Hi Neil
The design was perfect for the NiCad's of the 80's, why have a full wave rectifier when a diode would work just as well. A current limiting resistor is far simpler than a voltage regulator. And who needs smoothing caps, a NiCad does just a good a job of that.
But since then things have moved on a bit. Back then we were using 500mAH NiCad's, so a charger with a 50mA current would fully charge a flat battery in around 10 hours. When fully charged, the NiCad would easily dissipate the heat given off (50mA x 1.55V = 77.5 mW), so no cut-off circuit was required.
Fast forward to today and we are either using lithium based batteries with a UBEC or a NiMH battery, probably of 2000mA capacity or greater. For most of my sports models I use Panasonic Eneloop 2000mAH batteries. But the load on them is far higher than in the 80's as I have more servo's (typically 2x ailerons and flaps) and they are usually digital. And I don't recharge every session if I've only had a couple of flights. So when I come to recharge I probably need to put back 1,000 to 1,500 mAH of charge and I don't want to take all weekend charging at 50mA. But at least you shouldn't damage the battery at that charge rate, Panasonic recommends that the maximum trickle charge rate at 0.05C = 100mA for a 2000mAH Eneloop and their own chargers trickle charge at 50mA (when the fast charge is complete).
But why not invest in a modern charger? They nearly all use PWM for current control, so you still get you pulsed charging, albeit at a higher frequency. You can still slow charge if you want or if in a hurry do a quick charge. Better still you can occasionally do a full discharge - charge cycle to see their true capacity which is a good indication if they're still up to the job. And they're not much more expensive to buy than building your own geriatric charger© (I think I should Copyright that name
)
Cheers
Mike
