0
\$\begingroup\$

I have a HP laptop power supply 18.5V/3.5A that I intend to connect directly to LM7812 or LM7805 voltage regulator when I need to drop the voltage to 12V/5V. Since the laptop power supply (which is switched mode I assume) should be outputting a filtered voltage to the LM7812...are the input/output capacitors recommended in the datasheets truly necessary? could the LM7812 work in this particular case..without capacitors? what would be the disadvantage? I do have a couple of generic DC-DC Buck converter boards off ebay and these work, but am utilizing it for other purposes.

cheers, culain

\$\endgroup\$
  • \$\begingroup\$ Although the voltage regulator does not require the capacitors that much, it also depends how far away fro the transformer box the voltage regulator is. If the cable is long, then probably put the caps. But if you're running a basic circuit like an LED and you're under their max specifications by a fair amount, no need for the caps. \$\endgroup\$ – Bradman175 May 9 '16 at 4:26
  • 2
    \$\begingroup\$ ALWAYS put bypass capacitors next to any IC that shows them in diagrams. They reduce hum and noise and stabilize the IC so it behaves as expected. \$\endgroup\$ – Sparky256 May 9 '16 at 4:51
  • 1
    \$\begingroup\$ I wouldn't say they are "recommended" I would say they are "stipulated". It's not random advice. \$\endgroup\$ – Andy aka May 9 '16 at 7:34
  • 1
    \$\begingroup\$ I wonder why you want to avoid having capacitors. Is it just that you don't happen to have some in your stock ? Also, be careful that dropping 18.5V to 5V is a big step. If you don't want to use heatsinks, with the TO220 case which is about 1.5W max, it only allows a current of 110mA max. \$\endgroup\$ – dim May 9 '16 at 7:38
2
\$\begingroup\$

From the datasheet we read that the input capacitor is required if regulator is located an appreciable distance from power supply filter. If you are sure that there is no "too much noise" (to say in simple words) between your PSU and the 7812 you can avoid it.

Also, it is recommended if the output capacitance is large. Looking at the internal schematic of the circuit I suppose that is recommended to protect the input stage; I hope someone could confirm or correct this statement.

Although no output capacitor is need for stability, it does improve transient response.

These informations came from ST and Panasonic datasheet. Sometimes datasheet are a little obscure, sometimes explain things very well. ;-)

UPDATE 2016-05-11: In old regulator output capacitor was used also for stability purpose, but in modern datasheet this is no more required. Today I found an interesting (and OLD) document: NEC - Usage of Three-Terminal Regulators

\$\endgroup\$
  • \$\begingroup\$ appreciable distance = 1 inch, from previous questions. \$\endgroup\$ – Passerby May 9 '16 at 8:13
  • \$\begingroup\$ 1 inch,, so I definitely need input capacitors. \$\endgroup\$ – culain99 May 9 '16 at 8:53
  • 1
    \$\begingroup\$ @Passerby, only to say, The "National Semiconductor" datasheet say more than 3". \$\endgroup\$ – Antonio May 9 '16 at 9:32

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.