Thomas Bryhn wrote:
>>
I think the charger for a NiCd or NiMH battery pack should be a current
source, so a resistor in series would make no difference as long as the
charger has enough voltage overhead. If it doesn't have enough voltage the
charger would end up not charging at all. The correct way to modify the
current source would be to add resistance in the feedback loop, but I have
no knowledge of the actual circuit in the Olympus charger, so I can't
specify any further.
<<
I don't own either Olympus chargers but my understanding is they are very
crude and just "current limited" with a series resistor. (see also Mark
Dapoz's comments) Hence my recommendation of a crude resistor modification
within reach of hobyists. Many cheap consumer chargers don't even have a
series resistor they rely on the very thin wire used in the transformer to
act as a limiting resistor.
Even if Olympus were to use a current regulator an added resistor will
eventually limit the current when the supply runs out of compliance so a cut
and try approach adding a resistor will work if you have a few resistors
lying around and an amp meter to check current with.
If you want to make a "proper" current limited circuit use a 3 terminal
LM317T voltage regulator (available from RadioShack etc) connected as a
current source: Connect a resistor from the LM317 "vref" terminal to "OUT"
terminal. Connect the positive terminal of the battery to the Vref terminal.
Connect the unregulated positive supply to the "IN" terminal of the
regulator. The negative supply terminal goes to battery negative. This makes
a very precise and bullet proof current regulator since the device is
thermally limited internally. The resistor sets the current limit to 1.25/R
amps. For example a 120 Ohm resistor will give 10.4mA with a LM317. You need
an unregulated voltage input supply source of at least 2.9 volts more than
the fully charged battery voltage with this current regulator, since the 317
dropout voltage is about 1.6V and the ref voltage 1.25. See National
Semiconductor's website and the data sheet for the LM317 for tips on using it
as a current regulator.
Regards,
Tim Hughes
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