Hi Hans.? Tantalums should meet their manufacturer's derating voltage.? It's just not enough to say you're operating below the capacitor's voltage rating.? In aerospace applications, a typical voltage derating is 50% of rated value was used, and the capacitors had to be current surge rated.? Negative effects due to high surge currents must also be carefully considered, especially at startup.? Kyocera states, for their tantalum caps, "Voltage derating is necessary for tantalum and
niobium capacitors to prevent failure due to
excess current availability.?Tantalum capacitors can be safely used at 80% of
their rated voltage, but the MTBF will be lower and
leakage current higher.?If a tantalum must be used across a low
impedance source, consider incorporating a PFET
integrator to reduce risk of failure."
-Steve K1RF
------ Original Message ------
Date 5/12/2025 2:03:12 PM
Subject Re: [QRPLabs] Rev 1 vs. Rev 3 QMX Kit?
Hi Mike, all
The SMPS step issue wasn't quite the same thing as the capacitor voltage ratings issue.?
The tantalum capacitors used were rated 6.3V. they shouldn't ever see more than 5V. Indeed there is a 5.6V zener with a sharp cut-off as final protection, so I'd be very surprised if the capacitor could ever see 6.3V and exceed its rating.?
However, there were occasional field reports of the capacitor burning out (quite literally, tantalum capacitors catch fire). And not in any known circumstances such as sudden input voltage steps. So it seems that even though the capacitor's 6.3V voltage rating probably wasn't technically exceeded, it's still too close for comfort and perhaps some devices are a little lower spec than others.?
I subsequently changed to 10V rated tantalum capacitors which are available in the same size package and have not heard of burning capacitors since this change.?
Nevertheless if you have a Rev 1 board I wouldn't rush to change the capacitors, as I recall the fires were rare.?
73 Hans G0UPL
On Sun, May 11, 2025 at 02:15 PM, DL2ARL wrote:
humbly thinking that this statement is wrong. Wrong in so many ways...
Perhaps I did not include enough detail?
I'm referring to C106 and C107.
The parts supplied in Rev 1 were rated at 6V.
The parts supplied in later revs were rated at 10V.
Hans wrote a very detailed article on the overall issue here: ?
?
73, Mike KK7ER
?
