Saturday, February 18, 2012

Adding Resistance

Here is another email from a user that has a perfect question regarding power supply voltage... check it out!! This is the exact question (in blue) and my reply (in black)

Question ::

I want to make a hot wire cutter to cut rabbets in the edges of rigid styrofoam insulation. I have calculated my voltage and current requirements with the calculator on this site.

To heat my 1" 18ga Nichrome wire to 300F/600C, I need ~6.5A at 0.25V.

My variable power supply won't go that low, and because this is a one-time project, I don't want to spend $30-40 on batteries and another $30-40 on an appropriate charger.

I'm looking for suggestions for a voltage divider that would allow me to use an existing [variable voltage] 12VDC 10A power supply to power the cutter.

No problem at all, and great research! I always love when people like you utilize the website and have almost all your own answers!

You're close :) All you have to do is increase the resistance of your circuit to account for the lowest power supply voltage. 

Just play with ohms law to see how much you need to add, but here is the link to the main page for resistance modification

You'll be adding resistance in series with your nichrome wire circuit. 

Here is a quick series of calculations you can do, figuring the lowest voltage your power supply can output. Lets just use 2 volts (just guessing) as the lowest voltage your supply can provide. 
Resistance = Voltage / Current
Resistance = 2/6.5
Necessary Circuit Resistance = 0.307 ohms

Resistance of 1 inch 18 gauge wire = 0.035ohms

Unaccounted Resistance = 0.307 - 0.035ohms
Unaccounted Resistance = 0.272 ohms

So basically, all you have to do is add about 0.3 ohms and you can use a power supply that can only go as low as 2 volts (theoretical).

A REALLY cheap way to make a resistor is like this

I've never tried to pull more than a few amps through that wire, but theoretically it could handle quite a lot. I get a little wordy in the video, so I apologize, i haven't updated that video yet. But you should get the idea. I had also considered using a frayed telephone cable (which has 4x small cables in it) but you might melt that small of a cable. Anyway, a little playing around might save you some money.

Otherwise check out Digi-Key, this link goes straight to their resistor search page


On hand, I have 1ohm5Watt resistors. So placing a handful in series or combined series / parallel will provide a lot of circuits with the resistance and wattage they'd require.

I hope that wasn't too confusing!!