Can I use 220-12v,1.5 Amps Stepdown transformer along with 12V 2A (KBP200) bridge rectifier to build a charger for 12v, 7.5AH battery or I should use transformer with Higher output, maybe 15V

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    $\begingroup$ What type of battery are you planning charging? Is this a Lead Acid battery. If this is a Lead acid battery then the 15V might work better. What is your peak voltage? What is the voltage drop across the bridge rectifier. $\endgroup$ Commented Feb 23, 2021 at 13:05

1 Answer 1


You will need more than a 220V:12V transformer.

Fullwave bridge rectifier output:

$$ V_{DC} = 0.637\ V_{MAX} = 0.9\ V_{RMS}$$

This is the generic formula (textbooks), which gives you 10.8V. Higher current may cause < 10.8V. Lower current > 10.8V. As in: the actual voltage depends on the load current.

From Full Wave Rectifier.

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The peak voltage of a bridge rectifier has two diode losses (Forward voltage of diode depends on current).

If you do the ideal math:

$$V_{MAX} = V_{RMS} \ \sqrt {2} - 2 V_{F\ diode} = 12V \times \sqrt {2} - 2 \times 0.7V = 15.6V$$

which seems enough to make a 12V charger, but that is peak voltage (top of ripple), while actual voltage depends upon load current. Essentially if there is no load current, then maximum DC voltage is 15.6V.


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