forward drop over the diode. This gave me
35 mV when trickling and 00.6 mV when
no battery was connected. I turned the
gain down on the differential amps so that
the amplified voltage in the off state
would be less than the forward voltage
drop on the green section of the bi-color
LED.
Don’t have Versapaks? The charger
circuit in Figure 2 can be used to charge any NiCd or NiMH
3.6V (three cell) battery pack without modifications, or any
two cell pack (which I’m not sure anyone makes). It won’t
work beyond that on either end without changing the
voltage sense divider. The voltage divider for the smart
charger chips has to have the sense voltage in a certain
range (1.0V to 2.0V for the chips I use). It is also more
sensitive and works better if you use the high end of the
range (mine is set for ~1.85V). Commercial “universal”
NiCd/NiMH chargers you can purchase work from 7.2-12V,
so they work on the same principle.
If you modify the circuit in Figure 2 so that the voltage
divider formed by R1/R2 (and R11/R13) gives you around
1.85V when your battery packs are fully charged, you can
use the circuit to charge any NiCd or NiMH battery pack up
to 18V. This is the most the MC33340DG will handle. Of
course, you could isolate the MC33340DG and the LM224
behind voltage regulators and use high voltage adjustable
regulators (like the Texas Instruments TL738) then go quite
high with the voltage, but that is beyond the scope of this
article. I used the toner transfer method to make the board
shown in Figure 3. I can reliably make boards with 7 mil
traces and 10 mil spacing, though, truth be told, the toner
does spread out a bit in the process, so the traces are
slightly wider and the gaps narrower. I’m more comfortable
using a 15 mil trace clearance, but in order to get this thing
to route on the tiny board I had to bump the clearance
down a bit.
I have had a few of these boards professionally
manufactured, if any of you want to build your own
Versapak smart chargers without making your own board.
They would be available for a few dollars (cost would
depend on order size). If you are interested, just send me
an email at nutsandvolts@chebacco.com to ask about
them. A few words about working with surface-mount
components:
2
FIGURE 4.
• Use lots of flux. I like to coat the bare board with
flux, let it dry, then give it another coat just before I
solder on the components. Surface tension is your
friend. I use a rosin core flux pin exclusively.
• Use magnification and lots of light.
• A temperature-controlled soldering iron is best (set to
350°C), but you can get away with a small 15W iron
with a fine tip.
Parts List
Sacrificial (but to be greatly improved) stock Versapak charger
power supply: 12VDC @1A.
Components:
QTY DESCRIPTION
2 MC33340DG (smart charger chips in SOIC- 8 footprint)
1 LM224D (quad op-amp in SOIC- 16 footprint)
2 LM317LIPKG3 (any LM317 clone in the SOT-89
footprint should work)
LM317MDTX (any LM317 in DPAK footprint should
work)
2 TLUV5300 bi-color LED
Resistors, 2010 surface-mount
2 3.1 ohm
• A 1206 form factor is about as small as you want to
go; this is about the size of a match head. With the
proper tools and good magnification, you can work
with considerably smaller SMT components. I have
worked with 0603, which is about the size of two
grains of salt. Others work with even smaller parts.
However, the 1206 form factor is large enough to
have a decent power rating, heavy enough to not
stick to the tweezers, and almost solder-able without
magnification. Plus, it’s about the same price as the
smaller parts.
Resistors, 1206 surface-mount, 5% okay:
8 1M
12 10K
2 7.5K
2 2K
2 1K
2 510 ohms
2 33 ohms
2 1 ohms
2 0 ohms
Misc.:
Caps, 1206 surface-mount
2 10 µF
1 100 µF
Diodes:
2 CD214A-B160
2 FDLL4148_Q
50 SERVO 09.2010