Servo - September 2010

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.