# DIY heater with resistors? help please

1. Oct 23, 2004

### levi99

DIY heater with resistors??! help please

Hi physics gurus, I have some questions I'm hoping someone with a kind enough heart can answer for me!

I am trying to build a small circular heater (similar to a dew heater for a telescope) with resistors. Basically I have 22 gauge rated 300volts hook-up wire, and i want to line up (between 40-50) 330 ohm, 1/2 watt resistors across it, and power it so it gets warm. Then wrap it in either electrical tape or something insulating.

My problem is in powering it-- I really need this to be portable, I can't use a 12V car battery! I'm thinking along the lines of some kind of R/C car battery or something else rechargeable similar to that size. How can I do this?? Is it possible with my current configuration or maybe if I switch the type of resistors I can use a 6.0 9.0 or 12.0 V R/C car battery?

I really appreciate any help anyone is able to offer. Much thanks

Jason

2. Oct 23, 2004

### Integral

Staff Emeritus
You need to clarify how you are connecting the resistors. You stated the voltage rating of your wire, but that really is not a critical parameter, according to this page 22 ga wire has a max of .92A.

What you need to do is figure out how much power you need. Then using Ohms law and the power relationships, figure out a resistance that will give you the power you the desired power given your voltage and current limitations.

For example: you want 2 Watts and have a 12 V source.

P=IE tells us that your 22ga wire will do fine.

P = I2R tells us you need 72 Ohms resistance.

The fun part is now finding a combination of 330 ohm resistors which will yield 72 ohms.
For a parallel connection:
$$\frac 1 {R_T} = \frac 1 {R_1} + \frac 1 {R_2} +...+ \frac 1 {R_n}$$

For a series connection

$$R_T = R_1 + R_2 + ... + R_n$$

The parallel relationship tells us that 4 330Ohm resistors will be very close.

3. Oct 23, 2004

### levi99

this webpage is the basic model I am following:

http://www.gbronline.com/ronkeating/heaters/heaters.html

the pictures there show exactly how I plan on wiring them, in series (right?).

I'm not sure exactly how many watts of power I need. It's going to be similar to the telescope dew heater, but about 7-8" long and it will be used to heat something brass instead. I can't use the RCA plug as that webpage shows because thats meant to be plugged into a controller box, which I don't want nor need.

I need whatever power source I use to work for at LEAST 2 hours, anymore than 3 hours is not necessary. I am still confused as what to try for a power source before I wire all these resistors and find out I need to be using different ones. Much thanks for any help!

4. Oct 23, 2004

### Integral

Staff Emeritus
The link you provided, shows a parallel network. If you make an 8" heater as described it will consist of 12 resistors. A parallel network of 12 330 Ohm resistors will have a total resistance of 27.5 Ohms, on a 12 V battery it will draw .43A and produce about 5W. You could drive this with a bunch of D cells or as you suggest a motorcycle battery.

As for the battery rating you need .5 A for 3 hrs so you need at least 1.5 AHr. AHr or AMP Hours is a standard battery rating ask about it when you buy a battery.

5. Oct 25, 2004

### levi99

Ok so I soldered on 12 resistors onto the copper wire and bought an eight AA battery holder. So I now have a 12V power supply and 12 resistors wired in parallel. When i apply power to it they all heat up as they're supposed to. My next question is how do I fuse this power source?? I will be adding an off/on switch but I know it should be fused. Any ideas?? thanks in advance

I'm going to be hooking up 2 of these gizmos up at the same time, the smaller one being 9 resistors. I'm guessing I can just hook the top wire up to the top and bottom wire up to the bottom to connect the strands and power them both from one source