# How do I calculate the voltage needed to heat up a wire to a certain temperature?

The wire I am using is standard temp. nitinol wire which needs to be heated up to 45 degrees Celsius in order to be effective (move). What voltage do I need to achieve this?

• It depends on wire thickness - maybe a link to the exact wire you are using would get you an answer. Commented Apr 16, 2013 at 9:57
• .. and on length - how long is a piece of wire? Commented Apr 16, 2013 at 9:58
• @pjc50 - it's longer than a piece of string usually! Commented Apr 16, 2013 at 10:33
• Could you please share a link to the specific NiTinol wire you are using? For the several NiTinol wires used in my projects, the sellers provide approximate current for activation (70 and 90 degrees C). Commented Apr 16, 2013 at 10:46
• ebay.co.uk/itm/… Commented Apr 16, 2013 at 12:39

I don't believe you can do this with any degree of precision : it depends too much on the wire and its environment which determines its rate of cooling.

This 450 page book would give you a brief introduction to the difficulty; the TL/DR version is: Don't.

Which leaves you with the alternative problems of:

• determine the wire's temperature and control the voltage to achieve the correct temperature. You could measure the drive current and voltage, to determine its resistance, and compute the temperature from that
• measure the actual motion and control the voltage accordingly (programming a PID controller is the best way, but the response time of the wire makes this tricky)
• Experimenting to determine what voltage is "good enough" in your environment. This is by far the simplest and may be good enough for your application; you haven't told us enough to tell.

Your vendor does not seem to have good information on resistance and current values for your wire. Here is a table from a different place that can help:

Dynalloy tech data page

The numbers you want if you are using their -super heavy- 0.02" wire are

resistance per inch = 0.11 ohm / inch

apx current to actuate = 5 Amps


That gives you a starting point for your own testing. A 1.5V D cell battery, and a 3 inch length of the wire is a good place to start.

Their table is for 150 degree (F) wire, and yours is lower, so plan on a bit lower currents needed. On the other hand, the D cell probably will sag pretty much at 5A, so its a start.