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An open-loop adaptive filter, which can be used in various power electronics control systems, is presented. The purpose of the filter is to give as fast a step response as possible while retaining good noise canceling properties. The proposed filter consists of a first-order infinite-impulse-response filter, the coefficients of which are adapted according to the changes in the input signal. In particular, good performance is obtained if the filter must suppress a low-frequency narrow-band noise. The performance of the filter is verified through step response simulations and with a laboratory prototype of a welding machine. The control system of the prototype is implemented on a field-programmable gate array. The proposed filter is compared to two optimum linear filters: (1) the moving average filter and (2) a filter whose step response and white noise gain are both optimized. The step response simulations show that, with the proposed filter, the step response time can be considerably decreased compared to these filters. The prototype also shows that, when using the proposed filter, the step response of the welding current can be made better than that of the original analog control.