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I wish to build and get 3.3V from LM7805 voltage regulator with some extra parts like resistor and maybe a zener diode.

Typical regulator circuit

How can I achieve that, is it even possible to do that?

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    \$\begingroup\$ Yes, but whyyyyyyyyyyyy? \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 15 '15 at 0:49
  • \$\begingroup\$ What level of power (or current) do you hope to get on the 3.3V side? Actually, you can plug in the numbers yourself at hyperphysics.phy-astr.gsu.edu/hbase/electronic/zenereg.html \$\endgroup\$ – Fizz Oct 15 '15 at 1:01
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    \$\begingroup\$ I wish to use that 7805 and to get 3.3V out approx 250-400mA \$\endgroup\$ – beic Oct 15 '15 at 1:16
  • \$\begingroup\$ Just daisy-chain a LM3940 off of Vcc, add another 47uF and 0.1uF, and done. \$\endgroup\$ – rdtsc Oct 15 '15 at 1:46
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    \$\begingroup\$ @beic there is no more simple solution than using what you already have on hand... \$\endgroup\$ – Passerby Oct 15 '15 at 4:07
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What you want is probably something like this (3.3 ohm resistor, 1N4728A Zener):

enter image description here enter image description here

I used two loads 8 and 16 ohms to simulate approximately the current range you want. Beware that as the load current decreases, the power dissipation on the Zener increases.

Regulation is pretty "meh" as you can see.

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    \$\begingroup\$ More people need to specify technical terms in ranges of "meh" \$\endgroup\$ – Asmyldof Oct 15 '15 at 7:07
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    \$\begingroup\$ Given that the question didn't even consider/ask about regulation figures... \$\endgroup\$ – Fizz Oct 15 '15 at 7:21
  • \$\begingroup\$ In general, high currents and Zener don't play well together. For comparison, here's a simulation of a setup more similar to something I've used in the past: imgur.com/wajTWn2 Lower currents, flatter/better regulation. \$\endgroup\$ – Fizz Oct 15 '15 at 7:34
  • \$\begingroup\$ @RespawnedFluff, thank you for the simulation, I will try your solution and report back with results, for the rest of the users I would like to thank you for neg. reps and for down voting my question! (what a childish thing) \$\endgroup\$ – beic Oct 15 '15 at 11:49
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    \$\begingroup\$ @RespawnedFluff but if I replace 1N4728A with 1N5913B, then it would be safe? \$\endgroup\$ – beic Oct 15 '15 at 13:10
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If you're trying to do this you might as well build your own from scratch:
credits to www.google.com
Schematic

Alternatively, just put a 6Ω 3W resistor in series with the 5V output. That will keep it around 3.3V for a steady current like you described. Don't forget you can always parallel cheaper ones.

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  • \$\begingroup\$ He didn't say he wanted a pass transistor. \$\endgroup\$ – Fizz Oct 15 '15 at 1:11
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    \$\begingroup\$ No, but he did say he could use extra parts. \$\endgroup\$ – Daniel Oct 15 '15 at 1:12
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    \$\begingroup\$ Like one of the many 3.3V LDOs? \$\endgroup\$ – Fizz Oct 15 '15 at 1:14
  • \$\begingroup\$ Well that's obviously an option, but he's looking to build something from scratch it looks like. \$\endgroup\$ – Daniel Oct 15 '15 at 1:15
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    \$\begingroup\$ No zener, just the resistor to drop the voltage. If it's a constant current, you can just tweak the R value until it's 3.3V \$\endgroup\$ – Daniel Oct 15 '15 at 1:44

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