Measuring Current by rectifying or voltage-doubling a Current Transformer
A CT can be generally regarded as having a current output that is proportional to the current flowing through the primary. This is converted to a voltage by the burden resistor. So, if you apply a bridge rectifier between CT secondary winding and burden resistor you get a pretty good approximation to a full-wave precision rectifier: -



A word of warning; don't push it too far - you should be able to get a few volts peak-to-peak but don't aim to get a peak voltage higher than what would be obtainable with the burden placed conventionally because you might suffer a little bit of inaccuracy and, if you take it too far, you might start to overly saturate the CT's core. In other words, don't choose a burden resistor value that is significantly higher than that recommended by the data sheet.