Capacitors

[bacteria]

Building this circuit (its a regulator for converting into negative voltage); needs two electrolytic capacitors (no problem, have them, use them often); and it says use two 4.7uf ceramic capacitors. It states that 9.4uf of ceramic capacitors are needed (2 x 4.7uf).

**broken link removed**

Three questions:

1) can I use a 10uf electrolytic capacitor instead?
2) are the ceramic capacitors wired in series or parallel? (series I presume?)
3) what are the differences between electrolytic and ceramic capacitors?

 

[BrownOut]

Have the requirements come from the manufacturer? Is so:

1) Don't. Follow the MFs recommendations.
2) Parallel
3) Goodness. They are two different animals. Google would be a big help here, or someone who more time can write a thesis.

 

[antknee]

3) Electrolytic have polarity - put the negative lead and positive lead in the right way! Ceramics have no polarity put in either way and it doesn't matter.

 

[JimB]

1) can I use a 10uf electrolytic capacitor instead?
2) are the ceramic capacitors wired in series or parallel? (series I presume?)
3) what are the differences between electrolytic and ceramic capacitors?

1/ Probably, but try using a tantalum capacitor, they tend to have a lower impedance at high frequencies.

2/ When capacitors are wired in parallel, their capacitances add together.
When wired in series, the total capacitance will be less than that of either capacitor. The value you get will depend on the relative values of the two capacitors.
Google "capacitors in series".

3/ Electrolytics are good for providing a lot of capacitance in a small space at a reasonable price. But, they dont work well at high frequencies.
Ceramics are good at high frequencies, but dont pack as much capacitance into a small space.

JimB

 

[crutschow]

Electrolytics have higher capacitance for a given physical size, but also have more leakage and don't work as well at high frequencies (higher intrinsic inductance and ESR).

 

[bacteria]

Thanks for the replies, appreciate them - I need to get the right capacitors clearly, could you please confirm if this is the right item I need to purchase - described as "4N7 Ceramic Disc Capacitor 50V 0.0047uF 4.7N 4.7nF" as per 4N7 Ceramic Disc Capacitor 50V 0.0047uF 4.7N 4.7nF". I find this rather confusing, tried to work it out myself and failed, so your help would be appreciated! If you're interested, it's for this: https://www.electro-tech-online.com/custompdfs/2010/06/ptn78000a.pdf

Electronics is very complex subject, as you guys know; my skills are in different areas. If the item above isn't what I need, could I please ask you to post me an ebay link so I can buy what I need?

 

[audioguru]

You need 4.7uF capacitors, not 0.0047uF. The 0.0047uf capacitors have a value that is 1000 times too small and are sometimes called 4.7nF or 4n7 capacitors.

 

[bacteria]

JimB mentioned Tantalum ones, are these suitable therefore: "Tantalum Bead Capacitor 4.7uF 25 Volt" Link: **broken link removed**
**broken link removed**
Are Tantalum as good/better/similar to ceramic?

Before today, I didn't realise there were so many types!

 

[audioguru]

Are Tantalum as good/better/similar to ceramic?

The manufacturer's bulletin has a discussion of which type of capacitor to use and where. It even has a list of recommended capacitors.
They say:

 

[bacteria]

Ok, that put me back to square one. Any chance you could please post me a link on ebay for what I need to buy please, so I can buy them and complete my circuit, pretty please??

 

[audioguru]

Buy name-brand electronic parts from a local electronic parts distributor like Digikey, not cheap Chinese junk on E-Bay.

 

[bacteria]

Digikey are not local, I live in the UK; companies like Digikey charge a stupidly large amount of postage for anything. There are plenty of other suppliers than China for items.

All i'm asking from someone who wants to be helpful to a non-electronics person like me, is a link to the part I need to buy, preferably on ebay so I can get them easily and cheaply. Just a suitable link to the right component, that's all.....please?

 

[antknee]

Go to Maplin because you will be paying £3 in p+p even on ebay. There is a store in every town and the staff are paid and trained to be very helpful. They stock practically everything like that. It'll cost you less than £1.

 

[bacteria]

This is all I could find for Maplin : **broken link removed**

As you see, the pack doesn't have 4.7uf ceramic capacitors, and also, doing a store search, the two stores near me are out of stock anyway; so fail on both counts - also, these items are far more expensive than on ebay. The ceramic capacitors in their pack are 47pf, 470pf, 4.7nf, 47nf - as the only ones with "4" and "7" in them. BTW - ebay are cheap if you spend a little while searching - for example, got 200 recitifier diodes off ebay for about £4 delivered.

I just need an ebay link to the right item please.

 

[Mikebits]

The data sheet list recommended cap part numbers and manufacturers. See attached image.

 

[JimB]

Ceramic capacitors of that value (4.7uF) are not exactly cheap.

In the UK, Farnell have a good selection of capacitors with leads (for a price my boy!), have a look here:

Your Search Results | Farnell United Kingdom

RS Components seem to only have surface mount capacitors in 4.7uF ceramic.
Look here:

Passives | Capacitors | Ceramic Multilayer

JimB

 

[MrAl]

Hello,


Often capacitors like those have what is called a "Ripple Current" rating. The ripple current rating is a rating that is specified by the manufacturer and is an indication of how much ripple current the capacitor can take before its life shortens a lot.
By paralleling two capacitors, each capacitor gets roughly half the ripple current and so that means you can use a lower rated capacitor if you use two in parallel.
Of course there is a little more to it than that, so that say for example your circuit capacitor(s) had to handle 2 amps of ripple current you probably would not want to use two in parallel that were rated for exactly 1 amp each, as you may want to oversize just a bit and go wth two that are rated for say 1.5 amps each just in case the ripple current in each cap does not divide perfectly.

In power supplies, especially switchers, this is a big issue. Use capacitors that are rated too low and they could actually blow up (spewing lots stuff all about the place). Using two or more in parallel and they usually last a long time when the ratings are chosen correctly. In the circuit that was posted they most likely took all this into consideration, so if you wanted to replace two capacitors with a ripple rating of 1.5 amps and 5uf each, you would probably want to use one 10uf cap that had at least a ripple current rating of 3 amps if one was available.

Of course there is also the ESR issue, where two caps can provide a lower value where one cap may not be able to.

As Brownout had said, if the manufacturer recommends two caps then you had better use two or else know how to calculate the requirements of a single cap that would replace the two.

So the short story is that when two caps are paralleled it's not only the capacitance that is affected but also the ripple current rating
as well as the overall ESR, both of which are VERY big issues in power supplies.

 

[bacteria]

Thanks, i've learnt a fair bit today about this. I appreciate the input guys.

On Friday i'm going to see if I can buy some of the SMD caps from RS Components, they are about 10 miles from where I live. Murata 4.7uf ceramic caps X7R, 50v. I only want to put 7.5v-12v through them, but the 50v ones are far cheaper than 25v ones; and the SMD ones cost about £0.43 each as opposed to about £12 each for the upright ones (!).