The Learning Circuit 42: Replacing MLCCs with Polymer Capacitors

nmanjofo , sadly, that thread is a great example of why I avoid that forum. You have to be careful when changing out polymers in LDO-circuits and switching circuits. The significantly lower ESR can result in control loop stability issues that the original design did not consider. A Pentium4 design is on the verge of the timeframe where polymers were just entering the market. When I was at the capacitor company, we had issues where a supply chain manager swapped out a polymer for a traditional electrolytic in production, which resulted in failures in the field. (As engineers became aware of this issue, design rules were put into place to address that.)

So, for the big bulk electrolytics, you can probably safely replace them. They are usually input filter caps to other regulators and are less sensitive to ESR issues. If they are on the output of a regulator, there could be more issues, especially in the POL (point-of-load) converters that get closer to the processor.

Unfortunately, as that thread shows, there are many design engineers (or want to be engineers) who have never measured the control loop stability of a power supply design.

tldr; can you replace them in a P4 mobo? Maybe. I think the larger electrolytics are probably a safe bet. The smaller surface mounts near a switching regulator, I would be more cautious.

it is said that capacitance of the polymer should be like 1/3 of the original electrolyte cap. Is it like that?

Unfortunately, it is difficult to make a blanket statement like that. It comes down to the fact that the lower ESR results in better high-frequency performance. However, without characterizing how the control loop responds to that change in impedance is going to be difficult. When designing-in a polymer into a new design, this is one of the advantages. You can typically use a much smaller polymer because of the higher effective capactiance. But in that case, you're able to account for the difference in overall impedance.

Hey! Thanks for a great video. I was wondering if I can replace electrolyte caps on my old Pentium4 motherboard with polymers. Do I use the same capacitance or what parameters should I focus on? I've come to several ideas, for, example here

https://www.eevblog.com/forum/beginners/can-be-simple-electrolytic-capacitor-replaced-by-polymer-capacitor/msg750423/?PH…

it is said that capacitance of the polymer should be like 1/3 of the original electrolyte cap. Is it like that?

Thank You!

Thanks for the reply - I can definitely understand especially if you don't have an ESR meter to hand as it isn't necessarily easy to measure or interpret. In fact, I invested in one pretty early on as I felt it was going to be "useful" but have (admittedly) never attempted to measure leakage current directly in the way you have. Perhaps I should give that one a go someday .

- Gough

lui_gough  wrote:
 
I notice you measured leakage current (related with dissipation factor) to determine if the capacitor electrolyte is still healthy.
 

Gough, good catch. Something I should have more clear is that I was only trying to evaluate the health of the capacitor. I was not trying to assess the performance capability of the old capacitors. Thinking about what I said in the video, I can see how it might not sit well with some. I specifically said I was using the leakage measurement to determine the electrolyte's life. While a true statement, I might have said it too definitively.

Both ESR and leakage current is used by manufacturers to determine end-of-life for the electrolyte. So to be correct, I should have measured both. However, once one parametrically fails, the capacitor is considered end-of-life. I also like that leakage demonstrates both the dielectric's and electrolyte's health in a single measurement. A healthy dielectric will have a low leakage current, and, a healthy electrolyte will stabilize the current quickly.

All that said, the #1 reason I showed leakage is because it is an easy measurement to make and understand. All you need is a power supply and multimeter. For a video, it was a simple measurement to show. I, like most people, do not have an ESR meter. So to measure ESR, I would have had to use my scope and function generator. Initially, I thought about showing this measurement, but I felt like it slowed down the video too much to explain how it works. (But it is something I want to do a video on at some point.)

Another reason I skipped measuring the ESR is that I don't have any information on the original specs for the capacitors. I have no idea what a good or bad ESR measurement would be for them. With 30 years of electrolyte development, it is possible there is already an order of magnitude difference between the ESR of the original capacitors and their modern equivalent. One might say the same for leakage, but my experience from looking through old datasheets suggest that 1-5% of the CV has not changed much over time. And in this case, going from memory, the leakage after 5 minutes was in that range for the capacitor I measured.

I enjoyed the video along with the (at times) suspenseful moments looking for improvements. I know in the past, I've tended to do "shotgun" style repairs where I will replace capacitors wholesale, mostly with nearly-identical capacitance/voltage/technology specs but of a modern make which often results in better performance for vintage gear as modern capacitors generally do better on ESR and leakage.

Forgive my ignorance baldengineer, when you replaced the electrolytic capacitors of the C64, I notice you measured leakage current (related with dissipation factor) to determine if the capacitor electrolyte is still healthy.

I was wondering whether you measured the ESR difference between the old capacitors and the new capacitors, as reduced ESR (to my rather limited knowledge) should improve ripple voltage and frequency response of the capacitor and is the means by which I normally gauge whether an old capacitor would be any good (along with dissipation factor, although I note that that there are only rough guidelines for ESR as that varies from series-to-series so checking datasheets can be useful and sometimes varies quite significantly for different capacitor technologies especially over frequency).

- Gough

Good video.

As you point out, testing is key to understanding how different types of components affect specific circuits.

There is much more to engineering than just copying circuit diagrams and putting components together.

DAB

axials were intended to use with point to point wiring on solderstrips and wirewrap, the naming is similar to radial and axial agricultural crop protection blowers, the leads are attached like the stream of air would go (paralell to the axis-> axial). I only know that from having to remember when I studied viticulture...