I'm setting up a circuit to handle a 5V LED strip, which consumes between 500mA and 1A in bursts. The power source is a serial ATA cable which has 5V and 12V lines from the PSU. Without a capacitor and just using the 5V directly I see voltage drop down to 4.6V. With a 1000uF cap in the parallel, the drops look less dramatic / smoother and will stop at maybe 4.7V (just watching my meter without any real logging to observe). This is like 20 LED's on a WS2812B strip.

Pondering how to limit the drop so ideally there isn't one at all.

I'm thinking about taking the 12V line, using a step converter - believe the one I have is LM2596 based (It handles up to 3A and in past testing with a steady 1A, it gets slightly warm but not hot) - and use the 5V output from that to power the LED's instead. I'm not sure how this converters segment handles surges in current. If there is a surge will it smooth out the spike better?

Also, with regards to the capacitor(s). If I were to go the 12V route, not sure if I should wire one (or multiple parallel) caps in the circuit, before the step down? or after it's converted? I was using a 470 uF and then a 1000 uF cap, and now I'm wondering if I should try multiple caps (1000 is the largest I have on hand) to address my concern, instead of veering off into the 12V versus 5V question?

  • \$\begingroup\$ I would suggest asking an initial specific question, attempting more of your idea and then asking any follow up questions if you're stuck. This question is more a list of ideas and recommendations, which doesn't suit this site's format. \$\endgroup\$
    – ChrisD91
    Mar 17, 2021 at 23:35
  • \$\begingroup\$ Added 2 more question marks to help point out where questions are. [Comment edited by a moderator] \$\endgroup\$ Mar 18, 2021 at 0:00
  • \$\begingroup\$ Please read the datasheet to answer your question. Answer clearly shown in Figure 32 with values and even part numbers.. link \$\endgroup\$
    – Ernesto
    Mar 18, 2021 at 0:41
  • \$\begingroup\$ 0.4V drop at 1 amp implies a 0.4 ohms source resistance. What size wire are you using? How are you measuring? \$\endgroup\$ Mar 18, 2021 at 0:55
  • \$\begingroup\$ thanks @user1850479 --- the SATA cable appears to be 18 AWG and I'm using 22 AWG wire between components \$\endgroup\$ Mar 18, 2021 at 1:09

1 Answer 1


TLDR: If the problem is DC drop, then use the 12V line and have a voltage converter at the load. Also minimize the cable length. If the problem is AC droop you'll need decoupling capacitance. Not just 1 big cap but several caps due to the impedance profile of caps.

Explanation: The "Power Source" is not the ATA cable, it's your PSU. I will assume that your PSU is capable of supplying the amount of power required (i.e. VxI). There are several reasons why the voltage drop might be happening. Also, I'm not sure if you're talking about a DC drop or an AC droop so I'll answer for both.

DC Drop: No amount of capacitance will help your DC drop. As you may be aware, DC drop is due to IxR. Capacitors are used to battle AC droop when your regulator bandwidth is not high enough. If your ATA cable is too long, that's a problem because it adds both inductance and resistance in the line. How do we combat this? Well, at the end of the day you're delivering power not current and power is nothing more than VxI. If your ATA cable is too resistive (i.e. too long) then we you could use your 12V line instead of 5V because as you increase the voltage the current will drop to deliver the same amount of power. This will minimize DC drop. Also, a shorter cable might help.

AC Droop Capacitors store energy that can be delivered to the load when the regulator bandwidth is not fast enough. However, most people make the mistake of thinking that a very big capacitor will solve their problem. This is not the case due to the impedance profile of the capacitor (see below). Capacitor impedance decreases with increasing frequency (1/jwC). After a certain frequency the capacitor exhibits the impedance of an inductor (jwL). This means the capacitor's impedance increases with frequency. Furthermore, the capacitor has a minimum resistance ESR from it's wires. Without knowing you dI/dt of your load (i.e. the frequency) it is good practice to assume it varies. Therefore, always use several capacitors in parallel of varying magnitudes to minimize the impedance profile across frequencies.

The conclusion should be apparent. Use the technique of parallel capacitors for decoupling but also use your higher voltage line to minimize current (i.e. IR drop) and then use a converter close to your load.

Capacitor Impedance Images from: https://poulpetersen.dk/notes/cimp.html

enter image description here Images from: https://techweb.rohm.com/knowledge/emc/s-emc/03-s-emc/7669


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