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Thread: UL recognized potentiometer

  1. #1
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    UL recognized potentiometer

    Good afternoon,

    Currently we are in process of getting ETL listing mark for an appliance we manufacture (120Vac/25Amps). The testing laboratory is checking to see if the appliance complies to the requirements of UL197 standard (Commercial electrical cooking appliance) and CSA C22.2 no.109, which is a Canadian standard for the same. Accordingly to the standards requirements, all electric components in primary circuit must be UL listed or UL recognized.

    Now to the issue. The appliance has a heating element (1600W), the power supplied on which is regulated by adjustable SSR (phase control type). Basically, this SSR is a common dimmer controlled by a potentiometer. The exact model is FOTEK SSR-25VA (http://www.fotek.com.tw/Ecenter2.asp...8&class2_sn=18). There are no galvanic decoupling between control and load circuits in this SSR unit. In other words, the potentiometer actually is in primary circuit under main voltage (120Vac) applied on it, see the diagram below:

    Click image for larger version. 

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    So, the testing lab claims that we must use UL recognized potentiometer in this case; however, the problem is that I can't find variable resistors/potentiometers/rheostats at the UL database website (http://database.ul.com/cgi-bin/XYV/c...AME/index.html). Moreover, I asked many potentiometer manufacturers, and it has been revealed that they don’t offer UL recognized potentiometers. The manufacturer of SSR also couldn’t suggest any UL recognized potentiometer.

    It seems quite strange, because the SSR-25VA unit is UL recognized (UL file #E208225), and the UL database contains a lot of components, but there are almost no potentiometers itself. Kind of paradox: there is a UL recognized SSR unit but there are no UL recognized potentiometer to use along it.

    Please advise if someone have had similar experience with using UL recognized potentiometers.

  2. #2
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    Can the testing lab back up their requirement by demonstrating that such a part even exists?

  3. #3
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    I have asked them to recommend some particular potentiometer, and I even suggest to use a military qualified potentiometer made in accordance with MIL-PRF-94 specifications, the model RV4NAYSD102A by Honeywell; but they just keep saying 'please provide a listed/recognized one'.

  4. #4
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    Allen Bradley 800T and 800H potentiometers are UL listed under UL file NKCR.E14840
    __________________________________________________ ____________________________
    Many people are shocked when they discover I am not a good electrician...

  5. #5
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    Quote Originally Posted by exolon View Post
    Good afternoon,
    Currently we are in process of getting ETL listing mark for an appliance we manufacture (120Vac/25Amps). The testing laboratory is checking to see if the appliance complies to the requirements of UL197 standard (Commercial electrical cooking appliance) and CSA C22.2 no.109, which is a Canadian standard for the same. Accordingly to the standards requirements, all electric components in primary circuit must be UL listed or UL recognized.
    Now to the issue. The appliance has a heating element (1600W), the power supplied on which is regulated by adjustable SSR (phase control type). Basically, this SSR is a common dimmer controlled by a potentiometer. The exact model is FOTEK SSR-25VA (http://www.fotek.com.tw/Ecenter2.asp...8&class2_sn=18). There are no galvanic decoupling between control and load circuits in this SSR unit. In other words, the potentiometer actually is in primary circuit under main voltage (120Vac) applied on it, see the diagram below:
    So, the testing lab claims that we must use UL recognized potentiometer in this case; however, the problem is that I can't find variable resistors/potentiometers/rheostats at the UL database website (http://database.ul.com/cgi-bin/XYV/c...AME/index.html). Moreover, I asked many potentiometer manufacturers, and it has been revealed that they don’t offer UL recognized potentiometers. The manufacturer of SSR also couldn’t suggest any UL recognized potentiometer.
    It seems quite strange, because the SSR-25VA unit is UL recognized (UL file #E208225), and the UL database contains a lot of components, but there are almost no potentiometers itself. Kind of paradox: there is a UL recognized SSR unit but there are no UL recognized potentiometer to use along it.
    Please advise if someone have had similar experience with using UL recognized potentiometers.
    I see two problems.

    The most obvious one is the control system that you have deployed. . . . which leads to the other problem of being unable to find a UL listed/approved potentiometer.

    Generally, small electronic component specifically designed or operating at or well below 24 volts doesn't need a separate UL listing if it is an integral part of the appliance. The UL approval would be for the whole (complete) appliance.

    Your drawing, if that is the representative of the whole circuitry and arrangement, is telling me that the potentiometer or rheostat is isolated from the unit and as you mentioned is decoupled from the control and load. Since it is connected to the line voltage of 120 volts— it is well above the limit of low voltage 24 volts. That's the number two problem.

    What I can't wrap around my head is: why would you have the low voltage (24 volts) SSR component be controlled by the line voltage (120volts) potentiometer/rheostat instead of having it the other way around.

    If the whole assembly is in one enclosure without any component located externally eg with wire extensions, etc, you might be able to circumvent AHJ requirement.

    Just make sure you get UL or CE approval for the whole assembled unit.

    BTW: Potentiometers which include trimmers are different from rheostats--at least in our midst.

  6. #6
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    In a situation like that you would isolate the potentiometer from the knob with a non-conductive shaft extension then have the complete assembly UL listed. This is done every day with transmitters and HV power supplies.

    -Hal

  7. #7
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    Quote Originally Posted by Jraef View Post
    Allen Bradley 800T and 800H potentiometers are UL listed under UL file NKCR.E14840
    Thank you for suggestion. But, I forgot to mention, I've seen those before. The problem is that this kind of appliances (it is cotton candy machine, btw) usually uses not so expensive potentiometers. Since the net price of the whole appliance doesn't exceed few hundreds USD, it makes no sence to use a potentiometer that costs more than 200 USD (800T costs around 260 USD).


    Quote Originally Posted by myspark View Post
    Your drawing, if that is the representative of the whole circuitry and arrangement, is telling me that the potentiometer or rheostat is isolated from the unit and as you mentioned is decoupled from the control and load. Since it is connected to the line voltage of 120 volts— it is well above the limit of low voltage 24 volts. That's the number two problem.


    What I can't wrap around my head is: why would you have the low voltage (24 volts) SSR component be controlled by the line voltage (120volts) potentiometer/rheostat instead of having it the other way around.

    Actually there is no 24V at all. Here is the full circuit of the SSR unit:
    Click image for larger version. 

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    So, the potentiometer (VR1) works as a rheostat, and there is no galvanic decoupling; thus the variable resistor is under the same 120V line voltage.


    Quote Originally Posted by myspark View Post
    If the whole assembly is in one enclosure without any component located externally eg with wire extensions, etc, you might be able to circumvent AHJ requirement. Just make sure you get UL or CE approval for the whole assembled unit.

    By the way, we already successfully got the appliance certified per CE applicable requirements, with the same SSR unit and a few dollar potentiometer. For that point, CE certification has less strong restrictions regarding critical components.


    Quote Originally Posted by hbiss View Post
    In a situation like that you would isolate the potentiometer from the knob with a non-conductive shaft extension then have the complete assembly UL listed. This is done every day with transmitters and HV power supplies.

    Thanks for suggestion, I will check with the testing lab if it is possible to make that way. As far as I understand, there are different requirements for different types of appliances (household, commercial, medical and so on), and there may be complex issues mixing electric and mechanic features that interact to each other. It is a another topic for discussion.

  8. #8
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    Note that common run of the mill lighting dimmers use a line powered circuit and a cheap looking potentiometer (rotating or linear).
    Whatever they use must be sufficient to allow the final product to be UL listed in its category. In some but not all cases there is a plastic shaft connecting to the pot rather than a metal shaft.

  9. #9
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    Quote Originally Posted by GoldDigger View Post
    Note that common run of the mill lighting dimmers use a line powered circuit and a cheap looking potentiometer (rotating or linear).
    Whatever they use must be sufficient to allow the final product to be UL listed in its category. In some but not all cases there is a plastic shaft connecting to the pot rather than a metal shaft.
    True, and at first I thought if they can do it, it must be OK here too. But maybe the difference is in the requirements for a cooking appliance listing.

    Its been a while since I've done anything with Triacs and such. Is it possible to place a small control transformer in the circuit, and have the variable resistor connected to the transformer low voltage side? I'm not sure if that would work for triggering the SCR.

    Or what if a small fuse was placed in series with the pot? Does it still count as part of the "primary" circuit?

  10. #10
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    Quote Originally Posted by steve66 View Post
    True, and at first I thought if they can do it, it must be OK here too. But maybe the difference is in the requirements for a cooking appliance listing.

    Its been a while since I've done anything with Triacs and such. Is it possible to place a small control transformer in the circuit, and have the variable resistor connected to the transformer low voltage side? I'm not sure if that would work for triggering the SCR.

    Or what if a small fuse was placed in series with the pot? Does it still count as part of the "primary" circuit?
    I would think that the listing standard for your product would include necessary tests which, if successfully carried out, would negate the need for every component to be recognized??

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