> Sometimes the ground lead is cut to handle difficult cases

[Chris Carlen]

> The fundamental issue is that both pieces of equipment must have a
common
> reference - made common by wiring the chassis together. The few
millivolts
> of difference and pickup along the earth wire is handled by
the differential
> system. Otherwise, I promise you 1)audio oscillations
2)hum 3)distortion
> 4)RF breakthrough, 5)Intermodulation with switching
frequencies.

> If you look at the schematic of your differential amplifier,
you see that
> the input signals are taken in and subtracted by the common
mode rejection
> action of of an amplifier which operates *with reference
to earth*.
> Alternatively, one signal is inverted by an amplifier which
operates *with
> reference to earth* then the two signals are resistively
added. My point is
> that your amplifier operates with reference to earth
right at the input,
> before the differencing occurs and thus *requires*
that the earth be shared
> with the source device. If you don't expilicitly
make that required
> connection, then you must be *hoping* that it will
come from somewhere.
> Unfortunately, it won't.

> All this was hammered
out decades ago and appears wired to XLR connectors in
> broadcast, PA,
recording and stage gear.

> 10K in parallel with 0.1uF is better than
no connection at all, but there
> has to be a reason to choose this substitute
for a direct connection. This
> network appears as a common mode return
resistance - with interesting
> possibilities for degrading common mode
rejection - depending on your
> differential amplifier configuration.
Also depending on the resistances of
> your source outputs. On a stereo
amplifier, ground impedance could be
> involved with channel crosstalk.


> Roger
How to protect H-bridge from short?

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Greetings:

I have a H-bridge PWM amp using Apex SA-60. I have implemented an
average current mode control loop, thanks to assistance from Genome,
which makes it possible for the amplifer to handle a differential short
circuit fault.

The current sensing is done through two resistors on the low sides of
the bridge, going to a differential amplifier.

Trouble is, this current sense will read zero and thus attempt to push
even more current if there is a short from one H-bridge output leg to
ground.

How can I protect from this? I had considered floating the whole power
amp system, which already has a differential input. Then tie it to
earth ground via a resistor||cap combo. This would be fine except that
if one shorts an output leg, now the diff input saturates with excess
common mode input, resulting in an indeterminate command signal to the
PWM amp.

I can't have this because it is a DC amplifier driving a mechanical
system, that if the drive pegs for instance, will result in mechanical
things crashing together.

Thus, current to the load must cease if this short condition occurs.

I am inclining back toward keeping the thing connected to earth ground,
and figuring out some straightforward way to implement short protection.

My thoughts so far:

1. simply fuse each output, after the LC filter, and putting the
voltage sense after the fuses so their resistance doesn't cause voltage
drops.

But the PWM amp might just keep chugging along if one side blows, until
it attempts to reverse the output polarity at which point it will clip.
This isn't ideal.

2. fuse the power supply. The fuse will blow cutting power to the
whole thing if either side shorts. Trouble is, can a fuse blow fast
enough? The rate of current rise is slowed a bit by the output filters,
but still it's on the order of 5us to rise 15A with my L and supply V.

3. Add an additional current sense on the B+ supply to the H-bridge,
and shut down the amp via the SA-60 DISABLE input if the average current
exceeds 15A with a time constant of something appropriate.

This is probably the best solution, and also would help me if the
amplifier is configured as a straight PWM voltage amp without current
mode, which may be desirable in some cases do to better step response
performance.

But it's quite a bit more circuitry to add too something that has
already spiralled somewhat out of control.


Other ways?


Thanks for input.


--
_____________________
Christopher R. Carlen
crobc@bogus-remove-me.sbcglobal.net
SuSE 9.1 Linux 2.6.5

 

[Chris Carlen]

OOPs, ignore

 

[Winfield Hill]

Chris Carlen wrote...
Subject:
"> Sometimes the ground lead is cut to handle difficult cases, for example

where there is an alternative unavoidable ground causing heavy circulating
currents. For a single ended source, one of the differential inputs goes to
ground at the source end of the cable.

In the professional audio scenario I am describing, 10K input resistance and
10K input to ground resistance is quite OK. Going to 1M brings no benefit."

Click to read text article (7.4K)

Chris, you forgot to preface your Subject: line with the standard phrase,
"I want to ask you the most important question of your life."


--
Thanks,
- Win