Let’s get dirty with the ink. Tattoo machines are loads and Power supplies sources. Power supplies convert the 220V or 110V AC to few (2-18V) DC. This is normally done in 2 stages: first 220V AC to 18-19V DC and the 19V DC to the desired voltage. Most power supplies use an external “transformer” to rectify (get DC form AC source) and reduce to a workable voltage (first stage). What we normally call Power supply it is just a regulator capable of reducing DC voltage.
As you know, there are two main types of tattoo machines: coil and rotary tattoo machines.
The core of rotary machine is a DC electric motor. Any electrical motor has two parts rotor and stator. One has movement and the other is static. In the DC motor stator is composed by two magnets that generate a constant magnetic field. Magnetic fields have the force to move electrons in one way. Rotor has two windings that receives the voltage through brushes from the source and generates an opposed magnetic field. As they tend to align, the movement happens. When this alignment happens brushes exchanged positions and the polarity of the field changes so alignment occurs again. This is the rotation, a constant alignment between magnetic fields. Brushless machines use an extra circuitry to exchange the polarity on the rotor (same principle of alignment applies).
(CC BY-SA 3.0)
This kind of load consumes continuously current. The current is quite “clean” from the source point of view.
Coil machines are a bit dirtier. As you may know when current goes through, coils attract the armature bar and this disconnect the machine from the source. Current stops flowing into the machine and coils stop attracting, releasing the armature bar. When the front spring (attached to the bar) touches back the screw, the cycle starts again.
Coils are wires wrapped around a core (steel or ferrite normally). Any cable conducting some current generates magnetic field, in a coil, it is concentrated in the center of the windings and its core helps to concentrate more field.
If we go a bit deeper, we will see another component in the machine: the capacitor. Capacitor concentrates charge inside itself (like batteries). When it is empty allows current to flow until it is full. The function in the tattoo machine is to give current to the coils when the circuit is disconnected (bar is attracted). Coils generate big spikes when current stops suddenly flowing through them and this can cause harm in the components so capacitor avoids that.
When the current starts flowing the capacitor is empty so it first charges (with a big current requirement) once it is charged the current flows through the coils increasing like a ramp until it disconnects and the capacitor discharges itself on the coil (ramp down).
The current going out of the power supply looks like when it feeds a coil machine at 5V:
The big spike is the charging of the capacitor and the ramp is current going through the coils.
Power supply has to handle this peaks and interruption on consumption.
So what 3 or 5 A capable power supply means?
The rating in Amps refers to the RMS capability. RMS is a way for rating non-continuous waveforms in a continuous way. For example, peaks of 5A for a 50% of the time mean 2.5A RMS. So the RMS rating of a power supply give us information about how big the components are and which continuous power can handle. Big rotaries will require bigger RMS voltages. Big coil machines will require big RMS but also big current peaks.
This big current peaks need to be served fast. Power supplies transform energy with a certain rhythm which might not be that fast. So the key component is the output capacitor on the Power supply. This will storage energy and serve those peaks. Capacitors can be good or bad depending on its construction, some cannot serve fast peaks or cannot handle voltage ripple on its output. The bigger the capacitor the more stable current and voltage will serve. But depending on its quality will or not handle those peaks. Sometimes different type of capacitor are used to handle big storage and high peaks at the same time.
If those high current peaks are not served by the power supply, some weird behavior like voltage swings, instabilities, loose of strength on the mechanics etc can happen.
In power supplies, Amps are important but components inside are as important or even more important if you use coil machines.