Current (un)limited bench supplies


When using one of the common variable-voltage output test bench power supplies with adjustable current limits, be advised that many such devices effectively place a large output capacitance across the output terminals downstream of the current limiting function.

The consequence of this is that there is often a very substantial amount of energy available in this output capacitance available to do damage if the output of such a supply is ‘hot plugged’ into a load. As an example, a modern digitally-controlled bench supply set to 1.8V output and 100mA current limit was found to deliver a 25 amp surge lasting approximately 5 ms into a short circuit. The 40-amp limit of the current probe used was exceeded when the output voltage was set to somewhere between 1.8 and 2.5 volts, and with a 15-volt output setting, the wire segment used as contacts began to weld together upon closure–all with a current limit set at 100mA.

The moral of the story? Don’t hot-plug your bench supply. The output enable feature is there for a reason…