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Mains powered variable resistance

Discussion in 'Electronic Design' started by Raveninghorde, Oct 24, 2012.

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  1. I was given an interesting spec by a customer.

    He wants a wall mount RH sensor with a resistive output which emulates
    a 3k NTC thermsistor curve. So 0% RH is about 10k, 25% 3k, 50% 1k etc.

    However the only power available is 230V ac and there is no space for
    a transformer. And the design is cost sensitive as the quantities are
    good. The first batch of 5000 are needed in 8 weeks.

    So The design needs a variable resistance output which meets SELV

    My first thoughts are a capacitive dropper running a PIC which will
    provide a pwm output to an opto isolator which will control the
    resistance of a FET. Then all I need is a way of correcting the FET
    resistance but that is not easy as it's on the isolated side of the
  2. I've asked for a sample of the kit that this unit has to drive so I
    can check how it responds to different inputs.
    Maybe I wasn't clear.

    I am simulating the response of an NTC thermistor for a humidity
    sensor. So 0% RH gives the same resistance as 0C, 25% RH has the same
    resistance as 25C, etc.

    Temperature itself isn't a big issue.
  3. Guest

    use an NTCs control temperature with another NTC + FET as heater ?

    might want a few NTCs to get above ambient


  4. Hahahaha
  5. linnix

    linnix Guest

    I don't really get the joke. You need enough power to heat up the NTC and read it's resistance by something else. Of course, it's overkill or overspec. They are probably too lazy to redo the device reading the NTC.

    Alternatively, mount a 12V auto battery behind the wall to power it. Perhaps, they will get the joke of how ridiculous the spec is.
  6. Fred Bartoli

    Fred Bartoli Guest

    a écrit :
    Not a bad idea at all.
    Have two identical NTCs, one on each side of the PCB (for isolation).
    Say top = primary side, bottom = secondary side.

    Have a heating transistor on the top/primary side along with the sense
    NTC, while your "simulated" resistor lies on the back side.

    Have a small U or Omega shaped cut into the PCB so as to limit heat
    conduction to other regions of the PCB, and get as identical as possible
    temperatures on both sides.

    Except you'll have either to figure how to cool the PCB to 0°C or you'll
    need to use a higher resistance NTC so that at your max ambient, the NTC
    resistance is above your targeted 0°C resistance. Of course, if you need
    to reach the 100°C resistance at reasonable temperatures, you'll want a
    higher beta NTC.
  7. How much do you know about what is connected to the resistor?
  8. I'm just a fan of dummy parts, and stuff like that.

    microchip makes weirdo parts like the OP wants
  9. That would be an easy way- the "wiper" resistance will limit the high
    RH end and the resolution will also be crude at the high end (but RH
    measurements are pretty cruddy usually anyway) Probably want 1024
    steps .. eg. AD5161. The voltage of both of the rheostat ends has to
    lie within the power supply voltage limits of the digital pot. It
    would be easy enough to opto-isolate the SPI signals, but it might be
    easier to just use a tiny SMPS to generate a single isolated supply,
    since you'll need power on the "thermistor" side of the barrier
    anyway, and presumably the isolation from the mains will have to
    comply with safety standards.
  10. legg

    legg Guest

  11. Jamie

    Jamie Guest

    The only way I can see that happening in low part count is a photo
    resistor opto-isolator being PWM with a SIN to give you
    the log output you're looking for. THis would offer the isolated R you

    As for the cap supply, there was a thread here not to long ago on that
    and I came up with a couple of ideas that seemed to me, to be a viable
    application. If you're interested I can post that spice on that one.

  12. The customer says it is a thermostat. I'm waiting for a sample.
  13. I've not chosen the RH sensor yet. I already use a Sensirion SHT11 on
    another product. Easy to use and doesn't need calibrating.
  14. josephkk

    josephkk Guest

    Hmmm. Make the RH measurement and translate with a half-DAC switching a
    resistor tree (or maybe a r/2r ivy).

  15. Often thermistors for this kind of application are measured by
    alternately inserting them into an RC oscillator circuit with a single
    precision reference resistor- and a bit of math is applied which
    cancels out the supply voltage and capacitor value and linearizes the
    reading. The oscillator frequency would typically be in the kHz to
    handle the large dynamic range of the thermistor with reasonable
    resolution at high temperatures, so not so easy to apply this kind of

    Your suggestion might work if it was a simple ratiometric ADC reading.

    Best regards,
    Spehro Pefhany
  16. Jasen Betts

    Jasen Betts Guest

    how about a carbon pile resistor tied up with rawhide, catgut, or
    other hygroscopic material

    To meet the resistance target it'll probably need a special
    composition for the blocks, perhaps 90% cement, 10% carbon
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