There are lots of "how to" posts on the internet, decoder manufacturers' web sites and on Youtube telling you how to measure locomotive stall current to determine what decoder current rating is necessary to install in your locomotive. While the basic concept of this testing is usually provided correctly, most all of the information provided is lacking in sufficient detail to ensure accurate results. Most all show the use of a model railroad power pack of some sort providing power, stating you should run the test at a given track voltage, say 12 -14 volts. To do accurate stall testing, you must have a variable, regulated power supply or a regulated fixed voltage power supply that can deliver the necessary voltage and current to properly conduct the test. The power supply has to be regulated so that it will deliver the same voltage, regardless of current draw. Most "train set" power packs do not regulate the output voltage; they simply have a transformer that is connected in series with variable resistor (potentiometer) that varies the amount of current that can flow to the track. An exception to this is a power pack that includes a voltmeter and ammeter so you can see both voltage and amperage at the same time. More to follow.
