equivalent series resistance

3 Articles

[Dr Ali Shirsavar] drawing schematics and equations on the whiteboard

Calculating The Capacitance And ESR Specifications For The Output Capacitor In Your Switching-Mode Power Supply

January 18, 2026 by 13 Comments

[Dr Ali Shirsavar] from Biricha Digital runs us through How to Select the Perfect Output Capacitor for Your Power Supply. Your switching-mode power supply (SMPS) will require an output capacitor both to iron out voltage swings due to loading and to attenuate ripple caused by switching. In this video we learn how to calculate the required capacitance, and when necessary the ESR, for your output capacitor.

To begin [Dr Ali] shows us that in order to calculate the minimum capacitance to mitigate voltage swings we need values for Δi, Δv, and Ts. Using these we can calculate the minimum output capacitance. We then need to calculate another minimum capacitance for our circuit given that we need to attenuate ripple. To calculate this second minimum we need to change our approach depending on the type of capacitor we are using, such as ceramic, or electrolytic, or something else.

When our circuit calls for an electrolytic capacitor the equivalent series resistance (ESR) becomes relevant and we need to take it into account. The ESR is so predominant that in our calculations for the minimum capacitance to mitigate ripple we can ignore the capacitance and use the ESR only as it is the feature which dominates. [Dr Ali] goes into detail for both examples using ceramic capacitors and electrolytic capacitors. Armed with the minimum capacitance (in Farads) and maximum ESR (in Ohms) you can then go shopping to find a capacitor which meets the requirements.

If you’re interested in capacitors and capacitance you might enjoy reading about Measuring Capacitance Against Voltage and Getting A Handle On ESR With A Couple Of DIY Meters.

Exploring Cheap Tantalum Caps Of Mysterious Provenance

July 22, 2024 by 27 Comments

We’ve all heard about the perils of counterfeit chips, and more than a few of us have probably been bitten by those scruple-free types who run random chips through a laser marker and foist them off as something they’re not. Honestly, we’ve never understood the business model here — it seems like the counterfeiters spend almost as much time and effort faking chips as they would just getting the real ones. But we digress.

Unfortunately, integrated circuits aren’t the only parts that can be profitably faked, as [Amateur Hardware Repair] shows us with this look at questionable tantalum capacitors. In the market for some tantalums for a repair project, the offerings at AliExpress proved too tempting to resist, despite being advertised alongside 1,000 gram gold bars for $121 each. Wisely, he also ordered samples from more reputable dealers like LCSC, DigiKey, and Mouser, although not at the same improbably low unit price.

It was pretty much clear where this would be going just from the shipping. While the parts houses all shipped their tantalums in Mylar bags with humidity indicators, with all but LCSC including a desiccant pack, the AliExpress package came carefully enrobed in — plastic cling wrap? The Ali tantalums were also physically different from the other parts: they were considerably smaller, the leads seemed a little chowdered up, and the package markings were quite messy and somewhat illegible. But the proof is in the testing, and while all the more expensive parts tested fine in terms of capacitance and equivalent series resistance, the caps of unknown provenance had ESRs in the 30 milliohm range, three to five times what the reputable caps measured.

None of this is to say that there aren’t some screaming deals on marketplaces like AliExpress, Amazon, and eBay, of course. It’s not even necessarily proof that these parts were in fact counterfeit, it could be that they were just surplus parts that hadn’t been stored under controlled conditions. But you get what you pay for, and as noted in the comments below the video, a lot of what you’re paying for at the parts houses is lot tracebility.

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Getting A Handle On ESR With A Couple Of DIY Meters

November 14, 2016 by 11 Comments

Got a bunch of questionable electrolytic caps sitting in your junk bin? Looking to recap a vintage radio chassis? Then you might need to measure the equivalent series resistance of the capacitors, in which case this simple five-transistor ESR meter might come in handy.

Even if you have no need for an ESR meter, [W2AEW]’s video below is a solid introduction to how ESR is determined. The circuit itself comes from EEVBlog forum user [Jay-Diddy_B] and is about as simple as such a circuit can get. Two transistors form an oscillator that generates a square wave that drives a resistor bridge network. The two legs of the bridge feed matched common-emitter amps, one leg through the device under test. The difference in voltage between the two legs is read on a meter, and you have a quick and simple way to sort through the caps in your junk bin. [Jay-Diddy_B]’s circuit is only presented in breadboard form; no attempt was made to field a practical instrument. Indeed, [W2AEW] already built a home-brew ESR meter using hex inverters and op amps to which he compares the five-transistor circuit’s results. His intention here seems to be to clarify the technique of ESR measurement and evaluate an even simpler circuit than his. We think he’s done a good job on both counts.

We’ve featured plenty of [WA2AEW]’s work before, like this Michigan Mity-Mite transmitter or his primer on oscilloscopes. We really like his laid back style and the way he makes complex topics easy to understand. Check them out.

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