Ceramic multilayer capacitors

[sexton_mallard]

After many years, I am stocking up again for 0.1 and 0.47mfd caps and was intrigued to find these ceramic caps in the RS catalogue - the 0.47mfd 1000v model is in http://uk.rs-online.com/web/p/products/9060691/

I appreciate these are small but so is the price. Has anyone given these a 'go' in a old set?

[WaveyDipole]

That certainly is small for something designed to handle 1000v DC! They might perhaps look a bit odd in a valve radio and the legs are probably too short for practical purposes. I would also have reservations about trying to use one of these as a substitute for 'that capacitor'.

[sexton_mallard]

Thanks. I agree too, the spec seems almost impossible. I would have reservations using them in an old TV but I might give the smaller capacitance values a go in a radio first. I am not a fan of tiny components and prefer the chunky sized discrete components found in vintage gear. If I try some - watch this space.

[KeithsTV]

One thing to be aware of is that with some ceramic capacitors the capacitance can vary with the applied voltage. It depends on which dielectric material is used although I can't remember off hand which is the worst.

You may also find that because they are ceramic they may emit audible noise if subjected to pulsed voltages due to the piezoelectric effect. I had this problem with capacitors used in plugtop power supplies a few years ago.

Keith

[Hartley118]

My understanding is that we must differentiate between class 1 and class 2 ceramic capacitors.

Class 1 is the 'traditional' ceramic capacitor, typically using titanium dioxide as a normal insulating dielectric. These have very consistent performance and we're well accustomed to using them for RF decoupling and coupling. The dielectric permittivity is typically less than 100.

Class 2 capacitors on the other hand possess a degree of 'magic' by virtue of using a piezoelectric ceramic such as barium titanate as the dielectric. These can exhibit effective permittivity figures as high as 10,000 by virtue of retaining charge by the mechanical deformation which happens when a voltage is applied. So they're really electromechanical components and, as Keith indicates, have a capacitance value which varies with applied voltage. They're typically used for power supply decoupling where all the designer's looking for is a guaranteed minimum capacitance. They inevitably 'sing' with the applied frequency, though whether that's audible depends on the frequency and the capacitor mounting arrangement.

Using class 2 ceramic capacitors for restoring electronics designed to use conventional capacitors could be an exciting trip into the unknown!

Martin

[sexton_mallard]

Looking through the notes for these caps, they do indeed appear to be Class 2 and specifically quote... "X7R dielectric is not recommended for AC line filtering or pulse applications." Placement into a circuit therefore has to consider the function of that cap. I guess it does rule out use in a TV timebase.

[sexton_mallard]

I just watched this video at https://www.youtube.com/watch?v=2MQyQUkwmMk which shows the behaviour of X7R caps with bias voltage applied. I'll stick to the normal caps