Re:Servo braking resistor in series with motor armature

Nope, I don't see any separate freewheel diodes in the new design either.

How come recirculating the current thru the MOSFETS "internal" diodes could
be a problem while doing it thru external diodes is OK? I don't get the
difference.

I know you said the internal diodes are slow but yet the datasheet for the
IRFP264N states that Ton for the diode is negligible and depends on the
internal inductance of the Drain and Source leads, which is 5 and 13nH.

How many drives do actually have external freewheel diodes? I thought
everybody relied on the MOSFETS internal ones.....but this whole issue goes
to show how much I actually know... ;-)

Thanks!
/Henrik.

Henrik Olsson wrote:

> Hi Jon,
> AFAIK the UHU is a "50% PWM at 0 speed" drive and it does not rely on
> software for current sensing/limiting. BUT it does not sense the braking

or

> recirculating current....bummer....
>

Right, I have the schematic of the UHU, and at least for that
model, you are quite right. I was referring to the Rutex, which
has an additional mode where problems can show up, since it
shorts the motor when at rest.

The UHU drawing I have shows no separate freewheel diodes, and
if the body diodes get turned on during braking, that could be a
problem.

Jon