Hands On Radio
Experiment 121 through 130
Supplemental files and discussion for the experiments.
John, WB4LNM recommends a modification of the Figure 1c in January's issue. Instead of using the shunt diode to blow the fuse if the battery is reversed, I use a hefty Zener diode with a breakdown voltage just at or below the maximum rated input of the equipment. This approach also avoids the voltage drop problem you mentioned with series diodes.
Something like the 1N5353 is a good choice. It breaks down at about 16 volts, and costs just half a dollar or so in single piece quantity. With this in place, it will protect both reverse polarity hookups (blows the fuse) plus provides transient protection for short duration transients. I have used this for many years, and it seems to have worked quite well (no equipment failures from these mechanisms). I don't claim any originality on this approach - I'd seen it used long before I started designing equipment back in the 1970's.
The Excel 97-2003 spreadsheet for creating charts of digitized voltage measurements is available here.
Barry Shackleford W6YE points out that if you do decide to wind your own power resistor, the resistor can be made reasonably non-inductive by folding the length of wire in half and then winding the pair of wires after securing the middle as the starting point. The wires have to be insulated (enameled wire will do) and both terminals of the resistor will be at one end of the resistor.
Bob, W2DSB notes that "Sanyo makes an “eneloop” AA cell and claims 70% charge retention after 5 years or 30% discharge over 5 years. I figure this at 0.1%/month. www.eneloop.info/eneloop-products/eneloop-batteries/eneloop.html. Duracell also makes a low discharge series called “StayCharged" but does not specify self-discharge rate." Bruce KF6RAI also mentioned the eneloop cells and recommended the Sanyo AA NiMH superlattice cells and suggested thunderpowerrc.com for high-capacity LiPo cells and packs used for RC operation with very high discharge rates. He also cautions that LiPo cell packs MUST be charged,discharged, and handled properly to prevent dangerous results!
The full-resolution SimSmith screen shot of Figure 2 can be downloaded here.