As long as this voltage is lower than a certain threshold, determined by the setting of P1 and corresponding to a battery that is not yet fully charged, THY1 is not triggered and so leaves THY2 conducting for all the mains half-cycles. The voltage at the battery terminals is sampled by R5 and smoothed by C1 before turning on THY1 or not via P1 and D3. In that event, THY2 turns off the first time its supply voltage drops to zero, and no further current can reach the battery. This thyristor is triggered via resistor R4 for each half-cycle of the mains, except when thyristor THY1 is itself triggered. When thyristor THY2 is conducting, the battery is effectively charged, the charging current being limited only by resistor R6, which has to be calculated as shown below. Strange as it may seem, this is vital for it to work properly, because as a result the rectified voltage consists of a succession of sinewave half-cycles, and hence falls to zero 100 times per second.
The voltage supplied by our charger’s transformer is rectified by diodes D1 and D2 but is not smoothed. Let’s have a look at the circuit diagram. What’s more, it doesn’t use any ‘exotic’ components and is ridiculously cheap. So our proposed project is to build a charger that can act as both a standard charger, and a float charger that you can leave permanently connected without the slightest risk to your battery or fear of over-charging. The majority of dirt cheap ordinary chargers don’t include any regulation circuitry and so will over-charge a vehicle battery if you’re unwise enough to leave it permanently connected.
The solution of leaving an off-the-shelf charger permanently connected is not generally satisfactory, unless you’re lucky enough to have an ‘electronic’ one. Pushstarting is no longer recommended or even possible with modern cars, so a topped up battery is highly appreciated.
However, when we come to want to use the trusty vehicle again, it often happens that the battery shows serious, deeply worrying signs of being flat, sometimes to the point of preventing the engine from starting altogether. A Summer Circuits edition on ‘all things outdoors’ - good, but what of all the battery- powered circuits that remains indoors? Once the fine weather starts, the family car tends to remain increasingly in the garage – which is as beneficial to the owner, his/her bank account and the air that we breathe as it helps - to an extent to - reduce CO2 emissions.
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