 Calculations

Electrical Power   Before you start calculating your power supply, it is important to understand that it is the current that heats the wire, and it is the voltage that drives the current through your resistance.

You must therefore start out by determining the amperage requirement for a desired temperature, then use your resistance to determine the voltage.

It is this selected amperage and the calculated voltage, that become the minimum power requirements for your power supply.

It seems only proper to first discuss the two electrical laws that be applied throughout this discussion and other calculation pages.

Through these laws you will note how current (or amperage), voltage, resistance (or ohms) and power (or wattage) relate to each other.

By applying algebra, you can create a variety of other equations to determine your unknown value.

Taking this into account, you should now realize that it is more important to determine your circuit resistance and THEN find a power supply that will suit rather than trying to fit a power supply to your circuit.

But fear not, if you still want to retrofit your power supply to your circuit, it can be done. It just takes a little more finesse, but will require rather specific resistance requirements which in turn forces only a specific  NiChrome wire length and diameter. And certainly no offense intended, but there are few users that take the time to retrofit a power supply appropriately and end up burning out their power supply. (Ever hear of someone burning up a cell phone charger? or a doorbell transformer?)   Ohm's Law
Resistance (Ohms) = Volts / Current (Amperes)

Joule's Law
Power (Watts) = Volts * Current (Amperes)

The center ring shows which value you wish to obtain, the outer ring shows the equations you can use to obtain that value.

Use this diagram as a source for your algebraic equations.   When you use a power supply, as the current flow increases the internal components (transformer, resistors, etc) will naturally generate heat. This heat is dissipated to the surrounding air and this dissipation occurs at fixed rates depending on ventilation and the power supply casing.

In due time, if current is passed through the power supply higher than its listed rating the internal components will continue to heat up and will begin to fail. This failure is most easily detectable as a burning smell when the wire insulation, resistors and transformers start to melt.

The listed amperage indicates the maximum safe current to be pulled from the power supply without damaging the internal components. Granted, you may be able to pull more current through a power supply than it is rated for, but you run the risk of supply failure.

With either steady state power supplies or variable voltage power supplies, the voltage remains constant but the current pulled from the power supply is based on the electrical load or circuit resistance.

Ohms laws which states :

Current = Voltage / Resistance

or

I = V / R

Knowing that power supplies provide a constant voltage (whether a dial is set, or a solid-state or a battery) the only possible way that you can alter the current is by affecting resistance.

Just remember ohms law...

When you hook up a power supply or a battery to a hot wire foam cutter, the voltage applied by the power supply remains constant and the output amperage is dependent upon the resistance of your circuit.

The voltage written on a battery, solid-state fixed voltage power supply or the dial you select on a variable voltage power supply is this true voltage output.

The amperage written on a power supply is the maximum safe output for that power supply. But the ACTUAL output amperage is dependent upon the resistance of your circuit.

As you change the length of your NiChrome wire you are changing the circuit resistance and therefore the amperage drawn from that power supply will change and have no bearing on the output voltage.

Consider this :

Heres why :

If you are not changing the length of your nichrome wire (your resistance is constant), when you increase the voltage from your power supply (turning up the dial on a variable voltage power supply), you will increase the current.

Consider this :

Heres why :

Current = Voltage / Resistance

or

I = V / R

Because in this example resistance is kept the same, as you increase voltage, you increase amperage. The power supply is still outputting a constant voltage, but you simply changed the dynamics of the Ohm’s Law equation.

For instance, with a steady state power supply (battery charger) that is 12v/2a it will be able to supply your circuit with a constant 12 volts. The 2 amp rating is the maximum current that can be supplied without damaging the internal components.

For more discussion of power supply mechanics, visit the Power Supply Mechanics Page as well as the Over Current Protection Page. With this being the case, in designing an electrical circuit (your hot wire foam cutter), you should always choose a power supply that has the appropriate voltage but rated at a higher amperage than you require for your circuit. Many recommendations are 20% higher than you required amperage.

Don’t worry! This does not contradict any previous statements. The output voltage from the power supply still remains constant! With a variable voltage power supply, you have simply altered the output voltage and have thus selected the new constant voltage value.

BY changing the constant output voltage you’ve altered the output amperage.    