Page 57 - Robots Androids and Animatrons : 12 Incredible Projects You Can Build

to the calculated 6.25 ohms. This standard value is close enough to the
calculated value for proper operation. The C/30 resistor is R3 on the
schematic. Again the standard value of 10 ohms is close enough to the
calculated value for proper operation. Later on we will see that it’s
possible to fast-charge the batteries because of the voltage-sensing
capacity of the circuit.
V1 is a 5K-ohm potentiometer. It is set to trigger the SCR when the
NiCd batteries are fully charged. The SCR, once triggered, allows
current to flow through a DPDT relay.

When power is applied to the circuit, current flows through the
LM317 charging the batteries at a C/10 rate. Resistor R3 is shorted
by one-half of the DPDT relay. Current also flows through resistor
R1, which is a current-limiting resistor for light-emitting diodes
(LEDs) D1 and D2. Upon power-up, the red LED D1 will be lit. The
red LED indicates that the circuit is charging.

As the batteries charge, the voltage drop across V1 becomes
greater. After about 14 h, the voltage drop across V1 is great enough
to trigger the SCR. When the SCR is triggered, current flows
through the coil of the DPDT relay. The relay switches, causing
the red LED to go out and the green LED to turn on. The green
36 LED signals that the batteries are fully charged. The other half of
the relay switches, opening up the short on resistor R3. With R3
now in the current path, the current flowing to the NiCd batteries
is cut to a C/30 level. Diode D3 prevents any current from the bat-
teries flowing back into the circuit.
Determining the trigger voltage from V1
For the circuit to function properly, the SCR must turn on when the
NiCd batteries are fully charged. The easiest (best) way to do this
is to place depleted batteries in the charger, charge the batteries
for 14 h, and then adjust V1. When the batteries are fully charged,
slowly turn V1 until the relay clicks and the green LED turns on.
Design notes
When building a charger for your application, keep these points in
mind. The main considerations are choosing the C/10 and C/30
current-limiting resistors. Use the given formulas for selecting
these values. Current-limiting resistors should be rated around 2 W.
If the charging current is high (greater than 250 mA), heat-sink
the LM317. If the charger is switched on without the NiCd batter-
ies being connected, the relay will switch immediately, turning on
the green LED and providing a C/30 current.

Team LRN
Chapter three