■ FIGURE 6. An inside
view of the enclosure
with the printed circuit
about 65% at a 12 volt input, but is about 80% at a
10 volt input. Below 10 volts, the output drops below
18 volts. The reason for the early drop in efficiency is that
the lamp — being current-source driven — uses a constant
2. 6 watts at 18 volts. The extra voltage — 22. 5–18 or
4. 5 volts ( 68 watts) — is lost, primarily being dissipated in
the current-source MOSFET.
The doubler was an interesting diversion but I ended
up using an LM2577 based step-up boost converter
Jameco.com part #2159349 DC-DC converter; $7.95).
This type of voltage converter has the advantage of
working over a wide battery voltage range: - 3. 7 to 16 volts
at better than 75% efficiency. (NOTE: You’ll find voltage
converters on eBay that can operate constant current/
constant voltage. These all seem to be ‘buck’ converters
which are used to drop the input voltage, for example,
12 volts in and five volts out.)
The current source and battery low voltage detector
schematic is shown in Figure 4. At the far left is a voltage
reference based on the LM185Z-1.2. This is an
inexpensive band-gap voltage regulator diode that
maintains a constant 1.235 volts. It provides the reference
for both the current source and the battery low voltage
detector. (A 5.1 volt zener diode could also be used, but it
requires additional precision resistors and will have some
loss in accuracy and stability.)
The current source consists of U1B/Q1 in a standard
configuration. Variable resistor R2 is the dimmer control
and at the maximum clockwise position, the full 1.235
volts is applied to the + input of U1B (assume for the
moment that Q2 is in the off state, so R11 does nothing).
The output of U1B drives the gate of Q1 which is working
as a common drain (like an emitter follower). The voltage
across R5 is fed back to the – input of U1B as negative
feedback. This forces the voltage across R5 to be the same
as the voltage at the + input of U1B or, in the case of
maximum brightness, at 1.235 volts.
The current through R5 is therefore 1.235/8.2 =
150 mA, which must also be the drain current.
The 12 volt zener diode (any value from six volts to
15 volts is okay) from the Q1 gate to ground prevents the
gate voltage from exceeding the gate’s maximum rating of
20 volts. This could happen when the lamp load is not
connected since the output of U1B would rise to near the
20 volt supply as the op-amp desperately tries to get the
150 mA flowing through a now open circuit. If the 20 volt
supply happens to be set above about 22 volts, the gate
would exceed 20 volts without the zener protection.
Most rechargeable batteries don’t like to be heavily
discharged, so I use the U1A op-amp circuit to monitor
the battery voltage. U1A serves as a comparator with the
1.235 volt reference as one input and the R7-R8 divider
providing the second input. If the battery voltage falls
below the divider setting, the output of U1A will go high;
Q2 will turn on, thereby grounding the Q2 end of R11.
R11 together with R3 reduces the lamp current to about
one third. This should be enough dimming to warn the
user of the low battery condition. R11 could be changed
to zero ohms for a complete turn-off, but I don’t think it’s
a good idea to turn off the light during a power failure.
R9 provides some hysteresis to the U1A comparator.
The safe low voltage trip point depends on the battery
type. Li-ion batteries are somewhat delicate and a 12 volt
battery should not discharge below about 8. 5 volts. In
fact, the DC-12680 has an internal circuit that disconnects
the battery from the load at 8. 5 volts. I set my trip point at
9.0 volts, so the lamp will continue working at reduced
output for a while. Lead acid batteries are more forgiving
but should not be discharged to below about 11 volts.
Ni-Cad batteries should not be discharged to below
1.1 volts per cell. Check with your battery manufacturer
for their recommended trip voltages.
Like many electronic experimenters, I use a universal
solderless breadboard while designing. RadioShack has a
matching printed circuit board (PCB) with a buss across
the top and bottom, and 47 10-hole columns arranged
May 2013 33