How do we calculate heat produced in a resistor?
The voltage squared (in volts) divided by the resistance (in ohms) is equal to the power (in watts).
take the voltage dropped, multiplied by the current amp, multiplied by the number of hours, multiplied by 860.42 thermo chemical calories / watt-hour to get the number of heat calories.
Thermal energy (heat) in watt second is electrical energy, power x time.
The electrical power input is e x i (volts x current) watts. The thermal energy is in watts x time.
Measure the voltage drop and current flow (in amperes) in the resistance of your circuit and multiply them.
These are the watts (power) converted into heat. You can multiply the watts by 859.85 calories / hour. It takes 1 calorie to lift 1 g. of water 1 degree Celsius. Thus, a heating resistor 100 grams of water having 120 volts with a current of 5 amps would have 600 watts of power and would take about 4.5 minutes to go from 20 to 100 hp.
Current resistance squared works for direct current or direct current. Under pulsating conditions, we are seeing surprisingly large power dissipation. For short pulses (less than 100 microseconds), it is possible to average over time.
Depending on the thermal inertia of a resistor, a pulse width occurs where the peak power becomes the average power. Many wound power resistors can momentarily withstand 5 times the average power.
Metal film resistors should not be used for the management of high peak currents unless explicitly recommended by the manufacturer.
The carbon composition resistances of Allen and Bradley (ab) were once popular for their ability to handle high peak current. With intrinsically low inductance, a break probability has been reported for different peak energy levels and resistance values.
With dated manufacturing practices, production was finally stopped; Phenolic packages become parallel resistive elements at high temperatures.
Over dissipation chars phenolic packaging materials, causing ab resistance values to drop under persistent overheating conditions. Alternatively, a high pulse current causes the increase in carbon composition resistance.
Some system manufacturers may have achieved planned obsolescence by combining the two features to produce stable resistance until the end of the product’s expected life.
The old stock ab continues to circulate online and remains popular. Cheap imitations must be avoided.
Lookalikes can have tiny internal resistive elements or curved leads substituted for internal terminations.
There are also metallic look-alikes. Cutting samples is good inspection practice. I always check authenticity. Cheap imitations are still suspected.
Ceramic ground resistors have largely replaced carbon resistors for low pulsed high inductance applications in the lead device market.
Flat metal film resistors now support high peak power, also offering low inductance.
These range from the size of surface mounted devices to screw terminal boxes, some of which can handle hundreds of watts of average power.
Wire wound resistors made of silicon carbide and core-rib are available for 750 watts and up. The choice is made according to the tolerance to inductance.
Both can show an extremely high pulse resistance capability. Uncoated silicon carbide should not be used in oil.
The negative voltage resistance coefficient may be significant for globars intended to be used as heating elements (some have a varistor characteristic when subjected to a high voltage pulse service).
Many water-cooled metal film tubular resistors exist at an average value of 5 kW. I had problems with the ceramic on them breaking, flooding the building several times.
Be sure to exceed the required coolant flow rate with robust margins.
The use of small resistors at 1/3 of nominal power generally prevents overheating.
