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| Dave J.... |
 Posted: Thu Jan 08, 2009 9:50 am |
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Guest
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In MsgID<1s38l.2319$fE1.854 at (no spam) bignews3.bellsouth.net> on Sun, 4 Jan 2009
09:07:18 -0500, in sci.electronics, 'Deleted' wrote:
[quote:8e61b5a5e4]I'm having a problem with a circuit that I did not design. The unit uses
Lithium Ion batteries (laptop battery pack) when AC is not available. The
problem is, when the unit is turned on with the batteries, there is a large
initial spike (approximately 8 amps) which the charge controller sees as a
short circuit and immediately shuts down the battery. My first thought was
to use a capacitor to send the spike to ground. Then I realized that it was
the capacitors already in the circuit that are charging up to make it look
like a short. A resistor was added to attempt to reduce the impact of the
spike but that failed.
[/quote:8e61b5a5e4]
Why not just use an emitter follower, driven from a capacitor which
charges relatively slowly? Possible failure if the destination dislikes
intermediate voltages but if you keep the time constant to something
sensible (20ms or so?) then it would rise slowly enough to avoid the spike
but quickly enough to keep the destination circuit happy.
Main snag is the .6v drop that you'd be stuck with thanks to the BE
junction.
You *could* get around the above with a second transistor but that's
starting to head in the 'sledgehammer->nut direction.
Dave Johnson. |
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| Deleted... |
| Posted: Fri Jan 09, 2009 11:16 pm |
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"Dave J." <requiem at (no spam) freeuk.com> wrote in message
news:gk50bf$aic$1 at (no spam) news.datemas.de...
[quote:46177bb28f]In MsgID<1s38l.2319$fE1.854 at (no spam) bignews3.bellsouth.net> on Sun, 4 Jan 2009
09:07:18 -0500, in sci.electronics, 'Deleted' wrote:
I'm having a problem with a circuit that I did not design. The unit uses
Lithium Ion batteries (laptop battery pack) when AC is not available. The
problem is, when the unit is turned on with the batteries, there is a
large
initial spike (approximately 8 amps) which the charge controller sees as a
short circuit and immediately shuts down the battery. My first thought
was
to use a capacitor to send the spike to ground. Then I realized that it
was
the capacitors already in the circuit that are charging up to make it look
like a short. A resistor was added to attempt to reduce the impact of the
spike but that failed.
Why not just use an emitter follower, driven from a capacitor which
charges relatively slowly? Possible failure if the destination dislikes
intermediate voltages but if you keep the time constant to something
sensible (20ms or so?) then it would rise slowly enough to avoid the spike
but quickly enough to keep the destination circuit happy.
Main snag is the .6v drop that you'd be stuck with thanks to the BE
junction.
You *could* get around the above with a second transistor but that's
starting to head in the 'sledgehammer->nut direction.
Dave Johnson.
[/quote:46177bb28f]
Thanks for your response. I didn't want to go nuts with this design change.
Adding transistors is like redesigning the charge controller and I can't do
that.
Interestingly, my supply guys accidently gave me a brand new battery with a
larger capacity and it had exactly the same modification to it that we have
been using - that is, separate the load from the controller and tap power
directly from the batteries. Unfortunately, said battery is physically too
large for this particular project. I guess the manufacturer realized they
had a problem with their controller but chose not to upgrade this model of
battery for some reason. I don't speak Chinese, so I can't call them with
any questions...
Bart |
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