Resistance Heater for Al Brazing

Resistance Wire:  14AWG Chromel-C (AWS 675)
Resistance Wire:  Trade names
Chromel-C Heating Guide
Brazing Cycle

     The 1kW transformer shown below has had its outer winding removed and rewound to drive a custom heating element with AC mains isolation.  The first secondary winding test was six turns of #4AWG wire and put out about 7.2VAC (1.25V/Turn) or about 138A at 1kW.  The second and final secondary winding tailored for the final heater design consists of two 20 turn strip windings and puts out about 20V (Both windings in parallel) and will deliver about 50A.  The primary is driven with a 1kW dimmer control from Home Depot which sees the resistive impedance of the heating element reflected back through the transformer turns ratio and so operates as if connected to a resistive (Lamp) load.

     The heating element wire selected for this project is #14AWG Chromel-C from McMaster-Carr because it is easily obtained and has an operating limit of 1010C that is well above the target application of pre-heating aluminum for brazing at about 427C.  The final element design consists of two strands of wire (Equivalent to #11AWG) insulated with Cotronics 391 alumina cloth sleeving and inserted into a 5/16" OD - 0.243" ID 304 stainless steel tube bent into a circle of about 8.5" in diameter.  The tube is then welded on top of 10-32 x 1" SS pan head screws for leveling and thermal isolation from the base plate.  Driven at 40A, the unloaded surface temperature of the heating element in free air is 525C (The element resistance at this temperature is about 0.5 ohms).

    Working Temp
Material         Where Used                       Deg-C Deg-F
Aluminum Ring/Can (6061-T6/3003) 582 1080
Durafix Aluminum braze filler metal 389 732
Chromel-C Heating element wire 1010 1850
391 Sleeving Heating element insulation 1427 2600
SS Tube Heating element jacket 816 1500

Transformer winding Version 1:  6T #4AWG

Version 2:  2x20T 0.01x0.8 copper with 1kW dimmer.

Stainless steel tube bending jig.

Ring sizing and cutoff template.

Wire and sleeving pulled through SS tube.

Heater feed through base plate (5/16" id SS).

Wires ready to be terminated.

Aluminum ring to be brazed on heater.  Heater tube flatness adjusted with screws.

Center bar is welded to the base plate, terminals are insulated with 10mil Kapton sheet.

800W operation, 20VAC at 40A.  The heater levels out at 525C (977F) in 3 minutes.

Ready to braze:  When the aluminum gets to about 250C (482F), hit it with the MAPP torch.

Brazing stand plan.

Brazing Test 2.

  Any questions or comments?
This page last updated on March 30, 2010