I have been trying to build a simple induction loop amplifier. I am using TDA2040 as an output stage and it delivers 10 W (I limited it for that). I am using about 14 meters (1 mm thickness) loop.

My question is: How do I calculate a suitable resistance wattage to connect with the induction loop in series to the output of TDA2040?

  • \$\begingroup\$ look up AWG diameter and ohms/m then estimate thermal resistance of your geometry to cooling in ‘C/W to limit temperature rise. the wire Ampacity is for 85’C I recall but open straight cable , not magnet wire. what is your insulation? \$\endgroup\$ Feb 12, 2019 at 16:17

1 Answer 1


A good rule of thumb to remember is that 1m length of 1mm2 area copper wire has a resistance of about 17mohm at room temperature.

Your 14m of wire (1mm thickness, do you mean diameter?, if so area 0.785mm2) will have a resistance of about 17m * 14 / 0.785 = 0.3 ohms.

As far as the amplifier is concerned, that's pretty much a dead short, compared to the load it wants to see, in the 4 to 8 ohm range.

You therefore need a 10W resistor in the 4 to 8 ohm range, in series with your loop. This will limit the current to what the amplifier expects.

  • \$\begingroup\$ You forgot to mention that it will also throw away 96% of the power. \$\endgroup\$ Feb 13, 2019 at 4:34
  • \$\begingroup\$ Not exactly. I left it as an exercise for the reader that it throws away nearly 100% of the power. \$\endgroup\$
    – Neil_UK
    Feb 13, 2019 at 5:06

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