Re: [CAD_CAM_EDM_DRO] Smoking Allegro and blanking time

> 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@...
Instrument Maker, Horn Player & Engineer
22 Adrian Street, Somerville, MA 02143
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