I am attempting to run a Molybdenum heat coil. The best I can tell it is 0.3-0.4 ohms per rack and I would like to run say two racks at 230V at 3.9KW (so it doesn't trip my 230V 20 amp breaker).
Does that mean, assuming a value for two racks is a total of 0.8 ohms, can I add a ~12.7641 ohm resistor for a total resistance of 13.5641 to run it at 3900 watts at 230 volts?
If not, how is it possible to run these? Do I need a lower voltage supply with more current?
Answer
Using resistors is Very Bad Idea. Look at datasheet of Your heating element, this one is for example: http://heatingelements.isquaredrelement.com/Asset/Moly-D-technical-brochure.pdf On page 4 You can find a graph showing resistivity in function of temperature of heating material. At normal operating temperature (about \$1600^{\circ}C\$) molybdenum elements has resistance 7 times larger than at room temperature! If You have an element with \$0.3\Omega\$ resistance at room temperature, it will rise up to about \$2.1\Omega\$ at \$1600^{\circ}C\$. Connect them in series and You've got more than \$4\Omega\$ in operating condition.
I know, it's still too much to run it with 20A circuit breaker - only reasonable solution for such power is to use SCRs with proper driver. Such regulator gives You another two important features:
- Soft-start - limit current during start-up, when molybdenum elements are cold and have low resistance.
- Regulation of temperature - use more sophisticated regulator with feedback from some temperature sensor and you'll have an automatic regulation.
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