Re: How do I check the MOSFET's in my stepper driver

AWEM,

Fuses will not cure every ill even if they are infinitely fast.
Here's a scenario:

An overvoltage causes a MOSFET gate oxide rupture. The MOSFET fails,
over currents and causes the infinitely fast fuse to blow. The MOSFET
failure preceeds the fuse blowing.

Mariss


--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Andrew Mawson" <andrew@m...>
wrote:

>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...>

wrote:

> > Mariss Freimanis wrote:
> >
> > >1) Set your multimeter to Ohms.
> > >2) Disconnect the motor and power supply from the drive.
> > >3) Connect one meter lead to drive power supply ground.
> > >4) Measure the resistance at each of the 4 outputs.
> > >5) Move the lead from ground to drive + supply input.
> > >6) Repeat (4)
> > >
> > >Transistors invariably fail shorted. If one or more of the 8
> > >measurements you took show a short then the drive has 1 or more
> > >damaged power transistors.
> > >
> > >
> > I haven't seen this in 80 V stepper or servo drives, but in some
> 400 V
> > inverter drives I designed some years ago (very similar full-

bridge

> switch
> > circuits, though) I have on a few occasions had blown transistors
> that were
> > in the open condition AFTER a fault. Of course, these

transistors

> were open
> > in more than the electrical sense! They sounded like a pistol
> going
> > off, indoors,
> > and glowing pieces were embedded in the walls.
> >
> > (Yes, Mariss is correct above, the transistors shorted first,

then

> were
> > exploded
> > by shorting the energy in the filter caps.)
> >
> > Jon
>
> Years back when I worked on large UPS installations in critical
> applications (999 <911?> Fire, Police, Ambulance man mobilisation)
> the expensive semiconductors were protected with germanium fuses

that

> would go o/c before the semiconductor switching element was

damaged.

> They were VASTLY expensive, and I suspect nowadays it is much

cheaper

> to let the MOSFET go pop rather than the fuse!
>
> AWEM