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Thread: Heatermeter weird issue

  1. #1
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    Heatermeter weird issue

    Hi Guys,
    Im soldering the heatermeter 4.3 incl LCD and I have a weir issue.
    All seems to be working but the TC still is a PITA....
    after some re-soldering and testing, it is working except for one thing….
    as soon as I physically tough the thermocouple wire, temp goes thru the roof.
    I checked for bad connections but cant find any…
    Anyone got an idea what can cause this?

  2. #2
    TVWBB Diamond Member
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    Try an ISOLATED thermocouple....
    I created the Roto Damper, RD3, HMv4.2 sliding back case, "air-burner" and the "ping-pong" valve in my quest for delicious "set and forget" BBQ.

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    that would be the easy guess but since I have made 4 heatermeters and this is the only one that does this... I'm talking about the outside shielding of the tc probe that I touch.
    See pictures:
    https://ibb.co/x2NYd0Z
    https://ibb.co/4PCfMBs
    Weird thing is that there is no connection to the outer wire... I tried with other TCs that I have and they all have this. My other heatermeters don't do this so there is something wrong with the hm..

  4. #4
    TVWBB Honor Circle Bryan Mayland's Avatar
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    I'd check that 1K SMD resistor to GND in the thermocouple circuit and make sure it is well soldered. It helps to eat the noise you're adding when you touch the braid. If that's solid then make sure the 1n and 10n capacitors (differential and common mode noise filters) are soldered well. Finally a therocouple amp with a loose leg can cause this as well.

    Rejecting probe noise is a real problem with these mains-powered temperature sensors and there's always a little bit of magic. You can also upload the hm-noise firmware from the AVR Update page and that will add a new graph to the home screen that shows the raw ADC readings after a few seconds. That would tell you if there's always noise and touching it makes it worse, or if it is 100% fine until you touch it.
    I'm that HeaterMeter guy what ruins everybody's free time.

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    Thanks Bryan! I'll fire up the solder unit tomorrow to check the connections again. The amp is a real menace to get right, especially afterwards when all surrounding stuff is done.
    Will report back asap!

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    Ok, did some soldering last days (haven't gotten around to it earlier) and I have 10 units soldered.
    All 4.3 based on our zero w and the YC version.
    6 work fine as far as I can tell with just some small testing.
    1 doesnt indicate the tc but I probably have a loose connection somewhere I guess.
    Need to check that...
    The other 3 all have the same issue... When the tc is connected, the temp shows 25 deg (just above the temp of approx 20 it actually is). When I touch the tc wire it doubles to approx 55.
    This behaviour is identical to all 3...
    I really don't know where to look... Especially since the issue is identical on the 3 units.

  7. #7
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    I had this same issue when I was in Australia, operating on 220V wall power supply. Never really figured out the issue. try running it on 12V DC battery power, or try another power supply and see what happens?

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    Quote Originally Posted by WBegg View Post
    I had this same issue when I was in Australia, operating on 220V wall power supply. Never really figured out the issue. try running it on 12V DC battery power, or try another power supply and see what happens?
    I tried an 8pack AA batteries and you are right!
    The issue is gone then but it still adds about 5 degC to the temp at room temperature.
    Any idea which component does this or is influenced by the 220v adapter and isn't with a 12 supply?

  9. #9
    TVWBB Honor Circle Bryan Mayland's Avatar
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    The issue you see where the temperature increases when you touch the wires is due to some of the electricity of the circuit flowing through your body. The way the thermocouple works is by passing a small current through it and measuring the voltage drop. If some of that current flows through you and doesn't come back through the TC- wire, it counts as a drop and influences the temperature. You want to make sure that your probes are electrically isolated-- none of the circuit is exposed or uses the metal shield as a conductor as these will allow a path for the current to flow where it shouldn't. You can test this by checking for continuity from the TC+ or TC- wire to either the probe's braid or the tip of the thermocouple element. If it beeps, it's bad (for this circuit).

    It may also be helpful to install the noise firmware (AVR Firmware -> Online Repository -> hm-noise) then go to the home screen and wait for the second graph to show up. It should be flat or bouncing between two values. If it is erratic or shows a sine wave then you've got a noise issue which comes from the power supply, power source, or coming in on the probe wires. You can also see a noise reading on the probe by going into the diagnostics display on the device. Navigate the menus to "Manual Mode" then hold the left button to enter the diagnostics display.

    You might get a clue about the 5C offset there too. Note that the heat generated on the board influences the measured temperature as well, so if you're testing right after soldering the values will read high until the board returns to room temperature. I usually see a reading 3-4C high when I am testing a board when done soldering. If it stays 5C high even if left sitting overnight, eeeeee not sure where to go after that. Check the 3.3V at the "AVC" pin on the atmega. HeaterMeter expects that to be 3.300V because it is tying to calculate the absolute voltage coming from the amplifier. This can be accounted for if it is too far off by adjusting the mV/C setting int he configuration.
    I'm that HeaterMeter guy what ruins everybody's free time.

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    I will try to do it later today to see what I can figure out.
    But reading the above still makes me wonder why there is no effect when using battery power and also why, when I made 10 units and all the testing items are identical, the results differ.
    I don't think it's the tc for that reason since I use the same one...

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