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This paper describes the design and implementation of a computer controlled active load for characterization of the steady-state and the dynamic behaviours of polymer electrolyte membrane (PEM) fuel cell systems. The active load converter is based on a linear regulator topology. The non-switching converter is ideal for steady-state measurements. The control of the converter is implemented in Simulink and dSpace and uses a PI feedback loop. The active load is designed for characterization of a fuel cell system, and therefore an electric circuit model for fuel cells is introduced. The circuit model contains a highly non-linear voltage source describing the activation polarization and the concentration polarization, two resistors for the ohmic polarization and the membranes double layer capacitor for the dynamic behaviour. The circuit model parameters are obtained from the measured data. Test results show good similarity with the fuel cell theory and confirm that the fuel cell circuit model can be used for modelling commercially available fuel cell systems, and uncontrolled fuel cell stacks.