Choose capacitor.

Other Hardware General Electronics
1

Hello to all. Sorry for my poor English.
Below picture is the voltage between two across of capacitor that is 470uf.

Actually this is a simulation of it.
Does it burst if I want to use it in really situation.
How usually choose the amount of capacitors.
Another question. In somewhere I have seen that they use 2 capacitor parallel together instead of one. what is the reason?
Very thank you.

2

A capacitor by itself won't cause a voltage spike. A spike or ringing is usually caused by capacitance and inductance together.

A capacitor (in parallel with the source or load) tends to smooth-out the voltage. If you add a resistor, you have a [u]low-pass filter[/u].

[u]Here[/u] are capacitor charge and discharge curves.

In real-world high-speed circuits (such as a microprocessor data or address line) the inductance on the circuit board traces can sometimes cause ringing or overshoot, and sometimes it can be caused by an oscilloscope probe so it's hard to know if it's there when the 'scope isn't connected (unless the circuit fails whenever the 'scope is connected).

Is there something else in your simulated circuit, or just a battery or "ideal" power supply and a capacitor?

In a power supply, the power supply itself has low source resistance so the capacitor charges quickly. There are diodes so the capacitor only discharges into the (lower resistance) load, so the capacitor discharges more slowly than it charges.

Another question. In somewhere I have seen that they use 2 capacitor parallel together instead of one. what is the reason?

If you see two different types of capacitors in parallel (such as a 0.1uf ceramic and a 470uF electrolytic) that's because at very-high frequencies electrolytic and tantalum capacitors no longer 'act like' capacitors. It's possible that an added ceramic capacitor in your circuit would reduce or eliminate the ringing.

Otherwise, capacitors in parallel sum (resistors sum in series) so two 470uF capacitors in parallel is 940uF.

3

Hi,
What is the y scale, Volts?
What is the x scale, Seconds, mS, uS ? ? ?
Sorry but at the moment your trace is meaning less.

Can you post a picture of the simulator circuit please.
What do you do to the capacitor to get this voltage trace?

Thanks.. Tom.. :slight_smile:

4

have seen that they use 2 capacitor parallel together instead of one. what is the reason?

Two capacitors in parallel is equal to the one capacitor of a value which is the sum of each individual one.

However, these capacitors can be placed in physically different places despite being topological just in parallel. This is done with supply decoupling to distribute the effect across the whole circuit.

5

TomGeorge:
Hi,
What is the y scale, Volts?
What is the x scale, Seconds, mS, uS ? ? ?
Sorry but at the moment your trace is meaning less.

Can you post a picture of the simulator circuit please.
What do you do to the capacitor to get this voltage trace?

Thanks.. Tom.. :slight_smile:

This is a capacitor that uses in a Buck converter about 400 watts. Input voltage is about 70 volts and I want to control its amplitude,I want to control the speed of motor by tuning the amplitude of voltage.
y scale is Volts and X scale is Seconds.
Very thank you.

6

This is a capacitor that uses in a Buck converter about 400 watts.

I would suggest given the questions you are asking that this project is way out of your league. Its certain out of my league I would not attempt to make that and I have had 50 years of experience in electronics. You need to know a hell of a lot more before you could make one of these.

7

Hi,

  • What is the part number of the buck converter?
  • Links to data/spec.
  • Do you have a circuit diagram of the converter?
  • What type of motor.
  • Links to data.spec.

Tom.. :slight_smile:

8

Grumpy_Mike:
Two capacitors in parallel is equal to the one capacitor of a value which is the sum of each individual one.

However, these capacitors can be placed in physically different places despite being topological just in parallel. This is done with supply decoupling to distribute the effect across the whole circuit.

Well if you take into account ESR, self resonant frequency and all this stuff used in filtering high frequencies two capacitors are very different than one. I don't understand this topic so much to be able to say if two caps are better than one but I have seen people saying you need many caps of different values to get "proper" power filtering.

9

but I have seen people saying you need many caps of different values to get "proper" power filtering.

Which is indeed what I say in my decoupling page http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html

Given that we were not given any context for the question it is hard to say without looking at the specific situation they were used in.

One other use of two capacitors is in oscillator circuits where one has a positive temperature coefficient like a ceramic and the other has a negative temperature coefficient like a mica and by careful design of the relative sizes the two combined are very stable with temperature.