E - 12 Desktop trickle charger June 2003 IPN: M5000-00-105
Desktop trickle charger
The Tait Orca desktop trickle charger can
charge a 1100 mAh or 1500 mAh NiCd battery
overnight. It is designed to provide approxi-
mately 1450 mAh of charge in a 16 hour
period. Thus both 1100 mAh or 1500 mAh
NiCd batteries can be charged. Simple protec-
tion of the radio is provided in the form of an
open circuit voltage limit as well as short
circuit protection.
Figure E-5 shows the charger current profile.
Figure E-6 shows the circuit diagram for the
trickle charger.
Trickle charger operation
When the battery voltage is above approxi-
mately 8 V, the charge current is inversely
proportional to the battery voltage. This
characteristic is produced by Q100, Q101 and
Q102. The charge current is determined by the
current through R104, which is set by Q102, its
emitter resistors and the reference voltage. The
slope of the curve is determined by Q101 and
R106. The reference voltage is provided by an
8.2 V Zener diode (D100).
When the battery voltage is below approxi-
mately 8 V, the charge current is proportional
to the battery voltage. This is accomplished by
using Q103 to change the reference voltage in
proportion to the battery voltage. This chang-
es the current through R104, which changes
the charge current, as desired.
The maximum voltage available from the trick-
le charger is limited to less than 10.5 V by
R111, in conjunction with Q101, so that the
radio can not be damaged if the battery goes
open circuit. If the battery goes short circuit,
then Q102 is held off by Q103 and thus Q100
is off, and there is negligible charge current.
On startup, the state of the charger is similar to
that when the charger output is shorted. When
power is applied, V
IN starts to rise and the
emitter voltage of Q103 rises. However, the
base of Q103 is still at zero volts, so Q103 starts
to turn on. When Q103 is on, it will maintain
Q102 off and hence Q100 will also stay off.
Thus when V
IN has risen to its final value, the
circuit is in an off state, giving negligible
output voltage and charge current.
In order to activate the circuit, a minimum
voltage of approximately 2.6 V (a battery)
must be connected to the circuit to charge
C101 and turn Q103 off, thus turning on the
charger.
The LED is on whenever there is sufficient
charge current. Its brightness is proportional
to the charge current profile, and its turn-on
and turn-off thresholds are determined by
R105. Thus the LED is on under normal charg-
ing, dims when the battery approaches full
charge and is off under fault/no charge condi-
ions.
Figure E-5: Tait Orca desktop trickle charger current
charge profile
01
10
20
30
40
50
60
70
80
90
100
110
234567891011
Battery voltage (V)
Charge current (mA)
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