https://www.dropbox.com/scl/fi/191fe4xtipoxkr66g6y81/Cap_CV.jpg?rlkey=2gohwv8ksdc9z19znisalle6r&raw=1 So the 10u cap is better at all voltages, but the ratio is like 5:1 at 0 volts but only 2:1 at 10 volts.
X5R C-V data
Started by ●November 7, 2023
Reply by ●November 7, 20232023-11-07
On Tuesday, November 7, 2023 at 2:50:11 PM UTC-8, john larkin wrote:> https://www.dropbox.com/scl/fi/191fe4xtipoxkr66g6y81/Cap_CV.jpg?rlkey=2gohwv8ksdc9z19znisalle6r&raw=1 > > So the 10u cap is better at all voltages, but the ratio is like 5:1 at > 0 volts but only 2:1 at 10 volts.That's a curve of dQ/dV versus V_bias, I presume? That's the significant value when you're bypassing a DC bias voltage source. The capacitance could also be considered to be the total charge Q(V)/V versus V_peak, which would represent an AC current through the capacitor to ground better than dQ/dV does. Capacitance being an intrinsically linear property, the graph alone is ambiguous.
Reply by ●November 7, 20232023-11-07
On Tue, 7 Nov 2023 17:03:51 -0800 (PST), whit3rd <whit3rd@gmail.com> wrote:>On Tuesday, November 7, 2023 at 2:50:11?PM UTC-8, john larkin wrote: >> https://www.dropbox.com/scl/fi/191fe4xtipoxkr66g6y81/Cap_CV.jpg?rlkey=2gohwv8ksdc9z19znisalle6r&raw=1 >> >> So the 10u cap is better at all voltages, but the ratio is like 5:1 at >> 0 volts but only 2:1 at 10 volts. > >That's a curve of dQ/dV versus V_bias, I presume? >That's the significant value when you're bypassing a DC bias voltage source. >The capacitance could also be considered to be the total charge Q(V)/V versus V_peak, which would represent an >AC current through the capacitor to ground better than dQ/dV does. > >Capacitance being an intrinsically linear property, the graph alone is >ambiguous.It's the small-signal capacitance, literally measured as the time constant with DC bias and a small square wave from a 50 ohm source. I didn't trust my AADE c-meter with big caps like this. My Boonton only goes up to about 1 nF. As a power rail bypass, this is the number that I want.
Reply by ●November 8, 20232023-11-08
On 07/11/2023 22:49, john larkin wrote:> > https://www.dropbox.com/scl/fi/191fe4xtipoxkr66g6y81/Cap_CV.jpg?rlkey=2gohwv8ksdc9z19znisalle6r&raw=1 > > So the 10u cap is better at all voltages, but the ratio is like 5:1 at > 0 volts but only 2:1 at 10 volts. > > >Cool! Thanks, some manufacturers are coy about those graphs. So at 10V two 2.2uF would be almost as good as a single 10uF? piglet
Reply by ●November 8, 20232023-11-08
On Wed, 8 Nov 2023 10:23:18 +0000, piglet <erichpwagner@hotmail.com> wrote:>On 07/11/2023 22:49, john larkin wrote: >> >> https://www.dropbox.com/scl/fi/191fe4xtipoxkr66g6y81/Cap_CV.jpg?rlkey=2gohwv8ksdc9z19znisalle6r&raw=1 >> >> So the 10u cap is better at all voltages, but the ratio is like 5:1 at >> 0 volts but only 2:1 at 10 volts. >> >> >> > >Cool! Thanks, some manufacturers are coy about those graphs. > >So at 10V two 2.2uF would be almost as good as a single 10uF?It looks that way. I suspect that the root cause is the higher capacitance in a given package forces thinner dielectric films, increasing the voltage stress per volume of dielectric. Also, different dielectric compositions may be used. Joe Gwinn
Reply by ●November 8, 20232023-11-08
On Wed, 8 Nov 2023 10:23:18 +0000, piglet <erichpwagner@hotmail.com> wrote:>On 07/11/2023 22:49, john larkin wrote: >> >> https://www.dropbox.com/scl/fi/191fe4xtipoxkr66g6y81/Cap_CV.jpg?rlkey=2gohwv8ksdc9z19znisalle6r&raw=1 >> >> So the 10u cap is better at all voltages, but the ratio is like 5:1 at >> 0 volts but only 2:1 at 10 volts. >> >> >> > >Cool! Thanks, some manufacturers are coy about those graphs. > >So at 10V two 2.2uF would be almost as good as a single 10uF? > >pigletYes, less inductance too. Our original question was whether it's always better to use a cap with a bigger nameplate capacitance as a bypass. Probably yes. In this one case, the 10u always wins. I have some power pours that have noise from switching regs, in the hundreds of KHz range, 1.2 to as much as 10 volts DC. They are filtered by a ferrite bead and a lot of 1 uF bypass caps. The bead is a short at these frequencies. The fix is to use a real 47uH inductor and change the caps to 10u. https://www.dropbox.com/scl/fi/wt737ji10ap0o9beq6177/T660B_pcb.jpg?rlkey=bazxur2vfdt0765wmqy088lt3&raw=1
Reply by ●November 8, 20232023-11-08
On Thursday, November 9, 2023 at 5:42:03 AM UTC+11, john larkin wrote:> On Wed, 8 Nov 2023 10:23:18 +0000, piglet <erichp...@hotmail.com> > wrote: > >On 07/11/2023 22:49, john larkin wrote:<snip>> I have some power pours that have noise from switching regs, in the > hundreds of KHz range, 1.2 to as much as 10 volts DC. They are > filtered by a ferrite bead and a lot of 1 uF bypass caps. The bead is > a short at these frequencies. The fix is to use a real 47uH inductor > and change the caps to 10u.Ferrite beads and chips do have finite inductances. LTSpice has the Wurth range in it's list of models, and they go from 3nH to 14uH. 14uH and 10uF is resonant at 13hkHz - so a 14uH bead is not a dead short in that situation. It's a pedantic point, but one that I've had to worry about (and add a damping resistor to deal with). -- Bill Sloman, Sydney
Reply by ●November 8, 20232023-11-08
On Wed, 08 Nov 2023 10:41:45 -0800, john larkin <jl@650pot.com> wrote:>On Wed, 8 Nov 2023 10:23:18 +0000, piglet <erichpwagner@hotmail.com> >wrote: > >>On 07/11/2023 22:49, john larkin wrote: >>> >>> https://www.dropbox.com/scl/fi/191fe4xtipoxkr66g6y81/Cap_CV.jpg?rlkey=2gohwv8ksdc9z19znisalle6r&raw=1 >>> >>> So the 10u cap is better at all voltages, but the ratio is like 5:1 at >>> 0 volts but only 2:1 at 10 volts. >>> >>> >>> >> >>Cool! Thanks, some manufacturers are coy about those graphs. >> >>So at 10V two 2.2uF would be almost as good as a single 10uF? >> >>piglet > >Yes, less inductance too. > >Our original question was whether it's always better to use a cap with >a bigger nameplate capacitance as a bypass. Probably yes. In this one >case, the 10u always wins. > >I have some power pours that have noise from switching regs, in the >hundreds of KHz range, 1.2 to as much as 10 volts DC. They are >filtered by a ferrite bead and a lot of 1 uF bypass caps. The bead is >a short at these frequencies. The fix is to use a real 47uH inductor >and change the caps to 10u. > >https://www.dropbox.com/scl/fi/wt737ji10ap0o9beq6177/T660B_pcb.jpg?rlkey=bazxur2vfdt0765wmqy088lt3&raw=1Note that upper-left and lower-right are the little side-firing 3-color LEDs that were discussed in another thread.
