This is an automatic 12V battery charger circuit, they uses the power SCR and UJT are main parts.
And they will not work except the battery 12V to be charged with right
polarity, so prevent errors as well. The battery voltage controls the
charger and when the battery is fully charged, the charger will not
supply current to the battery anymore.
The battery charging current through the SCR MCR3818, when it is
triggered into the conducting state by the UJT 2N4851 relaxation
oscillator. And the oscillator is only activated when the battery
voltage is low. Vb2b1 of the UJT derive the battery voltage that to be
charged, and since Vp = Vd = Vb2b1 ; the higher Vb2b1, the higher Vp.
When Vp exceeds the breakdown voltage of the zener diode ZD1, the UJT
will stop to work and the SCR1 will not conduct.
T1 =Primary – 30 turn #22 , Secondary – 45 turns #22
R1 = Series resistance to limit current through SCR so use MCR3818 at 20 amps.
The setting VR1, we can charge the battery as needed. Electronics Circuit Application
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