Cap size

[Serge 125]

Hi all (again) as I said before I'm repairing/refurbishing vintage computer PS and I was wondering does the physical size of the cap maters as long as I have the same value. I have replaced some caps that are smaller but have the same value so I assume that it should still work, right? THANKS for any info!!! ;-)

 

[sagor1]

Not just the same value, but the voltage rating much be the same or "better". Other than that, it makes little difference about the size of the cap.
In a few cases, the temperature rating (85C or 105C) dictates how the caps can survive higher temperatures inside a circuit...

 

[Serge 125]

Not just the same value, but the voltage rating much be the same or "better". Other than that, it makes little difference about the size of the cap.
In a few cases, the temperature rating (85C or 105C) dictates how the caps can survive higher temperatures inside a circuit...

Yeah I knew about voltage but when I ment value I ment in general having the same rating (or better with voltage). I had 2 caps that were mid size to change and I had in my kit smaller ones with the same values so I thought I could still "swap" from mid to smaller size without causing any problems BUT that is why I asked here just to make sure before I turn the power on and OOPS. THANKS!!!

 

[gophert]

Electrolytic capacitors for power supplies have gotten smaller over the years as film-making processes and polymers have been developed to make thinner and thinner films. These metallized films, when wrapped into tight coils, result in more surface area (capacitance) for the same volume. Or, in your case, the same capacitance in smaller volumes.

 

[ronsimpson]

I looked at digikey.com for 470uF 16V aluminum electrolytic cap, polar, . Found about 400 different kinds.
Noticed temperature range. like Sagor1 said.
ESR=Equivalent Series Resistance. One cap =33 ohms. All the rest were below 1 ohm. Some (used in switching power supplies) went as low as 0.120 ohms at 120hz. I look at the resistance at 100khz because that is close to what the cap sees in a switcher. Some are as low as 0.056 ohms at 100khz. Switching power supply caps are rated for ESR at 100khz and 120hz. If it is rated only at 120hz it probably was not designed for 100khz. If there is no ESR rating don't use it.
A data sheet for a good power supply capacitor should show mix ripple current at 120hz and 100khz. There are places in a supply where ESR and current is not important but if the cap is one diode away from a transformer then think about current. Any cap on a output or input voltage should have a high current rating.
+/-10% or +/-20% is not so bad but some time the caps come in +50%/-10%. The error amplifier is critical to a stable supply. Adding 50% to a capacitor will cause delay through the system and may cause the error amp and supply to become unstable. Don't use caps that are different in value if possable.

Get the data sheet for the old cap. Find the ESR and Max current. Over stressed capacitors get hot inside and dry out.
I would like to tell you that power supply engineers choose the right voltage and current ratings. Some times they don't do it right. Some times they don't know how because they are just copying another design. Some times production chooses a low cost part not knowing that it will die in one year.
Most of the time the engineers do their jobs right and you should choose a part with equally as good voltage and current ratings. On the other side, if every things was right the supply would not need fixing.

Too long a story. Short = power supply capacitors have a current rating. Choose wisely.

 

[rjenkinsgb]

If there is extra space when replacing a cap, I generally try to find one with higher ripple rating and/or lower ESR.
That may mean using a higher voltage rating than the original

Also check the rated life and try and pick the types with the longest ones.

Using random junk box ones or unknown types in power supplies where they are under high load is not a good idea.