I have 1 circuit composed from 3 little very simple circuits I want to power them all from a 5V@1A power source.
All of these 3 circuits must be powered at different voltages.
Their current draw I measured for each of them.

A=4V @10mA, B=3.4V @1.6mA, C= 5V @24mA

What can you suggest?

Thank you.

(I re-edited)

  • \$\begingroup\$ If they all take identical currents you can put them in series and feed the composite from 12 volts. \$\endgroup\$
    – Andy aka
    Apr 24, 2020 at 18:06
  • \$\begingroup\$ You’re kidding, right? \$\endgroup\$ Apr 24, 2020 at 18:16
  • 1
    \$\begingroup\$ So, you want to supply circuit A, B and C using the same 5V power supply? Please hit the edit link and also state the required currents. \$\endgroup\$
    – Huisman
    Apr 24, 2020 at 18:16
  • \$\begingroup\$ @teodoric8. - Welcome :-) (a) Just FYI your comment above to Huisman didn't notify them, as there was a space between the @ and the username. To work correctly, the username (with no spaces, if multiple words) must immediately follow the @. On a desktop browser, you will be offered a choice of usernames after typing @ and the first letter. (b) Are you really sure that you need those specific voltages? They are unusual and might point to an X-Y problem, but without a schematic and more details, we can't validate for you. Please add a schematic, if you want readers to check. Thanks. \$\endgroup\$
    – SamGibson
    Apr 24, 2020 at 19:02
  • 1
    \$\begingroup\$ For 10mA and 1.6mA, Zeners are an option. \$\endgroup\$ Apr 24, 2020 at 22:26

1 Answer 1


For the 5V, connect directly.

For 3V and 4V, a linear LDO regulator for each one. Assuming they’re low current, linear would be fine.

The LDO (low drop-out) part is important. The regulator 'overhead' (or drop-out) voltage, needs to be less than the required Vin - Vout, with your worst case (lowest) Vin.

Why LDOs?

tl; dr version: the required overhead (drop-out) between Vin (5V) and the two Vout (4V, 3V) is too low to be satisfied by a non-LDO regulator like an LM317.


Assuming +/-10% on your 5V supply, your lowest Vin will be 4.5V. Then, the maximum overhead allowed for each of your supplies is:

  • 4V supply: 4.5V - 4V = 0.5V
  • 3V (3.3?) supply: 4.5V - 3V = 1.5V

Neither of these can be satisfied by a non LDO regulator like the LM317, which has an overhead (drop-out) voltage of 3V. That won't work for this setup.

So you need LDOs. These have overhead (drop-out) of about 150 - 200mV, lower than your required overhead. More about those here: https://www.analog.com/en/analog-dialogue/articles/understand-ldo-concepts.html


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