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I'm trying to find step-down DC-DC regulators that would take input voltages of ~30V and convert it to 5V and ~24V.

I was originally looking at linear regulators such as the 7805T, but the specific project I am working on would need to perform in a vacuum environment. As far as I understand, electrolytic capacitors cannot work in space as they have a high probability of popping due to the vacuum environment. Since the 7805T linear regulator and many other regulators require capacitors in order to function properly, I need to find an alternative option for a step-down regulator.

I searched online, but it appears to be that the vast majority of these voltage step-down regulators/converters require capacitors in one form or another. From breakout transformers to linear regulators, it appears that I can't find any without capacitor(s).

Is there a DC-DC step-down voltage regulator out there that does not use capacitor(s) in any way?

I also found a line of regulators from Pololu that are quite small, but I'm unable to determine whether it has a capacitor (there is something that appears as one on the boards of these, but I'm unable to tell whether it's a capacitor or not).

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    \$\begingroup\$ are there no vacuum compatible capacitors? \$\endgroup\$
    – Jasen Слава Україні
    Commented Feb 18, 2023 at 20:28
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    \$\begingroup\$ You can probably use solid tantalum or ceramic capacitors. They won't pop in a vacuum. \$\endgroup\$
    – JRE
    Commented Feb 18, 2023 at 20:30
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    \$\begingroup\$ Is this going to space, or is it running in a bell jar? heat is a big problem for vacuum electroncs. \$\endgroup\$
    – Jasen Слава Україні
    Commented Feb 18, 2023 at 20:31
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    \$\begingroup\$ Aluminum electrolytic capacitors aren't vacuum compatible -- the "electrolytic" in the name is actually a water-based goo. But there are capacitors that work in space. Possibly even wet-slug tantalums, if you shop for them carefully. But there's enough consumer-grade caps that'll work that it probably doesn't matter. \$\endgroup\$
    – TimWescott
    Commented Feb 18, 2023 at 20:35
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    \$\begingroup\$ Any regulator is going to require capacitors. How large a capacitor will depend on the design. But know that there are plenty of reasons to avoid linear regulators--in particular, they produce a lot of heat, especially when stepping down 30 V to 5 V. That itself will be very difficult in a vacuum, as there's no airflow to take heat away; you have to use radiative cooling alone. \$\endgroup\$
    – Hearth
    Commented Feb 18, 2023 at 20:46

2 Answers 2

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You cannot avoid using capacitors in any kind of real electronics design. Fortunately, people have been figuring out how to make electronics work in vacuum since at least the 1950's, as demonstrated by Sputnik, thousands of other satellites, and innumerable terrestrial applications in vacuum chambers or vacuum flasks.

Simply don't use aluminum electrolytics or other capacitors that aren't suitable for vacuum use, and be sure to choose chips that will function with the capacitors that you can economically use. Even in the 1970's, when the 7805 was introduced, one could, with digging, find suitable capacitors that will work in a vacuum. If the size and cost of the capacitors you find is objectionable, do some combination of shopping harder and finding circuits that don't need as much capacitance.

Suitable candidates that I know of at this moment are:

  • Solid-electrolyte tantalum or niobium capacitors.
  • Multi-layer ceramic chip capacitors.
  • (Possibly, and it'll be expensive) wet-slug tantalum, but double-check -- you want the ones that come in hermetically sealed packages.
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    \$\begingroup\$ There are some solid aluminum capacitors, called aluminum polymer capacitors, that might be suitable, but do research them first--I've never worried about vacuum-suitability so I don't know. I expect most film capacitors would work as well, but they would probably cost more in terms of size, weight, and money than any of the other options. \$\endgroup\$
    – Hearth
    Commented Feb 18, 2023 at 20:49
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    \$\begingroup\$ I'd be a bit worried about a film cap outgassing. Plastics can have all sorts of unexpected volatiles trapped inside them that come out and either leave behind unsuitable plastic, or that settle on their surroundings to the detriment of, well, everything. Of course, that's the case for just about anything that you want to work in an environment where pressure is measured in single-digit Pascals. \$\endgroup\$
    – TimWescott
    Commented Feb 18, 2023 at 21:19
  • \$\begingroup\$ Thanks for your reply; So would a solid tantalum capacitor like this work: digikey.com/en/products/detail/kemet/T110A474K035AT/… \$\endgroup\$
    – Runsva
    Commented Feb 19, 2023 at 2:00
  • \$\begingroup\$ The data sheet says nothing about working in a vacuum or high altitude. It does, however, say they're qualified to MIL–PRF–39003. Perhaps you could download that standard and look at what it says. \$\endgroup\$
    – TimWescott
    Commented Feb 19, 2023 at 3:07
  • \$\begingroup\$ Note that you've chosen a wet-slug tantalum -- that's inherently not as safe as a ceramic or solid-electrolyte tantalum, but as long as that hermitically sealed package stays that way, it'll work. \$\endgroup\$
    – TimWescott
    Commented Feb 19, 2023 at 3:08
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The question was whether there is a step-down regulator that does not require an additional capacitor to operate. (I had the same question, which is what brought me here.) Actually the 7805T datasheet linked in the question states that capacitors on the input side as well as output side are optional, depending on the application. Other datasheets for LM78-style linear regulators from ST and TI explicitly state that no external components are required.

So yes, they exist.

But, if you have a wildly fluctuating load, you may find that your load requires a capacitor.

I ended up using a Recom R-78W5.0-0.5 DC/DC converter because the package was convenient.

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    \$\begingroup\$ I don't see how this answers the question, other than that the module does not require any external capacitors, because the module already contains internal capacitors which are mandatory for switch mode regulators. The question did specifically ask for regulators that do not require capacitors because electrolytics do not work in vacuum. This module does not work in vacuum either, max altitude is 2000m. Simplest solution is, use any regulator and use other than caps than electrolytic. \$\endgroup\$
    – Justme
    Commented May 9 at 9:13

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