Greetings,

I am new here, so please forgive any faux pas.

I am trying to make a heating element for my electrochemistry setup, but I only need it to be slightly above room temperature, at a nice toasty 25°C. I have access to large spools of ~0.015in (~27AWG) ChromelC (~4Ω/ft), a variety of AC and DC voltage sources/transformers (including an old variac), and loads of components.

Basically, I need help with designing something that works! I've been researching and playing around with stuff, and from what I can find I need a low voltage, high current source and a decent length of wire. However, playing around with what I have in my shop, I haven't been able to get even a sizzle out of my wire. I've been working with ~12in of wire and a variety of step-down transformers and a 2.2M pot as a rheostat, but nothing has worked yet and I'm worried about blowing my pot.

My thought/question is: Is there a lower threshold for NiCr wire? Like, if I don't put X amount of amps through it, then I don't get any heat at all? I've been considering this issue and the notion of a PWM-controlled setup was discussed, but how can I run a sufficient current through a 555 timer without blowing it (datasheet says max=600mA)?

Thanks for any input you can give me!

 

What are you trying to heat to 25°C? Air in an enclosure, a metal plate, liquid in a test tube, or what? And what size and shape is it? Or do you simple want to get the nichrome wire to 25°C?

There is no lower threshold. The wire heats because of its resistance. The more current you put through it, the hotter it gets, however 25°C doesn't feel warm to the touch. How do you know that the wire is not being heated slightly?

What is the voltage you are applying to the 1 foot piece of wire? And how much current can your supply source?

 

What are you trying to heat to 25°C? Air in an enclosure, a metal plate, liquid in a test tube, or what? And what size and shape is it? Or do you simple want to get the nichrome wire to 25°C?

Well, it's a small glass 2-compartment electrochem cell filled with my analyte and solvent and all that, all of which I have nested in a lined and specially contoured box filled with air an packing peanuts. The whole mass of the setup is probably about 200grams, but the idea is to get the whole shebang to 25 degrees and then keep it there with intermittent corrections (basically a watchdog relay circuit).

How do you know that the wire is not being heated slightly?

I guess I don't, but I ramped up the current (read: turned down my pot) in order to see if I could feel anything, and I couldn't. On that 10v/600mA source I should have been at around 40 degrees, I think.

What is the voltage you are applying to the 1 foot piece of wire? And how much current can your supply source?

I was playing around with a couple of sources, first a 10Vac/600mA (Before I ad done any calculations), then a 30Vac transformer with dubious current limits, and finally a 6Vac/1A transformer, but that's when I decided I should start asking questions before I start setting fires.

As far as sources go, I've got carte blanche. I've got both AC and DC sources with high current limits and a range of voltages. The same with the wire length, although I've only got the one gauge to work with.

 

Well, it's a small glass 2-compartment electrochem cell filled with my analyte and solvent and all that, all of which I have nested in a lined and specially contoured box filled with air an packing peanuts. The whole mass of the setup is probably about 200grams, but the idea is to get the whole shebang to 25 degrees and then keep it there with intermittent corrections (basically a watchdog relay circuit).

You might benefit from this project. The TEC I used there was for cooling below ambient, but just reversing the leads would turn it into a heater. Point is, I really think you'll want a thermostat and not just a source of heating. A simple comparator circuit with a LM35 thermometer would do the job. In that project, I needed a bit tighter control than the comparator alone could provide, so I added an op-amp but maybe you don't need that.

 

First of all, nichrome wire is hard to work with. It's almost impossible to solder to, it has no thermal mass, and it can get white hot in a hurry. Are you determined to use it, or are you open to options?

I would consider low resistance sand resistors heated by low voltage DC. They are easier to work with and much less of a fire and burn hazard. For example, you could use a 24Ω 10watt resistor and a 12V power supply, which would give you .5A through the resistor, and result in 6W of heat, which is plenty for a container as small as you describe, and at the low temperature you want, would work well. Of course, you need a thermostat to maintain the temperature, but you will have to have that no matter what heating element you use.

 

Some math: 4 ohms
That means 1/4 amp per volt.
and you are using a 2,200,000 ohm pot to control it? Not likely.
So, if you try 6 volts, you will have 6/4 amps and (P=IE) 9 watts.
You don't need many watts for something that small. With enough insulation, you can start a fire with 1 watt.
Just trying to calibrate your thinking.

I can offer a nice, one chip thermostat that is designed for a 10k thermistor that is smaller than a match head.
http://forum.allaboutcircuits.com/blog.php?b=532

 

First of all, nichrome wire is hard to work with. It's almost impossible to solder to, it has no thermal mass, and it can get white hot in a hurry. Are you determined to use it, or are you open to options?

Oh hell yes I am open to options. The quicker/easier/safer, the better.

I would consider low resistance sand resistors heated by low voltage DC.

I had actually stumbled across something talking about using a bunch of resistors in parallel, utilizing their power dissipation as a heat source. I'm quite new to this stuff, so between that and using a forward-biased diode as a temperature sensor... mind=blown. I think I am going to look into this as my best option. Legit Thank You.

Of course, you need a thermostat to maintain the temperature, but you will have to have that no matter what heating element you use.

Indeed. I have a ton of options on that end of things, and I am pretty sure I can rig up that circuit with a thermocouple/diode/ic without too much hassle. I mean, I've recently just been shoving a glass-encased thermocouple probe right into my sample, so if it came to manually adjusting it that would be fine. I am just sick of sitting there with a hairdryer...

 

Some math: 4 ohms
That means 1/4 amp per volt.
and you are using a 2,200,000 ohm pot to control it? Not likely.
So, if you try 6 volts, you will have 6/4 amps and (P=IE) 9 watts.
You don't need many watts for something that small. With enough insulation, you can start a fire with 1 watt.
Just trying to calibrate your thinking.

Yeah, that sounds about like what I was trying to avoid. Probably lucky nothing worked. The online calculator I found only went down to about 1.3A and 93°C for my length and diameter of wire, so I was trying to extrapolate (but any math I try to do in my head is automatically at least a order of magnitude off...)

It's likely best for me to take tracecom's advice and use resistors instead. Much less risk for me to set fire to my face. Or my research!

 

Has anyone told you that 25 C is 77 F? That's "room temperature". Where are you that you need a hair dryer to get to room temperature in July?

 

Has anyone told you that 25 C is 77 F? That's "room temperature". Where are you that you need a hair dryer to get to room temperature in July?

HAHAHA, room temperature... not in Maine, my friend. I see you are from FL, as is one of my colleagues, who usually has about 2 sweaters while she is in the lab. Right now at the height of summer, at the warmest time of day, it is about 22°C in the lab. In winter/spring it gets down to about 16°C, and we have to thaw out our DMSO samples before using them.

 

I have this craving to drive to northern Maine this summer. I want to see the north woods (and escape the weather in Tennessee.)

 

I have this craving to drive to northern Maine this summer. I want to see the north woods (and escape the weather in Tennessee.)

It's absolutely beautiful there, one of the only reasons I stay in this podunk state. The weather likely isn't much better... less tornadoes but more humidity, and you'd better bring some serious bug spray, bug netting, and prepare to battle the largest black flies you've ever seen. Worth it though.

Thanks for all the help, this is coming together nicely.