Re: Smoking Allegro and blanking time

--- In CAD_CAM_EDM_DRO@y..., Chris Stratton <stratton@m...> wrote:

> > To prevent false resetting of the flipflop,
> > due to switching transients and noise,
> > a blanking time for the comparator can be
> > set by the user where tB _ 3600 x C in the
> > slow-decay mode or tB _ 2400 x C in the
> > fast-decay mode. For C between 100 pF
> > and 1000 pF, tB is in ms."
> > Ok so I read this and thought false resets would mean missed

steps

> > so I better try this. I plugged in 3.6k and 470pf and damn if it
> > didn't ruin my whole day when things went up in smoke. So I think

I

> > am learning from this that off time and blanking time are 2

different

> > things, and this was just the calculation, not what the actual

value

> > for the resistor should be. But I am still confused as to where

to set

> > the RC values for best performance, slow decay or fast, and no

false

> > resetting of the flip flop(I think that would be a bad thing).
>
> When the chopper turns on the transistors, there is usually a

voltage

> spike on the sense resistor. The comparator can see this and think

it

> means that the current has ramped up to the desired value, so it

shuts

> the transistor right back off again. Blanking time means that the
> voltage on the sense resistor will be ingnored for a brief amount of
> time during which you expect any spikes to die out. But since the
> comparator can't turn the transistor off again during the blanking
> time, no matter how high the current, it also sets a minimum on

time.

> If your blanking time was too long for your supply voltage and

winding

> inductance, then you could very easily put damaging currents through
> the motors. If you want to experiment with this, you will probably
> need a scope to see what is going on and some way to limit currents

to

> safe levels: a power supply with current limit perhaps, or simply
> lower the supply voltage until it is say 150% of the rated winding
> voltage, or maybe even fuse the supply or output.
>
> The calculation for blanking time isn't too clear about units... one
> could easily make a mistake there, which is why being able to check
> with the scope is nice.
>
> Sorry I can't help with the decay mode decision.
>
> Chris
>
> --
> Christopher C. Stratton, stratton@m...
> Instrument Maker, Horn Player & Engineer
> 22 Adrian Street, Somerville, MA 02143
> http://www.mdc.net/~stratton
> NEW PHONE NUMBER: (617) 628-1062 home, 253-2606 MIT

Thanks Chris, this is making more sense.
Like you say the calculation for blanking time is not too clear, and
even if it was I would not know what an acceptable blanking time
should be. I do not have a scope, this is just a low budget hobby for
me. Even if I did I am not sure I would know what to look for.
I was looking at the documentation of the Allegro 2962, wich is a
pair of 2961's packaged together. This has no user adjustable RC
components for blanking and off time, it is all preset. This kind of
tells me that there is an RC setting that will work for most
applications. Also it only chops the source driver, slow decay mode.
I could not use the 2962 because it is only rated to 3 amps.
Chris, those are good suggestions on limiting supply voltage for
testing. Hope I did not damage my motor, it is a spare however. It
did blow a 5 amp fuse when the chips fried. I probably should go down
to a smaller fuse though.
Does anybody have any suggestions as to how to set this up without
a scope, or know enough to calculate for me some RC combinations that
will work?
Lonnie