What kind of switch will mechanically break a circuit in response to a 12V signal - Page 2
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  1. #21
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    Mercury contactors are not perfect either. I used to work for a company that made industrial hot glue machines. They had a melter that would plunge a heated platten into 55 gal drum of glue and melt/pump it out of the drum. They used mercury contactors for temperature control and somehow they would stick on occasion.

    I this case I would add a latching power contactor for the main power and put a mechanical thermal cutout in series with the latch circuit (stop button) button. This way you have a separate contactor that’s tested to work each time the power is turned on and isn’t your main control l relay/contactor. You could even put a few of the thermal cutouts in series for extra protection.

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  3. #22
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    It's only a 4kw heater ! Personally, I'd use a SSR because the controller may be PWM out or at least switching on and off a lot, so an SSR will save the clacking of a relay.

    Typical circuit
    Attached Thumbnails Attached Thumbnails img_20210430_211958.jpg  

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  5. #23
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    I agree with the use of an ssr but you still need a safety shut down. And ideally it ought to use separate components. No matter how you control the heater it can fait some way.

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    Quote Originally Posted by dgfoster View Post
    One question that occurs to me is does anyone have experience to KNOW how much current is required to hold "on" a magnetic contactor. If they are designed to run a motor, at idle the motor will be drawing at least a couple amps of power. But in my application the only load on the contactor between segments of powering the heating coils would be perhaps milliamps to power the board and pilot light. So, I am suspicious the engineers might have designed the contactors to ignore such low power noise and open up. So from a computer logic point of view the circuit is closed (milliamps flow) powering the board and pilot light when the coils are not powered. But from a robust engineering point of view that power level might be "noise" and considered an open circuit.

    I can see the wisdom of the shunt breaker advocates, BTW. I am just in the gathering data phase right now and trying to fill in some holes in my knowledge.

    Denis
    The datasheet for a specific contactor should tell you what the required sustaining current for the coils is. Relevant jargon is "sealed" current. Just like motors and other inductive loads, the energizing (in-rush) current can be 6x to 10x the sustaining current.

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  8. #25
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    Quote Originally Posted by dgfoster View Post
    One question that occurs to me is does anyone have experience to KNOW how much current is required to hold "on" a magnetic contactor.
    Zero.

    The aux contact picks-up the power off the "closed" side of its OWN SELF.

    It doesn't actually know or care if the main contactor is even wired out of the box.

    If the main is running a 480 VAC motor's circuit, and its actuator coil is 24 VAC off a control transformer, you can see why a physically-slaved, but electrically isolated "auxiliary" contact is a necessity!

    ..and also why it is safe and easy to run that lower-voltage 24 VAC loop clear over to the doorway to the shop for "one of" the E-Stops, another E-Stop at the operator position, the 12V signal off the fault-alarm via a relay ....

    ANY of several minders throws a flag?

    OR "OFF" button is pushed?

    All play ceases!

    Hacker's Second Law
    :

    There are few things truly new in the Universe. When faced with a challenge, Job One is to ascertain how a previous SUCCESSFUL entity met it. Only THEN see if it requires adaptation, improvement, or merely adoption.

    Last edited by thermite; 05-01-2021 at 01:16 AM.

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  10. #26
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    Shunt breakers, relays, solid state relays, or any other device that you would like to consider will have an OEM's DATA SHEET. These are available from most standard suppliers (not E-bay or Amazon sellers). You can download them from the OEM's or supplier's web sites. FOR FREE! Get the data sheet for whatever device you are considering and READ and UNDERSTAND it. It will provide the needed design data and answer all such questions.

    I am designing an electronic device right now and have already downloaded several data sheets on the components that I will be using.



    Quote Originally Posted by dgfoster View Post
    One question that occurs to me is does anyone have experience to KNOW how much current is required to hold "on" a magnetic contactor. If they are designed to run a motor, at idle the motor will be drawing at least a couple amps of power. But in my application the only load on the contactor between segments of powering the heating coils would be perhaps milliamps to power the board and pilot light. So, I am suspicious the engineers might have designed the contactors to ignore such low power noise and open up. So from a computer logic point of view the circuit is closed (milliamps flow) powering the board and pilot light when the coils are not powered. But from a robust engineering point of view that power level might be "noise" and considered an open circuit.

    I can see the wisdom of the shunt breaker advocates, BTW. I am just in the gathering data phase right now and trying to fill in some holes in my knowledge.

    Denis

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    Quote Originally Posted by EPAIII View Post
    Shunt breakers, relays, solid state relays, or any other device that you would like to consider will have an OEM's DATA SHEET. These are available from most standard suppliers (not E-bay or Amazon sellers). You can download them from the OEM's or supplier's web sites. FOR FREE! Get the data sheet for whatever device you are considering and READ and UNDERSTAND it. It will provide the needed design data and answer all such questions.

    I am designing an electronic device right now and have already downloaded several data sheets on the components that I will be using.
    and drawings of the parts. I’ve looked some at shunts and have not yet seen clear data sheets for the item I’m interested in. But, working on it.

    Yes, I usually buy electronic components from Mouser or Digikey and they always provide data sheets and drawwings of those components. But in the case of breakers, the datasheets linked have so many subsets as to make it a challenge to sort out the information specific to the exact item of interest. Still working on that one.

    Denis

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    On another forum someone suggested putting an NC Latching relay in series with the main contactor. Seems like that should work OK. Anyone see a “fatal fault” with that idea?

    Denis

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    Quote Originally Posted by dgfoster View Post
    On another forum someone suggested putting an NC Latching relay in series with the main contactor. Seems like that should work OK. Anyone see a “fatal fault” with that idea?

    Denis
    Isnt that just the aux contact block that thermite wanted to use?

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    Fusible link/Thermal fuse used as an additional failsafe.

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    Quote Originally Posted by Rob F. View Post
    Isnt that just the aux contact block that thermite wanted to use?
    I think you are referring to post 13. I guess I am a lazy reader and failed to find the nugget wrapped up in all the frilly wrapping and bows typical of ThermitIc prose. ;-)

    Denis
    Last edited by dgfoster; 05-01-2021 at 10:36 PM.

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    Using a shop panel installed double pole GFCI as a shunt tripped circuit breaker.

    The ground fault circuit breakers are designed to trip when there is a current leakage of more than 5. milli amps. A double pole GFCI has three terminals. The two 120 volt leads and the neutral lead.

    Catalog description of the GFCI:
    https://images.thdstatic.com/catalog...7df69939fb.pdf

    The circuit breaker monitors the current through the three leads to determine if there is a ground fault. Suppose one were to add a 20 gage shunt wire that bypasses the neutral lead into the GFCI and connects directly to the panel neutral terminal. The 20 gage wire is connected to the input side of the neutral lead with a small 12 volt DC coil relay.

    The 12 gage wire supplying the 20 amp load has a resistance of 1.6 ohm/1000 ft. A 20 gage wire has a resistance of 10 ohm/1000/ft. Roughly 20% of the neutral current would bypass the GFCI and cause the breaker to trip.

    A small telecom. relay would work in this application and could be driven directly by the control board.

    This assumes that there is some current going through the neutral line while the furnace is running. . If there is not one can put a dummy load such as a light bulb between one of the hot leads and the neutral.

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    Using the kiln controller board overtemperature output may take care of a failed relay but you will still be at risk of controller failing and not triggering the overtemperature function.
    Risk of this happening may be quite low if the controller is well designed and well tested.

    Is a thermal fuse available in the temperature you need?

    Another option is an additional controller whose only job is overtemperature monitoring.
    If you custom build this, you could easily have a switch that must be rest before heater current will be allowed again.

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    Quote Originally Posted by pippin88 View Post
    Using the kiln controller board overtemperature output may take care of a failed relay but you will still be at risk of controller failing and not triggering the overtemperature function.
    Risk of this happening may be quite low if the controller is well designed and well tested.

    Is a thermal fuse available in the temperature you need?

    Another option is an additional controller whose only job is overtemperature monitoring.
    If you custom build this, you could easily have a switch that must be rest before heater current will be allowed again.
    Thanks, Pippin,

    Yes, a redundant boarde would work but would be an expensive (250-300dollar) option.

    The kiln controller is a "classic" US-made controller that is likely the most commonly kiln used control in the US. It has been in its present configuration for decades. So it is tried and true. I could be wrong about this, but it seems like solid-state boards, if they fail, usually quit putting out any signal. So, most likely the primary relay would simply open.

    To my knowledge there is no thermal fuse commercially available that operates in the 1800F range. I may stand corrected on that soon as I post this.

    It would be possible to jury rig a fuse using a wire of brass looped into and then out of the furnace. But that would not be easily done and adds one more level of complexity to the design. However, I will make some observations of the external temp of the kiln over the next few days to determine what temp its skin reaches when working through a normal cycle. It would not be hard to simply then apply to the exterior a fuse that provides a couple hundred degrees cushion. That seems very doable and addresses my primary concern which is fire that could result from a runaway furnace.

    Denis

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    Quote Originally Posted by Robert R View Post
    Using a shop panel installed double pole GFCI as a shunt tripped circuit breaker.

    The ground fault circuit breakers are designed to trip when there is a current leakage of more than 5. milli amps. A double pole GFCI has three terminals. The two 120 volt leads and the neutral lead.

    Catalog description of the GFCI:
    https://images.thdstatic.com/catalog...7df69939fb.pdf

    The circuit breaker monitors the current through the three leads to determine if there is a ground fault. Suppose one were to add a 20 gage shunt wire that bypasses the neutral lead into the GFCI and connects directly to the panel neutral terminal. The 20 gage wire is connected to the input side of the neutral lead with a small 12 volt DC coil relay.

    The 12 gage wire supplying the 20 amp load has a resistance of 1.6 ohm/1000 ft. A 20 gage wire has a resistance of 10 ohm/1000/ft. Roughly 20% of the neutral current would bypass the GFCI and cause the breaker to trip.

    A small telecom. relay would work in this application and could be driven directly by the control board.

    This assumes that there is some current going through the neutral line while the furnace is running. . If there is not one can put a dummy load such as a light bulb between one of the hot leads and the neutral.
    Very interesting thoughts. The possibility of using a GFCI breaker ran through my mind. But I did not (do not) have sufficient working knowledge to incorporate one as you suggest. I have a feeling that doing so would be easy for you given your evident knowledge of their workings.

    I will look at them in more detail as I'd like to understnd their workings in greater depth and because they might provide a very reasonable option.

    Regardless of what I individually do, I'd like to think that this thread has been generally informative and may provide useful knowledge to a lot of folks interested in safety circuit design for various applications. My thanks to those who have contributed so far.

    Denis

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    What am I missing? This seems so simple.
    The need for mechanical reset could be a push button in the logic?
    Is the world of designing simple relay logic lost now?
    BTW, SSRs on heaters circuits never turn all the way off as mechanical relays do. Leakage.
    Bob

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    Quote Originally Posted by CarbideBob View Post
    What am I missing? This seems so simple.
    The need for mechanical reset could be a push button in the logic?
    Is the world of designing simple relay logic lost now?
    BTW, SSRs on heaters circuits never turn all the way off as mechanical relays do. Leakage.
    Bob
    Geez, a button! I should have thought of that. ;-)

    Did you have any particular component in mind that is actuated by that button?

    Denis

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    Here is an inline thermal fuse (from ebay; $30.00). 1768 degree melting point. Put it between the power coming into the oven and the nichrome heating element.


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    Quote Originally Posted by David_M View Post
    Here is an inline thermal fuse (from ebay; $30.00). 1768 degree melting point. Put it between the power coming into the oven and the nichrome heating element.

    Haha, very clever. A pure silver bar does melt at about 1763 F. Since my target temp when annealing iron is 1750F, that would be cutting it too close. But, otherwise, not bad. I could actually use a brass or bronze bar in a similar way. But at those temps there are some technical issues with simply “ putting it inline” due to corrosion and expansion of physical connections. I is possible to loop a copper alloy wire in and out of the furnace with connections outside the furnace, but heat conduction and electrical insulation factors complicate that method too.

    An external “skin” temp fuse is practical with its temperature range selected after a few runs of the furnace and observation of the temperature rise of the skin with typical operation. Then I’d just add on a 150 degrees to the fuse and consider the system pretty well protected.

    Denis

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    In the final analysis I did order from Digi-Key a normally closed latching relay to place downstream of the primary relay. The cost was about 9 bucks plus 6 dollars shipping. (I could not find a suitable latching NC relay at Mouser.)

    The latching relay will pop open and remain open when an overtemperature condition occurs and is sensed by the board. I may add in a skin temperature fuse. In reality, the risk of fire due to failure of either the board or the primary relay is quite small as the furnace itself is not combustible as it consists of steel and ceramic wool. It will be located remote from combustible materials in a concrete shop basement that has bare concrete for floors and side walls and has a dry-walled ceiling. So, I think the latching relay is enough of a belt to go with the suspenders already in place.

    On a future build I think I might use the shunt-triggered breaker suggested in the second post. The shunt-triggered breaker seems to be the most robust purpose-made solution. Doing so now, however, would involve a moderate amount of additional time rebuilding of the control box I have set up. The gain in security is modest enough and the current addition so simple that I will go with the latching relay this time. Hacking a GFCI as suggested a couple posts up might also be a very good route to go.

    With repect to furnace control boards I will comment that I was very positively impressed with Bartlett in Madison Iowa. Binst They quickly and patiently asnswered my questions immediately when I called them. That sort of service is very rare in my experience.

    I really do appreciate the many helpful replies. It is so good to be able to get good help when you need it.

    Denis

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