I am looking at building a metal detector similar to the circuit below.
In the instructions, it is stated that 'Capacitors with values less than 1uF are polyester film type (+/-10%)'. I am planning to use SMD ceramic capacitors rather than film capacitors for ease of manufacture. I am aware that the capacitance of ceramic capacitors changes with the DC offset. I think this shouldn't be a problem for bypass capacitors and capacitors that form RC time constants. Are there any film capacitors in the circuit that should definitely NOT be replaced with ceramic capacitors? If so, why?
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\$\begingroup\$ Please edit to explain "PI" in your title. "Private investigator"? \$\endgroup\$– TransistorCommented Nov 4, 2020 at 21:23
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\$\begingroup\$ Pulse Induction it stands for @Transistor (a bit like a radar but not massively so). \$\endgroup\$– Andy akaCommented Nov 4, 2020 at 21:26
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\$\begingroup\$ @Andy. I never heard of it. Thanks. \$\endgroup\$– TransistorCommented Nov 4, 2020 at 22:26
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\$\begingroup\$ @mr_js - are we done with this Q and A now? \$\endgroup\$– Andy akaCommented Nov 17, 2020 at 9:11
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1 Answer
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I am aware that the capacitance of ceramic capacitors changes with the DC offset.
You can replace them all with ceramics but use X7R as a default choice because these don't drift much with applied voltage. Here you can see that X7R is the best in those graphed and the bigger body the have, the more stable they are so choose 1210 or 1812: -
Bottom line is RTFD (read the fine detail is one version of this acronym). So read the fine data sheet also.
There are several marked as 5% tolerance, the highest value being 22 nF so use C0G/NP0 dielectric to get decent stability over and above those shown in the graph. They all appear to be used in timing circuits and you don't want those values to change much.
Are there any film capacitors in the circuit that should definitely NOT be replaced with ceramic capacitors? If so, why?
No, they can all be replaced with ceramic capacitors but choose the correct dielectric as outlined above.
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\$\begingroup\$ Note that the physical PCB construction and layout will also be important due to parasitics and other effects - perhaps more important than the capacitor type. With so many variables, be prepared to 'scope the circuit and revise the design, possibly several times. \$\endgroup\$– rdtscCommented Nov 5, 2020 at 13:10
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\$\begingroup\$ @Andy aka - Thank you for your input. As stated in my question, I was already aware about this voltage dependence of the capacitance for ceramic caps. What I was looking for if there are any other factors in addition to this that should be considered. \$\endgroup\$– mr_jsCommented Nov 17, 2020 at 9:50
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\$\begingroup\$ @mr_js I've now explicitly addressed this question: Are there any film capacitors in the circuit that should definitely NOT be replaced with ceramic capacitors? If so, why? \$\endgroup\$– Andy akaCommented Nov 17, 2020 at 9:54
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\$\begingroup\$ @mr_js - are we done with this Q and A now? \$\endgroup\$– Andy akaCommented Nov 18, 2020 at 18:20