206 Modern Welding Copyright Goodheart-Willcox Co., Inc. Globular transfer voltages and amperages fall in the range between those shown for short circuiting trans- fer and spray transfer. Pulsed spray background volt- age settings are slightly higher than the values shown for short circuiting transfer. The peak current must be above the transition current. The currents shown for spray transfer are above the transition current. Figure 8-23 shows a welding machine on which welding variables are set and stored in electronic memory. The welding machine has a microprocessor inside. A microprocessor can be considered a small computer. The manufacturer programs the micropro- cessor with a set of welding values. The microproces- sor determines the best settings or parameters for the welding application based on a set of input data about a weld. The welding machine determines the welding parameters as well as the type of transfer. A welder using a microprocessor-equipped weld- ing machine enters information about the weld to be made, which includes welding wire type, wire diam- eter, type of shielding gas, and metal thickness. The microprocessor sets the voltage, wire feed speed, and possibly the slope, thus determining the type of metal transfer used. Standard welding values are prepro- grammed into the welding machine. Special welding parameters can also be saved as a program in the machine. This program, or any preset values, can be recalled at any time in the future. Because the welding values are stored electronically, the welding machine is set up exactly the same way each time. Some power sources allow the slope to be changed. Many machines have a preset value for the slope. Power sources that have the ability of pulse spray transfer have additional controls to set up. These con- trols include an on-off switch, a background voltage adjustment, a peak amperage adjustment, and some- times a “pulses per second” adjustment. The back- ground voltage is set relatively low, in the globular transfer range. The peak amperage is fairly high to cause spray transfer to occur. This peak amperage must be above the transition current. The pulses per second adjustment is used to set the number of times per second the current pulses from the low value to the high value. 8.4.2 Setting Up the Wire Feeder Wire feeders use a 24V–40V DC or a 115V AC supply. Figure 8-24 shows a complete wire drive unit with four drive rolls. Figure 8-25 shows a two-roll wire drive system. Two mated gears are located in the wire drive unit. One gear is driven by an electric variable- speed motor. A roll is attached to each gear. In a four- roll wire feeder, two rolls are driven. See Figure 8-26. The Lincoln Electric Company Figure 8-23. A microprocessor-controlled power source with a built-in wire feeder and digital displays. Electrode spool mount Drive rolls Welding gun connection The Lincoln Electric Company Figure 8-24. A complete wire feeder.