A Neat Little Variable Constant-Current Add-on for Power Supplies

Some of my DC power supplies do not have a constant current feature, that is, the output current cannot exceed a certain value or that cannot maintain a low value constant current. So I built this simple unit that plugs directly into the output socket of power supplies and that constrains the current to a specific selectable output - which is selected by a switch.

The constant current supply is based on an established circuit that uses the LM317 IC. The constant current is defined by the resistance value between the output and the adjust pin of the LM317. There are many online caculators that can be used to calculate the value of this resistance such as the one from DIY Audio Projects. I decided to use a switch to select the value of the resistance and therefore the output current instead of a potentiometer as I needed discrete and repeatable constant current values. I had a two pole 6 way switch so used 6 resistance values for 6 current values.

Note: The low resistance values require higher wattage resistors which I did not have so I paralleled higher value resistors to get the required wattage. For example, I soldered five 10 ohm 1/4W resistors in parallel to give me a value of 10 ohms divided by 5 parallel resistors = 2 ohms and 5 x 0.25W = 1.25W.

The wiring was simple, I used ring terminals in which I crimped and soldered the wires to connect to the banana male plugs and female sockets as shown. Rest of the connections were point to point.

Note: The center-to-center spacing of the plugs and sockets should be exactly 0.75 inches apart as that is the standard spacing on power supply sockets.

The resistors were soldered onto the switch, the LM317 was attached to a small aluminum plate that serves as a heat sink, the banana plugs and sockets were attached to the 3D printed box and the aluminum plate and wires were soldered to the different points. The aluminum plate was then attached with 4 screws to the little 3D printed box.

After assembly the actual current was measured for each switch position and a small label was etched into black acrylic, the etching filled with gold acrylic paint and then the label was glued onto the 3D printed box. The final current values differed from what was calculated due to not having the exact resistance values and maybe the internal resiatance of the switch.

The small module is plugged into an existing power supply's output sockets, the switch is set to the required maximum current and your device is then plugged into the constant current modules output socket.