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I need to reduce the rectified DC voltage. Is it possible to reduce the RMS value after the bridge rectifier i.e before the capacitor? Practical value (at the capacitor) is 34 DC voltage. I need to reduce this voltage.

Input is 24 VAC, output should be at least 26 V DC. Input voltage is fixed.

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  • \$\begingroup\$ For how much current? \$\endgroup\$ – Rohat Kılıç Sep 11 '18 at 7:09
  • \$\begingroup\$ 1.5 ampere max.(but practically am drawing around 800 milli amps)sir \$\endgroup\$ – Ali Lightwala Sep 11 '18 at 7:13
  • \$\begingroup\$ Since the required current is relatively high and the voltage difference is enough, I would suggest using a DC/DC step-down converter. \$\endgroup\$ – Rohat Kılıç Sep 11 '18 at 7:14
  • \$\begingroup\$ right sir.but i will be drawing only 800 m amps . i tried with L.M.2596(DC TO DC BUCK CONVERTER) but the problem is there is too much heating sensation across the ic \$\endgroup\$ – Ali Lightwala Sep 11 '18 at 7:19
  • \$\begingroup\$ The switch element in LM2596 is a BJT. That's why there's too much heat. Also please note that LM2596 requires a big copper area as heatsin. It would be better if you use an IC with internal MOSFET (like L5973 or TPS54340). \$\endgroup\$ – Rohat Kılıç Sep 11 '18 at 7:58
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You could simply add a couple of diodes to drop the voltage:

schematic

simulate this circuit – Schematic created using CircuitLab

each diode will drop about 1 V, you can add more if needed.

The "proper" way to do this is of course to use a transformer with the right output voltage. This diode trick is a bit of a "hack" but that might be all you need.

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  • \$\begingroup\$ Would a Z-Diode to GND also be working? \$\endgroup\$ – Alexander von Wernherr Sep 11 '18 at 7:59
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    \$\begingroup\$ @AlexandervonWernherr Only a zener diode: no. Imagine if the AC voltage from the left can deliver large amounts of current, then a large current will flow through the zener diode as it tries to limit the voltage. To solve this a series resistor (instead of D5, D6) would be needed. Even then the resistor and zenerdiode will burn a lot of power. It is a power inefficient solution. A zener diode in series can also work but it needs to be high power (a couple of Watt) and low voltage, these are hard to find. Using diodes is simple and easy. \$\endgroup\$ – Bimpelrekkie Sep 11 '18 at 8:09
  • \$\begingroup\$ An alternative would be a thyristor chopper circuit that would allow for adjustable output. I don't know how complicated OP is willing to go with this though. \$\endgroup\$ – alphasierra Sep 11 '18 at 8:12
  • \$\begingroup\$ thank you everyone i will try working on your points.appreciated your efforts. \$\endgroup\$ – Ali Lightwala Sep 11 '18 at 8:33

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