Ceramic capcitor capictance loss under DC bias - Page 1
Forum section: electronics / beginners

author: Ionforbes
author_rank: Contributor
timestamp: August 16, 2020, 04:18:58 pm
content: I just learned about class 2 ceramic capacitors losing capacity when a voltage is applied to them, but I don't know how this would affect RC time constants. Say I was charging a 1uf X7R capacitor to 16V through a 1k resistor. If there was no capacitance loss the time constant would be 1ms. However, if it lost say 60% of its capacitance at 16V, would I treat it as if the capacitor was 400nf and the time constant would be 400us? Or would it be much more complicated, somehow taking into account that the capacitance changes as it charges, being 800nf at 10V, 600nf at 12V (not real numbers, just hypothetical)? Wouldn't this also cause the charging waveform to look 'weird' compared to the typical curve?I would use my scope to find all this out for myself, but...Many thanksEdit: sorry for the weird formatting? I'm on mobile and can't change the font size

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 16, 2020, 04:47:00 pm
content: Yes, the time constant drops as it goes. You're right to be confused -- there may not even be a closed-form solution to the waveform. Such is often the nature of nonlinear differential equations.Since the capacitance drops, the charging curve can actually steepen; instead of a nice simple exponential curve, it can toe in, then curve down and back up (there's an inflection point). Depends on the ratio between the capacitor's C(V) curve, and the supply voltage. For small voltage swings, it tends towards exponential, at the capacitance at the average voltage; for large swings, it depends, and you can get shorter rising edges and much longer falling edges. Or lumpy edges when discharging through zero.This isn't much of an issue for small values used for timing (the voltage curve is usually pretty shallow), but for power supply values it's relevant for startup and hold-up time.Tim

author: Ionforbes
author_rank: Contributor
timestamp: August 16, 2020, 05:02:10 pm
content: Thanks for your answer, clears things up! I dream of a world of ideal components...

author: TimFox
author_rank: Super Contributor
timestamp: August 16, 2020, 05:08:06 pm
content: If you need an accurate time constant (e.g., in a low-pass filter) steer clear of the non-linear ceramics. For smaller values, choose NP0/C0G and for larger values choose polypropylene film. If the latter are too big physically, re-design your circuit for higher R and lower C. AN added SO8 op amp is much smaller than a 1 uF polypropylene film capacitor.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 16, 2020, 06:58:09 pm
content: Tantalum are fine, too: stable and linear, and the low currents in timing/filtering circuits won't ignite them (they are made of thermite materials, after all!).Electrolytics are sort of last resort, with drifty values and relatively high leakage and absorption; that might still be fine for, say, a thermostat cycle timer or something.For precision, the polypropylene is probably better -- it will have to be done that much more carefully, with very high value resistors, low bias op-amps, and very clean assembly (washed of soldering and handling residues, use of guard rings to control leakage).So the choice between these is driven by the impedance, which when low, you don't have to be careful and can use ordinary circuitry and stuff, but the capacitor values have to be humongous, but that basically limits you to electrolytics, so your accuracy is going to be pits.Which, on that note -- if you are using ceramics for timing application -- if the signal level is consistent (regulated supply or thresholds), the C(V) doesn't matter, it's factored out. Where it hits you is if V is varying between cycles. A 555 timer uses thresholds proportional to supply, so if the supply itself isn't regulated, that's a problem.If instead you're using them for signal filtering (e.g. audio), the change in capacitance causes distortion. Easy way to understand this: suppose there's a signal with high and low frequency components, and the filter is a lowpass type. The low frequency passes unimpeded, varying the capacitor's voltage (and value). The high frequency is in the asymptotic -20dB/dec cutoff range, so its amplitude is inversely proportional to C. Thus the high frequency component ends up amplitude-modulated by the low frequency component, and the modulation depth depends strongly on the level of the low frequency component (much faster than proportionally so). This isn't necessarily a distortion process per se (in terms of harmonic distortion), but it is a mixing or intermodulation process; if we consider signals of intermediate frequencies as well, we'll find they modulate themselves as well as others, and we have the usual, a mix of THD and IMD.So we again might have a case where, for low enough frequencies, it's better to use polypropylene and raise the impedance, to keep stability and distortion low, and pay the cost to noise level (low-bias opamps tend to have higher voltage noise), than to use ceramic or other types.These are really just the same thing, because a timer is just a filter in a positive feedback circuit. We just have the special case that, if the signal level in the timer is consistent, the output (timing) will be consistent; an assumption we can't make for an arbitrary signal.Tim

author: Ionforbes
author_rank: Contributor
timestamp: August 16, 2020, 07:41:02 pm
content: I remember one of my first projects being a speaker amp that used mainly X7R - now I see at leastonereason it sounded so distorted, among others!

author: David Hess
author_rank: Super Contributor
timestamp: August 17, 2020, 01:40:53 am
content: Low leakage aluminum electrolytic capacitors are suitable for timing applications, and might be more reliable than solid tantalum parts which cannot clear shorts and self heal in high impedance circuits.Where the voltage coefficient of capacitance is a problem in AC applications, either the capacitive reactance is kept low to minimize the change in voltage, like in audio applications, or a pair of capacitors might be used in series with high voltage bias applied between them.