Stall Current

How to Measure It

There is a lot of confusion and controversy about stall current measurements. Here is Mr. DCC's position: While you may never cause your loco to stall - that is freeze the wheels with power applied - you always start it. The first pulses of current delivered to the motor by the decoder are at "stall" current. That is why Bruce created this page, to show you how to measure the true stall current. Once you have this data, what you do with it is your decision. But at least you won't be shooting in the dark!

In its simplest form, what you are doing is measuring the current drawn at full track voltage with the motor not turning. 

Be aware some locos, Bachmann HO Shays come to mind, are geared so low that is difficult to impossible to stall them without physical damage. In those cases, I will press down hard enough to seriously slow the motion of the locomotive and take my readings. Then I apply a factor, 2x or 3x, to come up with an approximate stall current.

If you have a power pack with adequate output and built-in meters, then you just hook the loco up, turn the voltage up until you reach the level for your DCC system (12 volts for N, 15 volts for HO and 18 volts for O or larger are common values), stall the motor by pressing the loco against the track until the wheels stop turning and read the current, making certain that the voltage doesn’t drop from the set value. However, since most of us don’t have a big, fancy power pack  capable of achieving this, we need some help.

Two meters (one for voltage and one for current) are VERY helpful. Here’s how I do it on a medium power pack.  I connect the voltmeter across the power pack output and the current meter in series with the loco. I turn the power pack up to about 10 volts and stall the motor (by pressing the loco against the track until the wheels stop turning) and read the current and voltage. I then calculate the stall current at the DCC voltage with the formula shown below.

Find the Operating Current:

While I’m at it, I run the voltage up until I reach the desired DCC voltage and see what the operating current is.  Usually it is 1/2 of the stall current or less.

Stall Current Formula

Operating Stall Current =
Operating DCC voltage x
Measured Stall Current /
Measured Stall Voltage

Example:

As an example, if a loco stalls at 0.7 amps and 8 volts when measured as above and it will be operated at 15 volts DCC (Digitrax’ HO setting), then the Operating Stall Current is calculated like this:

Operating Stall Current =
15 volts x 0.7 amps / 8 volts = 1.3 Amps.

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