Servo - September 2010

• Put everything on one face of the board before you try to solder the parts. Just get them on top of the pads and oriented close to correctly; you can fine-tune as you solder. The wet flux will hold things in place until you are ready to start soldering.

• I hold the tweezers in my left hand in such a way that the little finger of my hand is resting on the work surface. I then move the components around by moving my arm, not my fingers. (This is how artists who work on master dies for coins do it.)

• The method for SMT soldering I’ve found recommended on several websites — “Put some solder on the pad, hold the component in place, then melt the solder” — works okay, but I prefer the following. I just put a tiny dab of solder on the end of my iron, hold the component in place with the tweezers, and touch the iron to the joint between the component and the pad. Surface tension does the rest. Use a very fine lead free solder; .020” size works for me.

• Don’t worry about having a little extra solder on the component, and don’t worry about the part being perfectly lined up. Hold the iron on the part for about a half second, then pull it off and give it a second to cool before letting go with the tweezers. Solder one pad on all components, then go back and finish that side of the board. Check your work under high magnification (at least 10X), flip the board over, and repeat.

• You will need to pull the battery connectors off the old board that comes with your sacrificial charger. I’ve found that the best way to do this is to simply load up some solder wick with flux and get all the solder to wick-out. Then, gently bend up the tabs that hold the clip in place and then heat the leg again while gently pulling on the part.

• You will need a new power supply to run your fast charger; the one that comes with the Versapak chargers doesn’t put out enough voltage (or current). Anything that puts out approximately 12V at 1A should work. I found a deal on some scratch and dent linear supplies, but a switching supply would work okay as well. Most of the 12V wall wart supplies I

have tested put out more than 12V, but you should be okay with a few extra volts. I wouldn’t go much higher than around 16V.

• Once everything has been assembled and you place the new and improved board into the Versapak charger/holder (Figure 4), you can plug in the power supply.

The LEDs should flicker briefly then go out. This is an unintentional feature, but it lets you know you have hooked up the power and everything is working. So, I didn’t try to fix it. When you plug a battery in, you will get an orange light (red for fast charge plus green for trickle charge equals orange) showing that the charger is working and attempting to charge the batteries. When the batteries are almost fully charged, you will see the light switch from orange to green and then back to orange periodically. This is the smart charger chip shutting off the fast charge circuit in order to get an accurate reading on the battery voltage. Given enough time, the LEDs will stay green, indicating that the batteries have been charged and the charger has changed to trickle charge only mode.