Method for powering nichrome heating element

[analogkid2digitalman]

I have a project where we need to heat a small part up for a few seconds in the range of 100-120C. The best approach looks to be a small coil made of a few turns of 18 gauge nichrome wire. Open loop control will probably suffice for this application. Guessing that power needed will be in the 10's of watts. Once I determine the power required it should not require adjustment.

What are the typical methods of powering low power resistive heating elements?

I am thinking of using either:
1. A toroidial step down transformer with just a low value series resistor.
2. Phase chopping SCR circuit on the primary of the step down transformer.

Any guidance or tips from experience would be appreciated.
A 20kW desktop furnace this is not!

-AK2DM


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"It's the questions that drive us"
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[MacGyverS2000]

Is there any reason not to just plug it into the wall? A 120V heating coil in a multitude of configurations can be had for a few dollars, an there's no engineering involved.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[VEBill]

Are you looking for an adjustable setting? If there're a lot of variables, then it might be difficult to hit the mark through prediction and ordering a X watt heater. It's not unusual to aim high and then adjust down to calibrate.

An $8 light dimmer might work, but it might not be suitable in some circumstances.

Once you determine the required power, then perhaps a fixed value heater could replace the adjustable version.

 

[VEBill]

PS: A variac variable transformer might be useful, depending on the details.

 

[analogkid2digitalman]

OK, an update and progress. We have a device here that heats a small coil of nichrome wire to burn polyimide off of capillary tubing. It is just on/off and glows orange hot, way too much for my needs.

Hooked it up to a variac and hit target temperature in a few seconds. Variac was set to 40VAC. I opended the case and found the primary 120VAC and secondary 6VAC coil sets in parallel. 30VA unit so 6VAC out at 5A max. The is also a 220mohm resistor between the xformer and heater. I'll start out with a 2-3VAC xformer, but will take a voltage and current reading of the current setup as a starting point.

Had problems trying to find an off the shelf heater coil. 3-4mm ID, 1cm long, 2-3 turns. Coil is nice because it is noncontact (optical scrath/dig requirement) and allows us to see the process within the part during heating.

Thanks for the tip on the variac, luckily we had one floating around.

Thansk again and have a good weekend.

-AK2DM


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"It's the questions that drive us"
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[OperaHouse]

You mentioned using a torroid. I have used these many times to build high current sources for test fixtures. It is quite easy to wrap ten or twenty turns of wire through the core to get the voltage you need. You may be dropping a lot of voltage in the existing windings now. With higher current wire the actual needed voltage may be only a volt.

 

[itsmoked]

If this is a one-off you should consider leaving it all alone and just using a PID controller and a cheap thermocouple. Otherwise you will have lousy repeatability and will be screwing up your tubing on a regular basis until you finally realize you do need some temp control.

Something like:

http://www.factorymation.com/s.nl/it.A/id.65050/.f?sc=2&category=157030

Or perhaps:

http://www.omega.com/ppt/pptsc.asp?ref=CN740_Series

And pick up one of these thermocouples:

http://www.omega.com/ppt/pptsc.asp?ref=IRCO-BW

Then just drive what you have now.

Keith Cress
kcress -

http://www.flaminsystems.com