Strange behavior when machine is off

[jram]

Not completely sure that this should be in troubleshooting but really don't know where else to post this. I was installing a fan shroud last night with the machine turned off.When I moved my effector plate down by moving my arms down the extrusions, the fan in the PSU would spin slowly and an LED light on one of my 80mm cooling fans would come on. When I stopped moving the arms, the light would go off and the fan would stop. I have an EVGA 650 watt PSU mounted outside the case right now. The fans are wired 3 in parallel coming from a SATA plug. on the back of the PSU .. Any ideas as to what may be causing this?

[jdurand]

The motors are acting as generators. On our clothes dryer spinning the drum with it "off" will turn it on, no need for the power button.

This is used for braking motors, short the motor windings and then try to turn it!

[jram]

Thanks jdurand, I had no idea that could even happen.

[mhackney]

You do have to be careful as you can blow the stepper drivers on your board (RAMBo). Try to minimize moving the steppers like that!

[Jimustanguitar]

I hear that it's a lot bigger deal on machines with non-ATX power supplies (probably the digital switch mode ones).

When an ATX supply is off, there's still a load across the output leads (the "internal resistance" of the 12v rail in the power supply itself). When you turn off other power supplies, it's an open circuit between + and - and the potential that you generate by spinning the motors has nowhere to go and the motor drivers pop first.

[Mac The Knife]

I hear that it's a lot bigger deal on machines with non-ATX power supplies (probably the digital switch mode ones).

When an ATX supply is off, there's still a load across the output leads (the "internal resistance" of the 12v rail in the power supply itself). When you turn off other power supplies, it's an open circuit between + and - and the potential that you generate by spinning the motors has nowhere to go and the motor drivers pop first.

I haven't had that problem with the switching power supply I installed, when moving the effector around, the motors just charge up the capacitors in the psu. Here's the one I'm using: http://www.amazon.com/Universal-Regulat ... wer+supply