Suggestions for a heating element

I am in need of a heating element that will produce heat on the order of 250-300 degrees Fahrenheit for a continuous period of time (several seconds). I will be using a temperature sensor to cycle the element on and off to maintain the desired temperature.

I would assume this element should be controlled by some sort of relay or MOSFET.

250-300 degrees is the vaporization point of THC by the way :).

Regards, Ownage

Hey, I just thought I'd throw a fun fact in there. This is for tobacco use only guys ;).

Really though how about something like this http://www.skycraftsurplus.com/ni-chromeresistancewire21awg.aspx

I could create a coil of wire with it.

Sure. You can also use constantan wires. On the other hand, you can buy flexible pre-made heaters from http://www.minco.com/

You can order those that have adhesive backings for easy application.

Wow that's a cool product but since I am going to be passing air through the element a solid solution will not work (convection heating). I am going to most likely use resistive wire.

Would it be ok to switch current directly through the wire (no resistors) and just cycle the current on\off based on the temperature sensor or should I wire a resistor in line with the heating wire?

It's not enough to know how hot you want the element to get: you also need to know what you intend to heat. I.e., how many Watts you'll need to transfer, how efficiently you can transfer heat to it, etc.

Hmm well what I am actually heating is a stream of air that is going to be piped through a small tube that is the diameter of a quarter or pill bottle The air which is heated to the desired temperature will be blown through the tobacco product. From personal experience with similar products a stream of air heated to the desired temperature and blown at a decently low speed past the product will cause vaporization. I am not sure on the exact CFM, but I can't see it being more than a PC fan

I do not have a strict tolerance. I assume that varying the power to the element on\off would work fine (this is how most vaporizers work). I just want to get in the ball park.

Here is a diagram of my intended design: http://img251.imageshack.us/img251/6725/diagramt.png

I will determine my distances between the element, product, sensor, and fan as well as fan speed experimentally ;).

I'd like to know some of the theory behind this though. Could anyone point me in the right direction of what equations I would use to calculate these things?

I would try to determine the parameters experimentally. Get yourself some wires, you'll know its resistance from spec sheet. voltage^2/R is power. You may just try say 10W and see if you can raise temperature fast enough and get to your desired temperature. If not, go higher in voltage. The way you are planning may be difficult (in the pipe). I would use flexible heaters and glue them outside or inside the pipe. Will that work? Maybe stick a hair dryer in the bottom and control its heater and fan?

Use a cheap heat gun. Use an SSR of the right amperage to cycle it and use a K type thermocouple and one of the thermocouple solutions people here have come up with so the arduino can read the output via an analog input. The Arduino can directly drive Omron brand SSRs and should be able to drive most of them if they don't need more than 20ma. Otherwise use a transistor as a buffer.

I checked the entire Omoron product line and I could not find a DC SSR that would comfortably switch up to 5 Amps of current. If you look at the resistive wire I posted about it will require 4-5 amps. This is a really dumb question but is it possible to use one of the AC SSRs for DC (The ones that are controlled by DC but use AC on the load side). The AC ones are cheaper and fit my needs as far as amperage goes.

The heat gun is a good idea but I prefer to keep the entire unit battery operated if possible. If it is not economical to do so I will probably going the heat gun route.

Is there any reason you suggested a thermocouple over a standard resistive temperature sensor?

is it possible to use one of the AC SSRs for DC

No - they use SCRs or a triac, once they are on they can only be turned off when the voltage across them is zero, which happens every cycle for AC and happens never (or when you turn it off) for DC.

Hmm would it be possible to use a DC-DC SSR designed for a higher voltage rating? Can I take a 30VDC - 5A SSR and try to push 9VDC through it?

DC-DC SSRs are kind of hard to find…

DC SSR capable of 5A is the problem? how about these - ANLY ASR-10DD with 3-32VDC input and 5-120VDC output for 10A (or 25DD/44DD with coresponding ampers)? (or anything else from ebay that oyu can find [ch363]sing "SSR 10A VDC")

Can I take a 30VDC - 5A SSR and try to push 9VDC through it?

Sure: the 30V and 5A are "maximum" ratings.

Just be warned that you'll lose some voltage going through the SSR. Probably about 1.4V, because they tend to use Darlingtons to switch the power. But read the spec sheet to be sure: maybe yours uses a power FET, instead.