Tips for Choosing the Right Power Supply for Micro Resistance Spot Welding Barbara Kuntz Successful micro resistance welding is achieved at the intersection of material choice, process development, and equipment selection. In our last blog post, we reviewed available options and imparted some guidance on How to Choose the Right Weld Head for Your Micro Welding Application. In this post, we’ll do the same for selecting the right power supply. Typical Resistance Micro Welding Setup A typical micro resistance welding setup (Figure 1) consists of a power supply, a weld head, electrodes, and, optionally, an external monitor. This is the same whether they are implemented for manual production or installed in an automated line. The power supply creates the energy, the weld head exerts the force, the electrodes make contact with the parts, applying the force and delivering the energy, and a monitor ensures the parameters set in the weld schedule are met. Figure 1: Typical Battery Welding Setup Resistance Welding Power Supplies Choosing the right power supply technology depends on the size, shape and resistivity of the parts to be welded. Available options include high frequency inverter welders (HF), linear DC welders (DC), capacitive discharge (CD) welders, and alternating current (AC) welders. Depending on the type of controller selected, the energy delivered is either “open loop” or “closed loop.” Open loop controls make no adjustments to current delivery during the weld, while closed loop power supplies include integrated feedback which enables them to make changes to the current during the weld. In Figure 2, the open loop and closed loop power supplies are categorized and compared. As a general rule of thumb, closed loop is generally preferred to achieve consistent results. This comes at a price, but for safety critical components the added cost is easily justified. Figure 2: Power Supply Feedback Types Let’s take a look at these four basic types of resistance welding power supplies for micro welding. High Frequency Inverter Figure 3: Current vs Time and Features – HF Inverter Power Supplies Considered one of the “resistance spot welding workhorses,” high frequency (HF) inverter power supplies are widely used in medical, automotive and electronic component welding applications. These power supplies get their name from the high frequency sampling rate of feedback. Though the definition tends to vary between different suppliers, in general, they have >10 kHz sampling range. Looking at a sample curve shown on the right, the frequency manifests itself as a slight ripple on the commanded current. HF inverters require three phase power input and feature closed-loop feedback that can be used in current, voltage, power, or combination modes. They provide high repetition rates and fast rise times for welding conductive parts. With the best control for automation and built-in monitoring, HF inverter power supplies enable operators to adjust weld times in 0.1 microsecond increments. Considering micro welding processes, they are best suited for resistive wires in diameters of about 0.002-0.072-inches (0.050-1.8 mm) and flat resistive material measuring 0.002 to 0.040 inches (0.050-1.0 mm). Linear DC Figure 4: Current vs Time and Features – Linear DC Power Supplies Linear DC power supplies use single-phase power and deliver a DC output that can be finely controlled, making them a good choice for small components and fine wires, particularly safety critical medical and automotive applications. Linear DC supplies are closed-loop, feature built-in monitoring, and can be used in current, voltage, power, or combination modes. Linear DC welding power supplies are appropriate for resistive wires with diameters ranging from 0.0002 to 0.036 inches (0.005-0.91 mm) and flat resistive parts ranging from 0.001 to 0.020 inches (0.025-0.50 mm). Capacitive Discharge Figure 5: Current vs Time and Features – CD Power Supplies Often used for battery tab welding and battery pack repair as well as for aerospace foil and ball tacking applications, capacitive discharge (CD) welders provide fast rise times with high peak current and are therefore a good choice for joining conductive parts. CD welders are open loop, so they provide no feedback during the weld. Rather, they have time controls on their pulse lengths. CD welders are commonly used to weld resistive wires with diameters ranging from 0.004 to 0.051 inches (0.10 -1.3 mm) and flat resistive parts ranging from 0.004 to 0.028 inches (0.10-0.71mm). For conductive materials, CD welders are commonly used to weld wires with diameters ranging from 0.004 to 0.036 inches (0.10 -0.91 mm) and flat conductive parts ranging from 0.004 to 0.020 inches (0.10-0.51 mm). Alternating Current Figure 6: Current vs Time and Features – AC Power Supplies A good choice for general purpose welding and brazing applications, AC welders are – for the most part – an open loop technology, though AC inverters offer closed-loop current control. For AC welders, time control is in cycles. AC welders are commonly selected for welding for resistive wires with diameters ranging from 0.004 to 0.051 inches (0.10 -1.3 mm) and flat resistive parts ranging from approximately 0.004 to 0.028 inches (0.10- 0.71 mm). Figure 7 provides a complete power supply technology overview, summarizing the above information. Figure 7: Power Supply Technology Overview At first glance these various types of power supplies look to have similar performance for similar wires. So what’s the difference and how do I choose which one is better? Indeed, the dimensions for wire and flat resistive part sizes are fairly similar for each of the power supplies. Determination between the best power supply also depends on manufacturing considerations and the production environment. Linear DC provides the best and finest control for the smallest of wires: dimensions < 0.008”. Because they typically run on single phase input power, they can be installed and operated virtually anywhere. However, they do not possess high firing rates, so production speed can be limited. Not surprisingly, they are often used for medical device manufacturing and micro-electronics where the part sizes continue to shrink and the need for closed-loop feedback is high. HF power supplies typically do not handle tiny parts as well as the Linear DC, but the 3-phase input allows them to fire rapidly, making them the best solution for high speed production. HF power supplies have closed-loop feedback. Examples of usage here include parts in the automotive and battery industries. For conductive parts, CD is often selected because the energy can be delivered in a very short period of time, limiting the effect of conductivity in the parts. CD technology is often utilized in the battery pack manufacturing industry where tab materials tend to be made of more conductive material. AC is the oldest welding technology in the group. It is best used to weld larger components that are not sensitive to heat input and/or line voltage fluctuations. Even though the unit is open loop, it is still widely used for various applications in automotive and larger scale production (think office furniture, water heater tanks, etc.). It is also the cheapest technology in the bunch. Summary We have reviewed 4 different types of power supplies used for micro resistance welding. Selection of the ideal choice comes down to the cross section between the parts, weld requirements, and manufacturing considerations. Of course, it is always good to confer with experts in the field and review the various facets of a resistance welding process. To learn more, refer to the following blog posts: Tips for Choosing the Right Weld Head for Micro Resistance Spot Welding Resistance Weld Monitoring Ensures Quality and Provides Traceability Why Weld Monitoring? 3 Reasons: Analysis – Stability – Yield Or view our webinar Resistance Welding Troubleshooting. Category: Other
