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    Default sizing SSR's?

    For a relatively low temp oven that might use heaters totaling 32A, but i might want to add capacity (elements, cu ft volume) later depending on actual temp & ramp speeds...
    Is there any reason NOT to use an 80A SSR? (Instead of the intuitive/typical 40A unit)

    IOW, coming from mechanical contactors, my gut tends to favor more ampacity as always being good
    Does this apply as well for solid state?

    Solid State Relays (SSR) : Auber Instruments, Inc., Temperature control solutions for home and industry

    Comparing SRDA80 vs SRDA40.
    Also, is there any benefit to the UL version? (same price in 80A)

    Thanks!
    smt

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    Quote Originally Posted by stephen thomas View Post
    For a relatively low temp oven that might use heaters totaling 32A, but i might want to add capacity (elements, cu ft volume) later depending on actual temp & ramp speeds...
    Is there any reason NOT to use an 80A SSR? (Instead of the intuitive/typical 40A unit)

    IOW, coming from mechanical contactors, my gut tends to favor more ampacity as always being good
    Does this apply as well for solid state?

    Solid State Relays (SSR) : Auber Instruments, Inc., Temperature control solutions for home and industry

    Comparing SRDA80 vs SRDA40.
    There is no reason not to use the 80-amp unit.

    With SSRs, the thing to worry about is current surges. If the load is low-temperature heaters (versus say incandescent loads), there won't be any surges to speak of. What is the ratio between room-temperature and operating-temperature heater resistance?


    Also, is there any benefit to the UL version? (same price in 80A)
    Purely a legal question. CE is plenty good enough for safety, but your local electrical code may require UL.

    By the way, what is the heater supply voltage?


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    There is no reason not to use the 80-amp unit.
    Great!
    Thanks for that.

    Purpose is to blow cast acrylic ("plexiglas") aircaft canopy.
    Working temp should be stable at 350 deg F
    Annealing cycle is "somewhat under 200 deg F" (depends on mfg literature and field opinions...)

    Supply is 220/240V single phase.

    Components are not spec'd yet. i'm sidling in that direction and certainly open to suggestions.
    Most likely (kitchen range) oven elements, 3600W ea, one at each end, with blower & ducting between for air distribution.
    The box does not need to heat rapidly. If acrylic sheet is installed and oven started at same time, it should take 20 - 30 mins for 1/4" sheet to attain working temp. Chamber is not exactly defined pending my dithering on ducting & efficient use of structural parts, but aprox 7' x 3'w x 2.5' deep. (53 ft^3) Not likely to be smaller. Might increase depth.

    Thanks much for any continued advice.

    smt

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    Make sure you check the datasheet and size the heatsinks appropriately, and determine what the ambient cabinet temp will be. You often cannot use SSRs at their rated current continuously without putting a lot of work into cooling.

    Buying SSRs with pre-fitted heatsinks is often easier as the manufacturer can do all the thermal calcs and put them straight in the datasheet.

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    Quote Originally Posted by SomeoneSomewhere View Post
    Make sure you check the datasheet and size the heatsinks appropriately, and determine what the ambient cabinet temp will be. You often cannot use SSRs at their rated current continuously without putting a lot of work into cooling.

    Buying SSRs with pre-fitted heatsinks is often easier as the manufacturer can do all the thermal calcs and put them straight in the datasheet.
    +1 on the heatsinks. Ton of "ordinary" work where a Crydom o/e "mini-brack" is OK just thermal-pasted to a modest thickness of Shiney-Wood, such as the cabinet wall or plate.

    "Serious" work, OTOH, the "Best Practice" heat sinks can be several times the size of the SSR.

    This is where I tend to go over to "Gigavac" sealed mechanical contactors - or "previoius practice" was happy with Mercury-Displacement units.

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    It might also be better to use an SSR per element, instead of one big one for all elements. This potentially allows for control of separate areas, and multiple smaller SSRs are potentially cheaper than big ones as the heat sources are a bit more spread out.

    E.g. a bank of these, although they're only good for 3.6kW/15A sustained at about 30C ambient, so you may want to go for the 30A variant unless you can point a cabinet fan at them.

    Pay attention to particularly page K-20 of the datasheet, with the derating curves - these are fairly typical. Non-heatsinked parts are likely to show current vs case temperature, and you then select a heatsink that can maintain that case temperature at the required dissipated heat - typically around 1W per amp.

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    Also, keep in mind that an SSR that is "off" will still show voltage on the output side of the relay contact (though passes nearly no current). That little bit of info cost me several hours of electrical troubleshooting until I finally called the SSR manufacturer's tech department.

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    Quote Originally Posted by Mike Henry View Post
    Also, keep in mind that an SSR that is "off" will still show voltage on the output side of the relay contact (though passes nearly no current). That little bit of info cost me several hours of electrical troubleshooting until I finally called the SSR manufacturer's tech department.
    Although one would think simple and obvious some overlook the fact that a SSR never turns all the way off as a hard relay does.
    This can lead to surprises or debug time that makes no sense.
    In the two choices mentioned it seems the leak to be specified to be the same which is strange for twice the power handling.
    Bob

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    The "SSR" turns pretty well off. There may be "R-C snubbers" in there as well, and those do pass some current through the capacitor.

    If nothing else, there is some capacitance through the switching elements as well. SSRs do NOT form a "disconnect".

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    I failed to check back and missed some of this.
    However, an 80A ssr, 100A cap fancooled heatsink, & dual input PID controller are on their way from Auber, other supplies from other sources.

    It is unlikely this unit will ever be provided with larger than the 2 x 3600 w oven elements (30A) arriving from elsewhere, for this initial design. I just get nervous about under capacity, especially with electrical control devices; but did not know if that prejudice was appropriate to carry forward from mechanical contactor experience.

    Thanks for the continued input!
    smt


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