goes over 25C, then you need a
    (80-25)/14.4 or 3.8 C/W heat sink (for each SSR).



    I hope this helps. Good luck.



    John
     
  4. James Lerch

    James Lerch Guest

    Thanks John and Fritz, I appreciate the feedback! (even though my POS
    ISP didn't list Fritz's reply, at least Google is on the ball!)
    After all these years on the internet, you'd think I'd look to see if
    Crouzet had a web-site :)
    Sounds like I may have found a use for a few of those old 486 cpu
    coolers I have laying about!

    Thanks again!


    Take Care,
    James Lerch
    http://lerch.no-ip.com/atm (My telescope construction, Testing, and Coating site)

    Press on: nothing in the world can take the place of perseverance.
    Talent will not; nothing is more common than unsuccessful men with talent.
    Genius will not; unrewarded genius is almost a proverb.
    Education will not; the world is full of educated derelicts.
    Persistence and determination alone are omnipotent.
    Calvin Coolidge
     
  5. Terry Given

    Terry Given Guest

    I meant to a few days ago but forgot...phone rang :)
    yep. Its most likely the baseplate temperature.
    Rjb = junction-to-baseplate - fairly common terminology with modules.

    The base plate not only acts as a heat spreader, because of its thermal time
    constant it may well average out the power dissipation - the 90A is still
    probably bullshit (like IR insists on rating FET Rdson at Tj = 25C - as if!)

    Note the reference to "G" series thermal dissipation curves for heatsink
    size (bottom RHS note 7)

    yes you are gonna need a heatsink
    Zero-Voltage Turn-on (ZVT) is perfect for resistive loads. For AC inductive
    loads it is exactly wrong, and gives rise to inrush current (the peak
    current will be 2x as high as you might expect). An AC inductive load should
    be turned on at the voltage peaks (troughs) where the inductor current is
    supposed to be zero. For a phase shift of < 90 degrees the optimum turn-on
    point is somewhere else.

    A quick laplace transform analysis shows this to be the case.

    Depending on the inductive load, the (almost) inevitable inrush current may
    cause the inductor to saturate, giving rise to inrush currents well above
    2x. most LF magnetic devices beat the hell out of their core materials, and
    os run close to (if not slightly in) saturation. This is why your
    transformer goes "boing" when you turn it on...

    Of course a 3-phase system can NEVER choose the optimum time to turn an
    inductive load on - so inrush is a real pain. The transformer saturates, and
    Inrush = 100% volts/%leakage inductance as the leakage inductance is
    effectively air-cored. A 5% transformer will therefore have a peak inrush of
    about 20x. The exact same argument applies for motors too. High-efficiency
    (and large - think SA/V ratio) motors have extremely low leakage, and
    therefore extremely high inrush current. pissy little motors are generally
    quite shitty, and often have > 33% leakage, giving inrush of < 3x

    capacitive loads are always a problem.

    Cheers
    Terry
     
  6. Terry Given wrote:
    (snip)
    A few years ago, I was involved in the calibration of the timing of
    big vacuum relays (electromagnetic, not solid state) used to switch
    capacitive power correction banks onto an 11 kV, 60 Hz 3 phase line.
    It turns out to be possible to zero voltage switch on into a 3 phase
    capacitive load (with separate angle for each phase), but it isn't
    easy. It is also loud.
     
  7. David Lesher

    David Lesher Guest

    How does this shut down the kiln?

    I'd consider a separate, hardware-based, system as well. ?RTD? sensor
    in room, connected to a NO contacts on main power; if it gets worried,
    it drops the power, period.
     
  8. GPG

    GPG Guest

    electronics? I plan to trigger the SSRs off a computer printer port.
    The spec for 84131021 shows 90 - 280VAC input
     
  9. James Lerch

    James Lerch Guest

    David,

    The Kiln Sitter is the original equipment kiln 'controller' for this
    kiln.

    #1 When the timer expires, it turns off the power to the heating
    elements somehow. (I haven't disassembled it, but I'd have to guess
    there is a big contactor inside the box)

    #2 When the pyrometric cone reaches its target temperature, it softens
    and bends under the load applied by a counter weight. When the
    counter weight is freed by the bending of the cone, an electric switch
    opens, again shutting down the kiln.

    More information on the kiln sitter can be found here:
    http://www.kiln-sitter.com/support.html


    Take Care,
    James Lerch
    http://lerch.no-ip.com/atm (My telescope construction, Testing, and Coating site)

    Press on: nothing in the world can take the place of perseverance.
    Talent will not; nothing is more common than unsuccessful men with talent.
    Genius will not; unrewarded genius is almost a proverb.
    Education will not; the world is full of educated derelicts.
    Persistence and determination alone are omnipotent.
    Calvin Coolidge
     
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