Confusing nichrome wire length calculations

Posted 19 November 2022 - 11:44 AM

So you have heater wire that is 10 ohms/ft but if you had 100 ohms per foot you would need 1/10th as much.  So I looked around and clearly Nichrome has the highest resistance of any "resistance wire" out there.  and you are right - to go above 10 ohms/ft the wire gets very delicate.

So you really need to lower the voltage.  You can use a voltage regulator.  If you lower to 5V then the power goes down by the square of the voltage.  (5/12)^2 is 6X so your wire could be 6X shorter.  e.g. 1 foot instead of 6 feet.  A voltage regulator is very simple - it has 3 pins and 2 pins go to ground and 12V and the third pin is the output.  You can get them for many voltages.  You have low current needs so they shouldn't get too hot.  In fact it will get about as hot as all your heaters put together if it's the old type of regulator.  You can get a switching regulator which is more efficient (still just 3 pins) and won't get hot at all.

Using a step-down resistor will work I suppose also.  But you need a different one for each heater in case you don't turn them all on at once.  You need to use the resistor divider formula.  So for example if you want to lower your resistance from 12 feet to 2 feet (6X) you need to lower the voltage to 5V so let's do the math:

2 feet is 20 ohms.  If you put 2 resistors in series with 12V across both and you want 5V across the 20 ohms I use this calculator:

https://ohmslawcalcu...ider-calculator

And I calculate that you want a 28 ohm resistor.  That resistor will have a power dissipation of power=v^2/r   = 7^2/28 = 1.75 watts.  That's not trivial.  You would want a resistor rated to at least 3 watts to be safe.  So you could get a 28 ohm 3 watt resistor.

Here are 28 resistors from digikey that are 3 watts and close to 28 ohms to choose from.  some are 74 cents in quantity of one:

https://www.digikey....5dPEISEBl6EVigA

I think it's much smarter and simpler to use a voltage regulator and that will cover *all* your resistors so you don't have to have a different balance resistor for each heater circuit.