Re: Would this work? ( remote sense power supply)

On Sun, 10 Aug 2008 12:38:30 GMT, Hammy <spamme@xxxxxxxxxxx> wrote:


The signal latch around U22A is triggered by either the input( ?? at
J2 ) or an output overvoltage. It is reset when both input terminals
are opened. The latched signal goes elsewhere, so effectiveness in
protection or ease of reset can't be assessed. The soft shutdown of
the regulator through pin#1, in the event of an overvoltage situation
caused by reglator failure, can obviously not fly.

I'm not sure I follow you here. I'm guessing you mean that the
protection shutdown wouldn't operate if the LP2975 were to fail this
is the most unlikely scenario. Designing protection for every possible
component failure possible would be a bit excessive. Instead the most
likely would be failure of the pass FET.

Would a soft shutdown of U2 provide overvoltage protection in that
event?

There are basically two options in overvoltage protection:
- employ an independant series switch to withdraw power from the
regulator. With redundant input power, soft or hard inhibition of both
would be neccessary.
- employ an independant shunt switch to crowbar the point of
overvoltage. This relies on the idependence/functionality of current
limiters, or the presence of fusible series elements. If the voltage
regulator was also responsible for current limiting, it cannot serve
this purpose.

It's funny how discussions on the likelihood of one failure mode
occuring vs another seem to take place only after a selected method of
protection is identified as being ineffective. I'd take this a few
steps farther back: Is the load threatened by unrecoverable damage in
the presence of the fault? If not, don't bother. If so, then effective
protection may be economically justified. If justified, then reliable
protection is warranted regardless of the failure mode. Waffling over
the issue is probably a waste of time.

One thing that an OVP latch will benefit from is a simple method of
reset - useful in testing and in troubleshooting. This may as simple
as two pads that are easily shorted by a screw driver.

The two parallel output lines are one more than the remote sensing
method previously discussed could handle.

And I did test the remote sense as I mentioned earlier,by placing a
4.7 ohm resistor in the positive sense lead and a 4.7ohm resistor in
the IC ground lead. I tied the IC ground to the load being sensed and
yes it does work. The reason I elected not to use it is because it
would require another wire.

An admirable first step. Don't you think there may be other
characteristics exhibited by a 200ft-long wire harness that might
warrant further investigation? If the hardware that was proven by the
previous circuit incarnation is still present, it could be a simple
matter to use it as a breadboard element.

Further description:

J4 is for protection circuits I have on the flyback (thermal and OVP)
if they activate they'll trigger U22A shutting everything down.

The second TLVH431 is just being used as an inexpensive comparator,
this keeps U16 (FET) blocking, until the LDO's output cap charges up.
I found this was necessary to guarantee predictable operation of the
SSR which is fed through J5. A signal already exists, so I used an SMD
2n3906 to disconnect U14 once it's served its purpose.

If the signal at 'delay' can be used to disconnect the loads, it may
be a suitable series element for output OVP protection.

U12 blocks the loads until the flyback takes over. I don't want the
battery supplying the loads so a cheap Si1302DL (90mV worst case drop)
is used to block the loads and an existing signal is used to gate it
at the appropriate time.

The time that the battery actually supplies this circuit including
50mA to an SSR is about 40ms.

In most redundant source systems, there comes a time when you have to
ask yourself 'Who's on first?'. There may be more than one answer, but
all must be anticipated.

If you're attempting to imhibit nodes in a protection scheme, you
don't want your protection circuitry to be dependant on those nodes
for continuous maintenance of inhibition. Hence the next question -
'Who's off last?'.

RL
.

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