Welding Current and Power Sources — Complete Guide
Introduction
The welding power source is the central piece of equipment in any arc welding operation. Its function is to convert mains electricity into a form suitable for maintaining the arc — with the correct voltage, current and stability for the process being used.
Understanding how electrical current works and the different types of power sources is essential for any welder, as the wrong choice of machine or parameters directly affects weld quality.
Sources: vocational training centres — Electrical Current (welding training module), Welding Machines (vocational training centres module), AWS Welding Handbook Vol. 1.
Direct current vs alternating current
Electrical current is the flow of electrons in a circuit. Two types are used in welding:
| Characteristic | Direct Current (DC) | Alternating Current (AC) |
|---|---|---|
| Electron flow | One direction (constant) | Reverses periodically (50/60 Hz) |
| Polarity | Fixed: DC+ (reverse/DCEP) or DC- (straight/DCEN) | Changes every half-cycle |
| Arc stability | Excellent | Lower (arc extinguishes at each zero crossing) |
| Arc blow | Can occur (especially with DC on thick workpieces) | Reduced |
| Main applications | SMAW 111, MIG/MAG 135, TIG 141 (DC), FCAW 136 | TIG on aluminium (AC), SMAW with certain electrodes |
DC polarity — Straight vs Reverse
Polarity defines the direction of electron flow and has a direct impact on penetration and heating:
| Polarity | Designation | Electrode | Workpiece | Effect |
|---|---|---|---|---|
| DC- (DCEN) | Straight | Negative (cathode) | Positive (anode) | More penetration, less heat on electrode |
| DC+ (DCEP) | Reverse | Positive (anode) | Negative (cathode) | Less penetration, more heat on electrode, cathodic cleaning |
Rules of thumb:
- DC- (straight) is used in TIG (141) on steels — protects the tungsten electrode and provides good penetration
- DC+ (reverse) is used in MIG/MAG (131/135) and SMAW with basic electrodes — melts the wire/electrode faster
- AC is used in TIG on aluminium — the positive half-cycle provides cathodic cleaning (removes aluminium oxide)
Types of welding power sources
Welding machines are divided into several types by technology:
| Type | Principle | Advantages | Limitations |
|---|---|---|---|
| Transformer | Converts mains AC to welding AC (low voltage, high current) | Simple, robust, inexpensive | Heavy, AC only, limited control |
| Rectifier | Transformer + rectification (diodes/thyristors) → DC | Stable DC, good for SMAW/MIG | Heavy, medium efficiency |
| Inverter | Rectifies → high frequency → transforms → rectifies | Lightweight, efficient, precise control | Sensitive to dust/moisture, cost |
| Engine-driven generator | Combustion engine drives electrical generator | Self-contained (no mains power) | Noisy, engine maintenance |
Note: Inverters dominate the current market (~90% of machines sold) due to their power-to-weight ratio and ability to produce both DC and AC in the same machine.
Static characteristic curve
The static characteristic curve relates the voltage (V) to the current (A) the source can deliver. There are two fundamental types:
Constant current source (CC — "drooping characteristic"):
- Current remains relatively constant even with voltage variations
- Used in: SMAW (111), TIG (141)
- The welder controls current; voltage adjusts automatically to arc length
- Typical variation: ±5A for a ±5V change
Constant voltage source (CV — "flat characteristic"):
- Voltage remains relatively constant even with current variations
- Used in: MIG/MAG (131/135), FCAW (136), SAW (121)
- Wire feed is constant; the machine adjusts current to maintain voltage (arc self-regulation)
- Typical variation: ±1V for a ±50A change
Duty cycle
The duty cycle indicates the percentage of time the machine can weld within a 10-minute period without overheating.
| Duty cycle | Meaning | Typical use |
|---|---|---|
| 60% at 200A | Can weld 6 min and rest 4 min | Maintenance, general workshop |
| 100% at 150A | Can weld continuously at 150A | Continuous production (at reduced current) |
| 35% at 250A | Can weld 3.5 min and rest 6.5 min | Heavy intermittent work |
Rule of thumb: For continuous production work (factory, shipyard), choose a machine with duty cycle ≥60% at the usual working current. For maintenance and repair, 35-40% is acceptable.
How to choose the right machine
| Criterion | What to check |
|---|---|
| Process | Does the machine support the required process(es)? (SMAW, TIG, MIG/MAG, etc.) |
| Current | DC, AC or both? (TIG aluminium needs AC; MIG/MAG needs DC) |
| Amperage | Is the maximum current sufficient for the material thickness? |
| Duty cycle | Suitable for the work pace? (production vs maintenance) |
| Power supply | Single-phase (230V) or three-phase (400V)? Generators in the field? |
| Weight and portability | Fixed workshop or field/site work? |
| Features | Pulsed, HF start, hot start, anti-stick, VRD? |
Electrical safety
- Open circuit voltage (OCV) must not exceed 80V DC or 48V AC (standard EN 60974-1)
- VRD (Voltage Reduction Device) reduces OCV to <35V when no arc is present — mandatory on many construction sites and shipyards
- Always use cables in good condition, secure connections and correct earth return
- In damp or confined environments, use machines with minimum IP23 protection and an isolation transformer
Sources and references
- vocational training centres — Corrente Elétrica (welding training module)
- vocational training centres — Máquinas de Soldadura (training module)
- vocational training centres — Ferramentas e Máquinas Elétricas
- EN 60974-1 — Arc welding equipment — Welding power sources
- AWS Welding Handbook, Volume 1, Chapter 3 — Arc Welding Power Sources