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When is a uF not a uF ?

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NY2KW:
I need to replace a tantalum in an old 555 switching supply.  I read that tantalum capacitors have a higher volumetric efficiency (CV/cc) when compared to other types of capacitors and you would need a 100uF aluminum electrolytic capacitor to replace a 10uF tantalum capacitor.  I checked both specs and a good low ESR aluminum has the same ESR about 2-3 ohms, so why do I have to go up in capacitance value?  The 555 switching supply is fed by 14vdc and outputs 33vdc (1v ripple) and uses a zener to drop to desired 24v.  The tantalum is rated at 35v right near its operating voltage so I would like to replace with a 4.7uF aluminum

MagicSmoker:
Good grief, this is a double-bad: never expose tantalum caps to high ripple current (ie - switchers) and don't run them anywhere close to rated voltage. And note that the most common failure mode of sintered tantalum (the only kind you are likely to ever see) is to explode then catch on fire.

Am I being hyperbolic? A bit, yes, but tantalum really shouldn't be used where high AC/ripple currents are expected and it definitely benefits from generous derating of voltage.

langwadt:

--- Quote from: MagicSmoker on October 03, 2019, 10:36:57 pm ---Good grief, this is a double-bad: never expose tantalum caps to high ripple current (ie - switchers) and don't run them anywhere close to rated voltage. And note that the most common failure mode of sintered tantalum (the only kind you are likely to ever see) is to explode then catch on fire.

Am I being hyperbolic? A bit, yes, but tantalum really shouldn't be used where high AC/ripple currents are expected and it definitely benefits from generous derating of voltage.

--- End quote ---

I have an old pair glasses with chip right in the front of one eye from an exploding tantalum capacitor

tautech:

--- Quote from: NY2KW on October 03, 2019, 10:30:34 pm ---I need to replace a tantalum in an old 555 switching supply.  I read that tantalum capacitors have a higher volumetric efficiency (CV/cc) when compared to other types of capacitors and you would need a 100uF aluminum electrolytic capacitor to replace a 10uF tantalum capacitor.  I checked both specs and a good low ESR aluminum has the same ESR about 2-3 ohms, so why do I have to go up in capacitance value?  The 555 switching supply is fed by 14vdc and outputs 33vdc (1v ripple) and uses a zener to drop to desired 24v.  The tantalum is rated at 35v right near its operating voltage so I would like to replace with a 4.7uF aluminum

--- End quote ---
Who knows unless we can examine the schematic.

If it's used for timing purposes you wouldn't want to change the value and if for local bulk capacitance replacement with just a higher voltage rating would be a satisfactory improvement.
An ali cap can be subbed in too but the designer used tant for some reason, most likely speed or size.

NY2KW:
The schematic is attached.  The switching supply is at about 30kHz and the specs (Rockwell Collins) calls for a 4.7uF @ 35v tantalum even though the switching voltage output to the cap is 33vdc.   Can you explain what other characteristic besides ESR makes the tantalum required.  Thanks in advance

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