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I designed a circuit that consumes about 0.7 to 0.9 amperes depending on the state of its components.

I use a 7805 regulator to feed the circuit with 5V and the regulator is connected to a 12V adapter.
The regulator has to drop about 7 volts at 0.9 amperes maximum and it means about 6.3 watts of heat generated in the regulator. Changing the adapter is not an option because this circuit is a part of another circuit that needs 12V.
My question is: Is it possible to use a resistor to drop a couple of volts before the regulator? This may split wasted power between resistor and regulator. Am I right?

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  • \$\begingroup\$ Yes, you are right. \$\endgroup\$
    – Andy aka
    May 13, 2020 at 16:41
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    \$\begingroup\$ Why not use a buck regulator instead of the 7805? \$\endgroup\$
    – Ron Beyer
    May 13, 2020 at 16:49
  • \$\begingroup\$ Is this a common way to maintain situations like this? Is there any advice? or a better way? \$\endgroup\$
    – Mehdi Maazi
    May 13, 2020 at 16:51
  • \$\begingroup\$ I have to keep my circuit as simple and cost-efficient as possible. By the way, I have a huge stack of 7805s and I have to use them too! \$\endgroup\$
    – Mehdi Maazi
    May 13, 2020 at 16:53
  • \$\begingroup\$ This is a common, if old school, method. Be sure to NOT drop too much voltage in the resistor or the 7805 may lose regulation. \$\endgroup\$
    – mike65535
    May 18, 2020 at 11:55

1 Answer 1

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The regulator needs rougly 7.5V-8V on the input, so you can drop 4-4.5V with a resistor. Rough calculation to drop 4.5V at 0.9A means about 5 ohm resistor, and it needs to handle dissipation of 4 watts. That can be split into multiple resistors too. But regulator would still dissipate about 3W which is a lot. Consider a 7805 compatible switch mode regulator module.

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  • \$\begingroup\$ These 7805 compatible replacements just install; no need to change the PCB. The 7805 is a legacy part and should be retired. Add some extra capacitance (ceramic and electrolytic) after the resistor but before the regulator if strong transients (surges and/or switching noise) are expected, to prevent it from "bottoming out." \$\endgroup\$
    – rdtsc
    May 13, 2020 at 19:01
  • \$\begingroup\$ Unfortunately sanctions made these kind of parts unavailable. I live in Iran... I have to stick with 7805. \$\endgroup\$
    – Mehdi Maazi
    May 14, 2020 at 8:59

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