How to wire in resistors to decrease the temperature of a heating element

[lil_vinn]

I have a 15v 3.3A ac to dc converter that is hooked up to a heating element (nichrome disc) the disc has a resistance of 2.6 ohms, I want to decrease the temperature of the disc, it is currently around 390 degrees C and I want to get it down to around 290 degrees C, I keep trying to hook up different resistors to decrease the temperature but the resistors keep burning out (shorting out). When it doesn't burn them out the heating element does not heat up and no voltage goes through the heating element. I'm not sure how to wire it so that it can drop to the appropriate temperature and not burn out the resistors or POT's that i've tried. I'm farely certain its because the amperage or volage is too high but I don't know how to make it work.

Thanks,
Justin

---------- Post added at 21:00 ---------- Previous post was at 20:31 ----------

Also, I can get the resistors hooked up correctly so I have the voltage desired (around 11.6v) however, when i hook up the load (heating element) the reading goes right to zero and won't heat up

 

[enjunear]

The problem with a nichrome element is that it's not a constant resistance. If you connected an ammeter in sereis with your nichrome, you'd see that, initially, you'll have a very high current (the wire will be cold, i.e. low resistance, as you mentioned, 2.6 ohms). However, when the nichrome heats up, it's resistance increases, and the current drops. It's this non-linear effect that you are running into.

Determine how much current the nichrome disc uses when it's hot (in air, max load, minimum current). Then, use the nichrome to cut something and see where the current moves to (should be up, since the wire is cooling, lowering it's resistance). From those current's, determine what your resistor needs to be. Don't forget to proper size your resistor for POWER HANDLING. You can't use little 1/2 or 1/4 watt carbon-comp resistors in this scenario. I'm thinking you'll need something along the line of a wirewound or "sand" resistor, like these.
wirewound 1
wirewound 2
"sand" resistor

Here's a site with some high-power parts, Surplus Sales.com. Google for other vendors.

 

[BradtheRad]

1.

The resistor needs to be rated for over 25 watts. A resistor is the easiest way to accomplish what you're asking, though it is not the most efficient method.

2.

Or a more efficient method is to switch current on and off by using a transistor or mosfet. Power waste is slight. This method will take work. It cannot be used if you only want uninterrupted DC going through the disc.

3.

Or can you get at the innards of your DC adapter? If so then you might remove windings from the transformer. This is not easy. Or if it has a center tap then you might rearrange connections / windings to obtain the voltage you want (also to waste less power). Might take some experimenting. Opening up the adapter probably won't be easy.

4.

Is it vital to supply the disc with DC? Or is it okay to send AC to it? Then it would be easy to drop the AC voltage with reactance instead of resistance. Reactance generates hardly any heat. You can experiment with either a capacitor or inductor (although a capacitor may not withstand having a few amps going through it).

This would mean using only the transformer in your adapter. No resistor is necessary.

Example, if you were to use a 15VAC transformer at 60 Hz (instead of your DC adapter)...

You might drop to 3/4 of supply V by using a capacitor value 2000 uF. This range of value is usually made as a polarized electrolytic. However you must use either (a) a non-polarized type, or (b) put two 4000 uF caps in series with each other at opposing polarities, then in series with your disc. Using a capacitor may not be the most workable method.

Using an inductor could be workable. 4 mH will drop your AC supply to about 3/4. The coil must be substantial enough to endure 1.3 amps (or whatever amount heats your element to the proper temperature). Any ohmic resistance it has must be factored into the voltage drop.

Again method #4 is usable only if you can run AC through the disc.