We have 470pF 500VDC caps (Kemet part number C1206C471KCGAC Ceramic, SMD, MLCC, Ultra-Stable, Low Loss, Class I, 470 pF, +/-10% Tol, 500V, C0G, -55/+125C, 1206, 0.1% DF, Dielectric Strength=750V) on 110VAC 400Hz signal and chassis ground (I think for EMI purposes after main design was completed) in our Controller circuit. This should be okay. Is it or can it cause problem since these caps are DC rated? What would happened if they mistakenly placed 100VDC rated caps with the same spec in place of 500VDC caps?
DC rated caps for AC signal
accapacitordc
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Here's such a device, with some diagrams: the MAX1474.
The MAX1474 is a fine-line (geometry) electronically trimmable capacitor (FLECAP) programmable through a simple digital interface. There are 32 programmable capacitance values ranging from 6.4pF to 13.3pF in 0.22pF increments. The quartz dielectric capacitance is highly stable and exhibits a very low voltage coefficient. It has virtually no dielectric absorption and has a very low temperature drift coefficient (<33ppm/°C). The MAX1474 is programmed through two digital interface pins, which have Schmidt triggers and pulldown resistors to secure capacitance programming.
First let me say that I agree with all the comments made (Matt, Brian et al).
My concern about this question is safety - mains voltage can and does kill.
There are good reasons why only fully qualified and trained technicians and engineers should attempt to work with mains voltages in projects. The fact that so many questions are being asked about the comments made above, together with the poor quality of drawing and a fundamental error in drawing a simple bridge rectifier, does not fill me with confidence.
(1) The bridge rectifier you have drawn is wrong and will not work.
(2) A water heater element is electrically isolated from its metal casing and normally fitted with some form of thermostat (usually a bi-metallic switch) to prevent it getting too hot. The metal casing surrounding the element must be suitably earthed to prevent shock in the case of a fault.
(3) Both DC and AC are able to give electrical shock. At 220V either can be fatal. (I'm certain we all remember the fate of topsy the elephant - http://en.wikipedia.org/wiki/Topsy_%28elephant%29 )
(4) A simple circuit only requires a single break to prevent current flowing. For AC the switch is usually placed in the 'live' wiring as the 'neutral' is considered to be near earth potential. An example of this would be household light switching.
(5) Isolation switching is when the live and neutral lines are both switched off. The first switch to open switches the circuit off. It is not critical that both switches operate at exactly the same time. If you think the supply is shorting through the water then there is a serious (and dangerous) electrical fault.
(6) There is no advantage in rectifying the AC supply to feed a resistive water heating element. Each half cycle will heat the water and the the mass of the water will 'smooth' out the heating effect. It will not make it safer. Switching the element on and off is best done at (or near) the zero crossing of the input to reduce surge current. This can be done with a suitably rated opto DIAC/TRIAC (solid state) switch. For example -
Best Answer
The dc rating should be the same as the peak ac rating. A 110Vac supply may be expected to be as high as 125Vac and the peak voltage seen could therefore be approaching 180V so no, I would change the caps rated at 100V to higher voltage ones such as at least 250Vdc.