Electronic Product Design » Capacitors

RC Filter

admin — Thu, 17 Jun 2010 22:27:39 +0000

RC calculation

T=RC, so:


          Time

  Resistance   Capacitance


      mS	  

  Kohms   uF


    Seconds

  ohms      F

The result gives you 1T. This is 1 time period. So for 1000uF and a 1K resistor T = 1000mS

For charging and discharging a capacitor:

1T 63%
2T 86.5%
3T 95% (this tends to be the default usage with 95% considered basically there)
4T 98% (this tends to be considered as 100% charged / discharged)
(in theory 100% is infinite)

Voltage is irrelevant to this. If a 12V supply was connected via 1K to a 1000uF cap it would take 1000mS to reach 7.56V, 3000mS to reach 11.4V. Its exactly the same for discharging a capacitor.

Ceramic

admin — Thu, 17 Jun 2010 22:24:29 +0000

Multilayer ceramincs are great for applications where voltage is constant (ie. your smoothing off a voltage). However they are not good for use inline for AC voltages (i.e. audio and AC signal filters). The reason is that ceramic capacitors will give a piezo electric effect to an AC signal, which causes noise. For audio use a Poly of some type (Polypropelene is best for audio signal pass through).

X7R, X5R

Better than Z5U and Y5V. Typically maintain a capacitance range within +-20% of nominal over the full operating voltage and temperature ranges. As a result they are typically larger and more expensive than Z5U and Y5U types.

Z5U, Y5V

Capacitance drops off severely with applied voltage. A typical Z5U or Y5V capacitor can loose 60% of its rated capacitance with half the rated voltage applied to it.

Z5U and Y5V also exhibit a severe temperature effect loosing more than 50% of nominal capacitance and the top and bottom ends of the temperature range.

Tantalum Capacitors

admin — Thu, 28 May 2009 11:27:21 +0000

Tantalum capacitors have a couple of important design advantages:-

They can be excellent at removing noise (for instance placed across power rails) and improving the EMC performance of a circuit.
They are physically small

The main downside is that they are more expensive than say an equivalent electrolytic capacitor.

Voltage

It is important to derate tantalum capacitors voltage by 50% (e.g. if placing across a 0V and +5V supply rail select a 10V or higher rated tantalum capacitor). The reasons for this are complex, but it was discovered when first developing the capacitor that it needed to be used for a significant period of time to reveal any faults and their life was greatly extended by derating down. This is one of those rules you should just follow as a designer to ensure the longevity of your manufactured hardware.

Faults

Tantalum’s can cause really strange problems if they fail or are inserted the wrong way round. If you’ve got something very odd going on its often a good idea to check and even try removing tantalum capacitors to see if they are the cause.