Copyright PSC
2000781K MkII MLB Operating Procedure
Page 63 of 68
16.
Caution - Unintended Electrical Paths
By modifying the Power and Measurement Circuits, several customers have caused anomalies while operating
the Lightband. They created unintended electrical paths that bypassed limit switches and/or shorted motors. This
in-turn permanently damaged, or unexpectedly deployed, the Lightband. The customer shall be extremely
confident in any changes to the Power and Measurement Circuits prior to operating a Lightband. The most
common causes for these unintended electrical paths include, but are not limited to:
1. During the Stow and Deploy
operations, the Lightband’s switches will change state to a temporary
intermediate position as shown in Figure 13-1.
2. The Lightband motors share a common pin 9 that can connect them under certain instances.
3. Most oscilloscopes have a common ground. Therefore, all
‘negative’ probes are connected to each other.
Consider this when connecting the voltage probes. Reminder PSC strongly recommends using an
oscilloscope with isolated channels.
4. The user changed the voltage probe connection pins during Set-For-Flight from those recommended by PSC
and inadvertently bypassed the limit switches, Deploying the Lightband.
5. The user connected the oscilloscope ground to earth ground and inadvertently bypassed the limit switches.
The Lightband thus Deployed instead of the expected Set-For-Flight.
6. The user chose to measure current not by current probes but by measuring voltage drop across a resistor and
inadvertently bypassed a limit switch.
The figure below shows a Set-For-Flight operation performed using an oscilloscope with common probe grounds.
Current can bypass the first open switch and run through the voltage probes to the second Stow switch. The
resulting voltage and current profiles will make it appear as if both switches are synced even though they may not
actually be. Although this is not detrimental to the Lightband, it is an example of unintended electrical paths.
In the circuit below the Stow A limit switch changes to NC, but power can still flow from pin 4 back to pin 8 and
through the Stow B limit switch to pin 9 until the Stow B switch changes to NC.
Figure 16-1: Set-For-Flight oscilloscope common probe warning
