Last year's design was rather complicated so this year I thought I'd present a simpler one for those who want to charge an electric lamp without knowing the slightest about electronics. The 'circuit' is shown in Fig. 1 (well, I said it was simple).

Fig. 1.

It is not constant current or constant voltage but it will do the job. Resistor R1, which will get hot, can be made from a length of electric fire element (try about 50cm) wound round the old former. The current is adjusted by moving the crocodile clip along the element, and should be about 2 Amps for a NiFe or NiCad battery, or about 0.5 Amp for an Oldham or similar battery. Further charging details were given in last years journal. The current should be checked and adjusted frequently since it varies with the state of charge of the battery. For the technically minded, R1 will need to be between 5 and 15 ohm and should be rated at about 15W.

The next stage in civilisation is shown in Fig. 2. Current is limited by R1 which should be about 1 ohm, 5 W. It is wise to include an ammeter since the current depends heavily on the sort of battery being charged. The output must not be shorted or the diodes will be damaged. The current is governed, approximately, by the equation: I_mean = [we'll need a graphic here...]

Where R is the sum of R1 and the other circuit resistances (usually negligible), U_b is the battery voltage plus the drop across the diodes, (about 1V per diode at the currents involved here), and U_t is the transformer RMS voltage. The power dissipated in the resistor is I²R. The mean current should be close to (and not greater than) the rated current for the transformer or the equation will not apply.

It is possible to replace two of the diodes with thyristors as shown in Fig. 3. Should the output be shorted or the battery be connected in reverse the thyristors will not turn on, and the circuit and battery will be protected against damage.

STOP PRESS - IMPORTANT Figure 3, page 46

Apart from the fact that the thyristors have been drawn back to front, the triggering circuit is missing! Anyone in dire need of the completed circuit should contact David Gibson. Meanwhile, don't miss next year's exciting instalment....

It is possible to go on for ever like this (and I probably will). In the next journal you can look forward to the results of my experiments with jelled electrolyte batteries, and a cigarette packet sized charger that will do the job in half an hour. (I am also working on a nuclear power station that fits in a matchbox and .....)

References:
1 Cambridge Underground 1979 pp 56-61 2 Cambridge Underground 1979 pp 51-55