Tantalum cap question

HI,

I have a pile of 50v tantalums that are "marked" as 6.8uF but all of them seem to be testing at 8.0uF - 8.3uF on both my meters. (one is a cheap digital LC meter and the other is a DMM)

Is this a measurement error (ie something to do with measurement freq) or is it normal for tantalums to read higher than their marked value, or is this just a bad batch or a mislabel ?

6.8uF up to over 8uF seems a big tolerance jump to me (I thought they were 10% tolerance)

Any thoughts ?

Are they SPECIFIED at 10% tolerance? Most tantalums (and electrolytics for that sake) I have are -10/+20%..

Jakob E.

(btw, wrong subforum, methinks - probably belongs in the Lab)

All Ears said:

HI,

I have a pile of 50v tantalums that are "marked" as 6.8uF but all of them seem to be testing at 8.0uF - 8.3uF on both my meters. (one is a cheap digital LC meter and the other is a DMM)

Is this a measurement error (ie something to do with measurement freq) or is it normal for tantalums to read higher than their marked value, or is this just a bad batch or a mislabel ?

6.8uF up to over 8uF seems a big tolerance jump to me (I thought they were 10% tolerance)

Any thoughts ?

Click to expand...

I think you would need to use a different measurement procedure. I would suggest charging the caps at some known value and discharging it with a known resistor. If your DMM is 10 Meg input resistance, that would make a 68seconds nominal time-constant (Tau = RC product). Observing the discharge, the voltage should be 0.37 the start voltage afetr one time-constant (68 seconds). Or you could use the half-life formula. The time taken to discharge 50% is 69% of the time-constant.
I don't recomment methods using an AF signal and observing the cut-off frequency, because:
A) the capacitance is not linear with level/frequency
B) tantalum caps hate reverse voltage