electric underfloor heating tester

Discussion in 'The Projects Forum' started by pally, Oct 19, 2011.

  1. pally

    Thread Starter New Member

    Oct 19, 2011
    Hi all,

    I am planning to design a hardware circuit to test electric underfloor heating (ideally without a lcd display or the use of a microcontroller)

    The inputs to the system will be LIVE, NEUTRAL, EARTH (not live).

    The resistance of the cable between LIVE AND NEUTRAL should not fall below 1Mohm
    If it falls below 1Mohm i want my circuit to alert me by a buzzer or LED. But how can i design a system to trigger a ON state if it falls below 1Mohm. Output needs to be either ON or OFF (ie not a half lit LED)

    All suggestions welcome,

    Thanks in advance!

  2. someonesdad

    Senior Member

    Jul 7, 2009
    A basic solution would be to provide a constant current source, then use a shunt resistor in series with the heater resistance. The current source would be hooked across the shunt resistor in series with the connection to live and neutral. Then a comparator would monitor the voltage across the shunt. You'd adjust the circuit to trigger at 1 Mohm resistance. You could set things up so that e.g. a green LED is lit if the resistance is above 1 Mohm and a red LED lights up if it's below 1 Mohm.

    However, it's not clear to me why you said 1 Mohm between line and neutral. If it's a heating element, shouldn't the resistance be around 5 orders of magnitude smaller? How else would it heat? Did you actually mean that it should measure the resistance between line ("hot") and ground? Note resistance between neutral and ground will be a low value.
  3. pally

    Thread Starter New Member

    Oct 19, 2011
    Thanks for the reply someonesdad.

    Sorry my mistake, thats exactly what I meant, the resistance between Live and Earth should be above 1Mohm.

    I'll have a go at your suggestion and get back.


  4. strantor

    AAC Fanatic!

    Oct 3, 2010
    no, it really shouldn't and if you try to measure it with power applied (i.e. while the heater is on) that's a bad idea