A powder spray coating system includes a power source and an electrical supply line connected to the power source. A spray gun sprays powder in a spray pattern onto a part. The spray gun includes an electrode which charges powder as the powder is dispensed from the gun toward the part. The electrode is connected to the electrical supply line. A first current sensing device is located in the electrical supply line measuring gun current between the power source and the electrode. An ion collector or ABI probe is mounted with the gun for collecting free ions produced by the electrode. The collector has a forward portion positioned near the spray pattern and is spaced from the electrode. The collector is connected to a ground line. A second current sensing device is located in the ground line measuring back current between the ion collector and ground. A controller is connected between the power source and the electrode to control the gun current. The controller is also connected to the first and second current sensing devices for controlling the gun current in accordance with the difference between the measured gun current and the back current. By automatically controlling the current between the gun and the part, the present invention controls the field strength and the current between the tip of the gun and the ABI probe, making unnecessary adjustment of the position of the ABI probe in order to maximize the benefits of the probe in situations where the distance between the gun and the part changes.