Nichrome Wire Circuitry

[MrCrowley]

So I'm trying to use a car cigarette lighter as a heat source (for something related to microscopy). I want the nichrome wire in the 12v cig lighter to heat up to several hundred C (doesn't need to be that hot) and have it stay hot for a fairly extended period of time (up to 30 seconds, I suppose). I'd prefer to run this on DC as I'm not so confident making my own circuits that run off mains power (otherwise I'd just get a hair dryer or soldering iron).

• How much current will I need to accomplish this? I have a few 240VAC - 12VDC 3A transformers but will that have enough juice? I don't need it to heat up as quickly as in a car, but I don't want to be waiting too long either.
  
  How could I go about controlling the temperature? I figure a pot. would do the trick but I'd have no idea which pot. to buy or if I'd need any other components on top of that. Makes it difficult when I have no idea what gauge the nichrome wire is in the cig. lighter.

Also, I have a 12VDC 4A transformer but I replaced the 0.5A fuse (blown) with a 4A fuse and sparks started flying from the windings/core as soon as I switched it on (not under load). The guy at the electronics store said a 4A fuse would be fine so perhaps the transformer is faulty.

[POLAND_SPUD]

I used a 9V battery to heat up a lenght of nichrome wire - so you don't really need a lot of current

How could I go about controlling the temperature?

PWM signal driving a transistor

A circuit that can do it can be built using 555timer or pretty much any micro-controller. I've even seen ICs that are thermostats - they were quite cheap actually

[mark.f]

What PS said. I posted a simple 555 PWM circuit for driving a fan/LED/etc., although the switching frequency can be very low for something like a piece of nichrome wire. I think it was in the Offtopic-posts thread. All it needs is a pot and diode for duty cycle selection and the capacitor size dictates switching frequency.

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[Technician1002]

Nichrome wire comes in many sizes much like electrical wiring. The current needed will vary depending on wire size and heat load. For example a typical hair dryer will draw about 10 amps on high on 120 volts. 240 volt versions will use 1/2 the current for the same wattage.

If you remove the fan from a hair dryer, they will rapidly overheat due to the lack of a thermal load. As such you can get the same temperatures on much lower voltage and current.

Not stated in your original post is what is your thermal load? This dictates your heat requirement. Maintaining a warm oven in a thermos bottle is a low power load. Melting the ice off your pond is a high wattage load.

Ohm's law with a resistive load will give your voltage and amp requirements when you use a Nichrome wire table. Ohm's law states Volts = Current divided by Resistance.

On DC Watts = Volts times Amps. Use above to calculate the current with the provided voltage.

Nichrome wire does change resistance with temperature, so the current drawn when cold is much higher than when hot. Your circuit must allow for this.

http://en.wikipedia.org/wiki/Nichrome

[MrCrowley]

Thanks for the help guys. I was thinking about a PWM but they start at about $35 here and it's a bit too complicated for the circuit in terms of what I'm trying to achieve and how much I want to spend on it. Essentially, I need a small heat source that can radiate a bit of heat (300-500C range) to dry out liquid samples on microscope slides. I don't want to use anything too hot since it can damage the slides/specimens (more than I want, anyway) and I need it to be relatively small and portable.

I tried a bunch of transformers that were either around 12V 1A or 5V 1.5A to 3A. None of them could heat the wire beyond warm-ish. I also tried a ~15V 7A laptop transformer and that started beeping when I applied the load (car cigarette lighter).

Unfortunately, I have no idea what gauge wire the nichrome is and my analogue voltmeter won't reset to zero after adjustment when I try to measure resistance. That's part of the difficulty I'm having, I have no idea what voltage and amperage transformer I need nor what pots would be suitable for adjusting the temperature of the nichrome wire.

Because of these setbacks, I'm pursuing another idea for now. Will report back if I continue down this path. Looks like I'd have to buy a transformer at the minimum and that's another $20 or so unless I scrounge one from somewhere.

[POLAND_SPUD]

ohh come on you could build one using 555timer and a couple of passive parts that cost 2-3$ total

[Gun Freak]

I'd ship a 32V transformer to you but seeing as youre trying not to spend much I guess the shipping cost would rule that out...

[Technician1002]

For drying slide samples, only a Watt or two is all that is needed to warm it. I would ditch the nichrome wire due to the unknowns with your sample. Use Ohms law and simply use a resistor from Radio Shack or Mouser, Digi-Key or other supplier.. This gives you a known value.

Say you want 1 Watt of heat.. You want Volts X Amps to equal 1 Watt. Given 12 volts from your lighter you need 0.083 Amps of current.

Using Amps = Volts divided by Resistance, 0.083 = 12/144 or about 120 or 150 ohms in a standard value resistor. Use a 2 Watt or larger resistor to prevent overheating. A square 10 Watt resistor should work nicely.

Plug it in, lit it warm and dry your sample.

FYI, soldering irons depending on size are between 15 and 26 Watts for ones without a thermostat. 1 Watt should be plenty for drying slides.

[MrCrowley]

Thanks for that Tech, that does seem like a more suitable and safer option. Sorry about the late reply (and brief one here), just been a bit busy lately.

On a related note, does copper release harmful fumes when heated to 200-400 degrees Celsius? I was heating some fittings the other week with a heat gun and afterwards I had the smell of ammonia in my nose for the rest of the day. I assume it's probably related to metal fume fever from heating some galvanised fittings (with inadequate ventilation it seems) but I did heat up a copper fitting or two as well.

[Technician1002]

If you didn't heat the copper beyond soldering temp, you should be fine. No normal brazing activities creates an ammonia smell. Overheated Zinc will cause problems. Most likely the effects of Zinc is the issue.

If you heated used refrigeration tubing, and it contained some oil, the oil can release dissolved refrigerant that will break down into nasty compounds with Fluorine and Chlorine compounds on contact with overheated copper..

Most refrigerants are a chlorinated fluorocarbon.

In short copper doesn't release nasty stuff by itself. Hot in contact with refrigerant is another story.

[jimmy101]

If you use the 240VAC to 12VAC transformer you can plug the HV end into a standard household light dimmer. The dimmer will drop the power to the transformer and hence the power to the wire so you can use the dimmer to control how hot the wire gets. 3A is enough for small gauge nichrome wire but heavier gauges will draw more than 3A and toast your transformer.

Google for a "hot wire cutter" circuit. That should do what you want without needing a 555 or other complex stepdown voltage supply.
http://jacobs-online.biz/power_supply_design.htm


Tech said: "Nichrome wire does change resistance with temperature, so the current drawn when cold is much higher than when hot. Your circuit must allow for this."
I think there is a typo in there somewhere. Nichome wire is a conductor (vs. a semiconductor) and a characteristic of a conductor is that it has a positive tempco (temperature coefficient of resistance). When you heat a conductor the resistance goes up. This is what makes a incandescent light bulb so simple, they self regulate their current draw. (Semiconductors are the opposite, as temp goes up resistance goes down and they draw more power resulting in release of the device's magic smoke.) IIRC a standard 100W incandescent has a room temperature resistance of a couple tens of ohms. In operation the heating ups the resistance to perhaps 200 ohms.