Go to your local Home Depot or Lowes and buy a 100 foot roll of #12-2 or 12-3 copper wire. #12 wire has a resistance of .159 Ohms per 100 feet so if you hook the Black and White conductors together on one end and then put your power supply on the other end you will have a high powered resistor of .318 ohms. If the supply is at 12.7 volts it will draw 39.94 amps and if the voltage is 13.6 it will draw 42.77 Amps. I suspect the voltage will be below 12.7 at 40 amps so just use Ohms law to calculate the parameter you need. Use a good digital meter for the voltage as the metering on most power supplies is pretty poor. The good part is, the wire is inexpensive and can always be reused for your next hamshack wiring project. I guess if you wanted to try a mid scale reading you could tie the white and the ground wire together on one end and tie the Black and ground together on the other end. Then put the loose Black and White ends on your power supply and you would have a .477 ohm resistor. That would draw 27.67 Amps at 13.2 Volts, with an accurate digital meter on the voltage you can then calculate the actual current value being drawn.
Make sure the joints are secure where you fasten the conductors together as the full current will be going through them. # 12 wire is rated at 20 amps 24/7 so running 40 amps through it would not do any harm for the amount of time it takes to calculate the current. You could also do this with #14 wire just use .253 Ohms per 100 feet, or .506 Ohms for 2 conductors. With #14 .506 Ohms will draw 26.09 Amps at 13.2 volts. When I worked at Motorola we used this trick all the time to test high powered supplies, just remember to keep the joints tight. I prefer to give them a few twists and solder them. One thing to check if you have access to a scope is the droop when you connect up the load and the over shoot when you remove the load. Depending on the stiffness of the supply you may see some pretty healthy excursions. Good luck, Jack K6YC ----- Original Message ----- From: Don To: [email protected] Sent: Monday, March 05, 2007 10:03 PM Subject: [Repeater-Builder] DC Power Supply Test Load Sometimes I wonder how in the World did I get a Ham Radio Lic , 25 Yrs ago , Anyway here is what I need Help on I bought a 40 Amp Power Adjustable Power supply with Volt and Amp Meters , I would like to make something to test it Before putting it on a Expensive piece of Communications Equipment Like My Kenwood TS-2000 , I tried a Car Battery Tester but that is a 100 Amp non Adjustable Load , Pined the Amp Meter and Voltage Dropped to 6 Volts . To Much Load, Maybe I will try a Car headlight but that will be Unknown as to how many amps at 13.8, and that is what I need to know Exactly a load that will tell me that, anyone that has done this or has any Ideas I would appreciate it Thanks Don KA9QJG
