Resistance Measurement of Heating Element on High Voltage Side

Discussion in 'Test & Measurement Forum' started by Mezzer26, Feb 26, 2018.

  1. Mezzer26

    Thread Starter New Member

    Jan 11, 2016
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    Hey Everyone,

    I am currently working on a project that has a heating element used in it and would like to vary the current that can go through it based on the power supply that we connect. In order to do this the most logical way seems that we could take the resistance measurement of the element while a master relay ensures the supply is disconnected; additionally, I'd like to know the element resistance for maintenance purposes. However, the point of contention is how to protect the measurement circuits from the high voltage that will manifest on the line when the relay is closed for regular operation. The current plan is to use a micro on the high voltage side, digitize the signal, and then optically transfer the value over an isolation boundary. The max voltage that the element will see is 240V from two 120V supplies 180degrees out of phase. Since the micro will be the only thing I need to power on the high voltage side (besides the element) and am isolating everything, I plan on using a capacitive dropper to power it and the measurement setup. I think I can remove most of the noise and get good accuracy down to my goal of .1ohm.

    If you guys think I'm going about this in the completely wrong way just let me know! The design is still open to be modified.
    Best Regards,
    Mezz

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  2. Reloadron

    Distinguished Member

    Jan 15, 2015
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    One problem is you can remove all power, you can even just lay the heating element on a bench and measure the resistance. The problem is that measured resistance will change dramatically when your heating element heats up. I have a 120 VAC incandescent light bulb laying here, it has a tungsten filament. Measuring the resistance I get 9.1 Ohms. Using that 120 Volts / 9.1 Ohms = 13.187 Amps yet the reality is 100 Watts / 120 Volts = 0.833 Amps which is the current a 100 watt incandescent lamp will draw. So with a cold resistance of about 9.1 Ohms when the filament heats up that resistance will increase to about 120 Ohms, well over ten times the resistance I measured.

    The only way you will know the hot working resistance of your heating element is to power the element and measure the actual current draw. Then do the math. Also with the power known you can do the math. Measuring the cold element resistance won't really tell you anything.

    Ron
     
  3. shortbus

    AAC Fanatic!

    Sep 30, 2009
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  4. Reloadron

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    Not really but yes :), this forum has a section on doing 4 wire or commonly called Kelvin resistance measurements. This follows along with the Wikipedia write up. The problem with using a 4 wire measurement with a heating element is we would need to pass enough current through the element to get it up to temperature. Kelvin measurements are typically used for low resistance measurements, for example below one ohm where the lead resistance comes into play.

    When we apply the working voltage to a heating element, as it heats up, the resistance of the element increases. The cold measured resistance no longer figures into things. When the elements rated voltage is applied it will draw it's current for it's wattage. So if we wanted to do a Kelvin measurement we would need a source compliance voltage exceeding the rated voltage of the element and a source current which can provide enough current for the elements rated power.

    Apply the rated voltage of 240 volts and measure the current with the rated voltage applied.

    Ron
     
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  5. KeepItSimpleStupid

    Distinguished Member

    Mar 4, 2014
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    What your trying to do is way to much trouble. Just use a DC power supply or two of them in series (Master/slave) connection with a 0-5V isolated output and input. Everything, then becomes so much easier. I did just that. Ionly needed voltage control, so 0-20 mA isolated current out works best with a resistor on the control input.

    USB, GPIB and RS232 control may be available. The 1980's, when I did it, was different.

    If the waveform is phase angle fired, then it;s going to be really tough.

    You can buy SCR units from Eurotherm which will give you the info you need. Some will do voltage and power control and have a partial element failure.
     
  6. Mezzer26

    Thread Starter New Member

    Jan 11, 2016
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    Yeah, so I'm trying to include this feature for a couple reasons. The first is that when making kiln elements its far easier to design the coil to a cold resistance (10 to 35 ohms typically). Upon startup I would like to poll this cold resistance for maintenance purposes to tell the user if the coils need to be changed or just to allow them to get the approximate value of the coil if new ones need to be manufactured and tell them what the initial resistances were.

    Second, the exact source voltag for this system could vary. It will either be the 240 I mentioned previously or single phase 120V. In addition to that, the number of elements will vary depending on what I'm trying to fire. As such, I was thinking that the only user input I would take at the controller would be the amperage of the breaker and then I could measure the input voltage and element resistance, determine the resistance at temperature through material constants, and then modify the phase angle of the driving TRIACs to limit the maximum current and let me know what levels of firing would be achievable.

    I do still plan on having monitoring for the mains voltage and current sensing, maybe hall effect for isolation? Not sure on that yet but that is my thinking. Perhaps it's too complex since I sometime have the tendency to overthink it but these all seem like reasonable things to include.

    Mezz

    EDIT: I should also mention that I was going to use this cold measurement to tell if an element is connected to a driver. That way if I get an open value I know not to turn power on to that element connector. This isn't especially critical but I was thinking I would include it for more peace of mind so that mains voltage isn't present on any open lines.
     
    Last edited: Feb 28, 2018
  7. Reloadron

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    Jan 15, 2015
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    Well alright then, enter the kiln. :)

    My dad had one when I was a kid, he was developing high temperature nozzles and did the prototypes in a small kiln. I have also worked with several specialized tubular furnaces where the elements were sort of woven into the firebrick wall.

    The obvious merit to using a voltage like 240 volts over 120 volts is the current. When we double the voltage we half the current which means any current carrying conductors can be smaller and lower in cost. High temp wire which connects to elements, depending on how you go about it, can be expensive. You start getting into high content nickel wire with glass on glass insulation.

    That said measuring cold resistance is cake. This is especially true of resistances in the 10 to 35 ohm range you mentioned. Just about any decent quality DMM will measure the cold resistance. A nice feature is a meter that allows for "relative" resistance measurements. You short the leads and hit a button to zero the meter and then measure your element. This removes the normal 0.2 Ohm lead resistance.

    Kilns can be peculiar animals in that like electric fired furnaces as the elements age they draw less current and after a period of use the kiln will not reach the temperatures it once did. Popular wire choices in furnaces for wire were nichrome and there was also an iron chrome aluminum alloy but I remember little of this stuff. As to measuring the hot element resistance? You can build a current measuring circuit but depending on budget and if you are building a test bed or test stand for these things I would just get an off the shelf current transducer solution. Measure the voltage across the element and measure the current then do the math. Matter of fact you could collect data from a few sensors like voltage and current and dump the numbers into an Excel sheet and let Excel do the math. Anyway with the voltage and current you can derive the hot resistance and also the power expressed in watts the element is using at full power or any part thereof.

    Do you have any idea how much current you plan to draw or the approximate wattage?

    Ron
     
  8. dendad

    Well-Known Member

    Feb 20, 2016
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    Use current transformers to detect the current.
    https://www.electronics-tutorials.ws/transformer/current-transformer.html
    There are lots on Ebay or other on line stores.
    These give you isolation and if you have your control circuit powered via a step down transformer, monitoring the secondary volts will give you a voltage reading then combine both to work out power and heater resistance.
     
  9. Reloadron

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    I was going to suggest a CT (Current Transformer) but before that if you have some idea of the current that would help in suggesting a suitable CT and meter to compliment it. It would also help to know if this setup will be used to test multiple kilns which have multiple power consumption?

    Ron
     
  10. ebp

    Well-Known Member

    Feb 8, 2018
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    What dendad said.
    A decent current transformer can give you thousands of volts of isolation. You can measure the voltage applied to the heater with good accuracy using a very small iron core step-down transformer. A transformer might be an issue if you are using phase angle control and the half cycles are asymmetric, but even with phase angle control you can get the info you need about conduction angle from the CT and use a voltage transformer from the uncontrolled line. Just ignore the voltage signal when the current signal is zero. It is safe to assume that the heater is a pure resistance, so you can still calculate everything easily. You'd possibly need to fiddle with the "gain" to allow for voltage drops in connecting wires, etc.
    There are some very nice, fairly reasonably priced isolated current sensor ICs on the market. I can't remember who makes them. As I recall, some are usable up to 200 A, but I may be wrong on that. I'll root through my library later.
     
  11. ebp

    Well-Known Member

    Feb 8, 2018
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    Allegro is the mfr of the Hall sensor - just up to 100A for the one I have data for. I haven't looked at them for years, so I don't know what else may be available. ASC756 is one p/n.

    Coilcraft has a line of current sense transformers. I've use some of their high-freq types in switchers.

    CTs for 50/60 Hz typically have a great many turns on the secondary. Some of those from Coilcraft I believe were 1000 turns, which is a lot to wind on a small toroid.
     
  12. dendad

    Well-Known Member

    Feb 20, 2016
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  13. ebp

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    regarding hobbyist boards - I've seen photos of many many hobbyist boards that are grossly badly done with regard to adequate safety spacing between low voltage and line voltage parts. I remember seeing photos of a board with one of the Allegro parts with just stupidly done routing that would be grossly unsafe and it could have been done in a way that could probably meet VDE rules.

    Oops - should have looked first, yapped later. That is exactly like the board I saw before with the stupid poured copper that contributes nothing and makes the thing unsafe for line voltage use. I would certainly meet most safety regs were it not for the poured copper and the plated big holes.
     
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  14. dendad

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    Feb 20, 2016
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