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I've got a Bosch cooktop (part number PKF645E01A) and the inner-ring of the double-burner element has stopped working (the outer ring works fine). I've opened up the cooktop and notice that the double-burner element has 3 wires.

I've checked the resistance between the wires and have found the following:

  • The first and second wires have zero resistance:

    enter image description here

  • The first and third wires have 33.3 ohms of resistance:

    enter image description here

  • The second and third wires have zero resistance:

    enter image description here

It seems that the element is faulty, but before I go ahead and order a replacement burner, I'd like to confirm that this is in fact the case, which brings me to the following questions:

  1. Based on the resistance values posted above, does it seem as though this burner is defective? If there's not enough information to know this, what else can I check?
  2. Is there a way I can connect this burner directly to a power source to test the burner elements so I can either confirm that the inner element is faulty, or demonstrate that it works?
  3. What's the purpose of the three wires?

Thanks!

Update 2023-12-30T00:53:27.168Z:

Thanks for all the fantastic responses! I followed the advice from ecnerwal and ordered a replacement double-burner element from here for €50 (plus €75 for shipping!) and after swapping out the old burner with the new one, here are the new meter readings:

  • The first and second wires have 100Ω of resistance:

    enter image description here

  • The first and third wires have 34.7Ω of resistance:

    enter image description here

  • The second and third wires have 66Ω of resistance:

    enter image description here

And I'm happy to report that the new burner functions as expected:

  • Before

    The inner ring doesn't work:

    enter image description here

  • After

    Both rings work:

    enter image description here

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  • In my experience the controller switch at the knob is fails as often or more often than the heating element. 33 Ohm is in the expected range for one element corresponding to a 1700 W element (maybe nominally 1800 W). P = V^2/R = 240^2/33.3 = 1730 W. The open circuit may indicate a burned out element. The wires to a combination burner would be one unswitched phase hot call it L1, one switched hot from the other phase L2, and a third switched hot from L2 from a different switch or different part of the switch. Commented Dec 4, 2023 at 12:08
  • 1
    You should not need to power up an element to verify it is heating or confirm it is not, the ohm reading should tell whether it is working or not, but if you want to you could exchange the switched hot connections at the combination burner, then turn the breaker back on, then turn on the burner that was not working. But I dont think this is necessary and could go wrong. Rather put the power on and use the voltage mode of your meter. With the burner controlled OFF check the voltage differences of the connections at the burner relative to ground. Then turn the burner on high and check again. Commented Dec 4, 2023 at 12:59
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    The results of the resistance readings and the voltage readings should tell you if the burner is out or it is the switch (so called "infinite" switch, misnomer for continuous) that is bad. In your case it appears to be the burner. (A neighbor recently called me to check his glass top resistance stove. One surface burner was not working even after he installed a new burner. With a multimeter I showed him that the control switch at the knob was faulty. He installed a new switch and the burner worked.) Commented Dec 4, 2023 at 13:04
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    You're reading your meter wrong. "0.L" means infinite resistance, (an open circuit) not 0. 0.0 would be 0 instrumart.com/assets/Extech_EX330_Manual.pdf but at least you provided pictures so that could be seen. +1
    – Ecnerwal
    Commented Dec 4, 2023 at 13:13
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    Considering the relatively low cost of buying a replacement burner (especially one that can be returned), vs the potential cost of futzing around with trying to hook the burner up directly to mains power, I'd just buy a new burner and see if that magically fixes it. If not, either keep it as a spare or return it.
    – FreeMan
    Commented Dec 4, 2023 at 17:19

1 Answer 1

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A 2-element burner with 3 contacts is normal and expected. One contact is common to both elements, the others are the opposite ends of the two elements. Could be done with 4 contacts, but if the manufacturer can save 0.50 € by using one less contact, you can be sure they will.

Your actual resistance readings are infinite, 33.3Ω and infinite. 0.L on the Mega-Ohms scale is an open circuit. So your burner looks like this, electrically (ignoring the burner controls between the line and the elements):

{2}--- ---{1}---33.3Ω---{3} ...| Inner ring | Outer ring | ..L1 <-240V-> L2 <-240V-> L1

The element between 1 & 2 is broken, thus an open circuit, thus infinite resistance between 1 & 2 and 2 & 3

While you could hook it up to power, there's no need and some hazard from making jury-rigged connections. The infinite resistance reading on the 1-2 element is a sure sign that it's broken. And it's been hooked up to power in your stove and not worked. I suppose if you hooked up a 12V supply (car battery charger, say) capable of 360 or more mA you might feel the warming from a whopping 4.3 Watts on the outer ring, and nothing on the inner ring, without getting into hazardous voltages. Or you could use your meter in current (Amperes) mode to see the current, and lack of current. Just pay attention the fact that current is done with the meter in series with the load, not in parallel like voltage measurements.

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    One warning on current measurements is that it is easy to blow the expensive fuse in the meter. What happens is that a user subsequently makes a voltage measurement by switching the mode selector knob to volts, BUT forgets to change the test lead from the amps socket. Touch the test lead to a hot contact, a small spark tells you a $25 fuse was just blown. Commented Dec 4, 2023 at 19:14
  • Where do you get the Greek omega symbol, degree synbol, etc? Do you recommend getting a current clamp so one just clips onto the circuit? Commented Dec 4, 2023 at 19:58
  • Well @JimStewart I'd be happy to be your new, lower cost fuse provider. I'll sell them to you for only $12.50 and make a mere 90% or so profit on them, buying them retail. If you have enough need that I can buy them wholesale, I might consider dropping your price to $10 - IOW, you need to shop at different suppliers for your meter fuses if you think they are $25. On a Mac, ° shift-option-8 and Ω option-z if you have a PC look them up.
    – Ecnerwal
    Commented Dec 4, 2023 at 22:03
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    @Ecnerwal He's right. It's a safety issue, and using a $2 fuse will get you killed. For a proper CAT IV multimeter, the fuse needs to have a minimum interrupt rating of 10-20 kA and a voltage rating of 1 kV in order to withstand distribution-level transients, e.g. lightning. That's why they're big sand filled things that cost $20 a piece. Yes, you can find them cheaper on Amazon, but then you get into fakes. Not worth your life.
    – user71659
    Commented Dec 4, 2023 at 22:13
  • @user71659 I suggest you pay attention to the meter in the question, which is not whatever you have in mind. Link to the instructions is right there, use it.
    – Ecnerwal
    Commented Dec 4, 2023 at 22:22

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