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Converting a cheap welder to a high-current battery charger I modified a low-cost Kenstar 200A IGBT inverter welder so that it can be used as a welder or a battery charger. An added switch and relay allows either of the two functions to be selected. The circuit uses the common UC3846 switchmode controller chip. Its output voltage can be varied from 11-22V, with the typically ~14.4V charging voltage for 12V lead-acid batteries well within this range. Generic IGBT or Mosfet welders are cheap to buy and usually easy to repair or modify. If they fail, it is often the auxiliary ±24V power supply module that is faulty. I bought my welder for $7 in a nonworking condition and repaired it for a few dollars. For any engineer used to working on mains-powered equipment, it is easy to modify it to charge a car battery. The high-voltage side does not have to be changed – only the lowvoltage control board. I purchased a low-cost LED voltage/ current meter online to display the battery voltage and charging current (see our article on these meters in the De- cember 2020 and this issue on p102; siliconchip.com.au/Series/306). It is necessary to add a current shunt resistor to sense and display the charge current; most meters come with the shunt. Both are shown in my circuit diagram. The modifications for switching between charging and welding are as follows. I added a 12V regulator on the +24V rail which generates a voltage to switch the coil of the added relay via the charge/weld switch. I cut the connection between pins The modifications marked on the welder – remember to be careful when working on mains-powered equipment. Radiating test antenna for AM radios Most transistor radios, and many later valve models, use ferrite rod antennas. While some of these provide external antenna/Earth connections, most don’t. While it is possible to connect a signal generator directly to sets that do have antenna connections, proper alignment demands the use of a ‘dummy’ antenna to mimic the capacitive nature of the few metres of wire typically used. This radiating antenna solves several problems. It will work with all sets using ferrite rod or wire loop antenna circuits, needs no electrical connections to the set being tested and, for ferrite rod antennas, gives an actual 76 Silicon Chip sensitivity calibration in microvolts per metre (µV/m). The antenna uses an ordinary 200 x 9.5mm ferrite rod. The coil is seventeen turns of wire, 0.4-0.65mm diameter (22~26 B&S gauge), spaced over 100mm. The two resistors, 100W and 82W, are carbon types. The antenna can sit in a simple non-metallic cradle of timber or plastic. Placed close to the set under test, it’s possible to inject IF signals at a high enough level for IF alignment, even for the low-sensitivity Regency TR-1 with its unusual intermediate frequency of 262kHz. This eliminates the difficulties of injecting to the converter base in compact sets. Australia's Australia’s electronics magazine For broadcast alignment, set the radiating antenna 600mm from the receiver’s antenna and parallel to it, and align at the usual frequencies of 1610kHz for the oscillator trimmer, 600kHz for the local oscillator coil and antenna and 1400kHz for the antenna trimmer. Once you have completed alignment, you can easily determine your set’s sensitivity for 50mW output. With the radiating antenna 600mm from the set’s antenna rod, divide the generator output by 20 to get the field strength at the receiver. For example, a signal generator output of 20mV RMS gives a field strength of 1mV/m at the receiver. The radiating loop gives useful results on sets with frame antenna loops, but the radiating loop must be aimed perpendicularly at the frame antenna for measurement results. This design is based on information in Mingay’s Electrical Weekly, October 18, 1963: Pye Caddy Transistor Portable Receiver Service Data Sheet. Ian Batty, Rosebud, Vic. ($80) siliconchip.com.au