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Hi I am working on a project which involves heating a resistor with a 9v battery. I am heating resistance because I want to increase the temperature of a temperature sensor so that I can do something with arduino after the sensor reaches a specific temperature.

But when I connect the resistance across the battery the voltage drop comes out to be 0.5 volts instead of 9v. And the resistor is not heating up.

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    \$\begingroup\$ Calculate how much current you would get for 9 V and 5 Ω. Then look up the data sheet for the battery and see how much it's rated for. Also look up "battery internal resistance". Then figure out what happens to battery voltage when you draw current. Welcome to SE. \$\endgroup\$
    – Transistor
    Feb 3 '16 at 8:07
  • \$\begingroup\$ Small batteries can only supply a limited amount of current and they have significant output resistance too. \$\endgroup\$
    – Andy aka
    Feb 3 '16 at 8:10
  • \$\begingroup\$ Four AA cells or a couple of Li-Ion 18650s will supply 1A or so for a couple of hours. Either option is better than a PP3 9V battery. \$\endgroup\$ Feb 3 '16 at 12:38
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Consider Ohm's Law. V = I * R. Depending on V & R, I changes. But batteries and power supplies are not perfect. Different battery chemistries have different ESR, Equivalent Series Resistance. As the current through the battery increases, the voltage drops, because its as if an extra resistor is in the way. You're creating a voltage divider, and part of the voltage is dropping in the internal ESR resistance.

And it depends on the Battery. A GOOD 9V battery is an Alkaline or Lithium. A CRAPPY 9V battery is a Carbon-Zinc or Zinc-Magnesium or ""Heavy Duty"" battery. Rechargeable are in-between.

Notice the voltage and current on the 5Ω resistor in each of these circuits. An fully charged Alkaline or NiMh 9V will produce a few watts. The Carbon-Zinc can't even do 1 Watt.

schematic

simulate this circuit – Schematic created using CircuitLab

And that's before you even consider if the battery is drained or dead. 9V batteries in general have a very low capacity. It doesn't take much to drain them. If you are just doing some testing, consider a couple of AA in series, or buy a good Alkaline 9V. If you are designing this into a circuit, rethink what you need completely.

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Try increasing your resistance. That 5 ohms resistance is so low that it's possible you're well below the 9V battery's internal resistance, in which case you'll end up heating the battery far more than the resistor (and maybe end up blowing up a battery).

I'd reccommend starting with a resistance of about 50 ohms, then go up or down from there based on heat output & voltage drop noticed.

Assuming you have a good/full 9v battery, any resistor that gives less than 5V drop whan connected is too small & damaging the battery.

As you increase the resistance, heating lf the resistor will increase until you reach a maximum point (which depends on the exact internal resistance of your battery), then will begin decreasing again ax you increase rezistance further. For best safety & battery performance, please select a resistance that is on this "down slope" for your final version.

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