Capacitor Selection

[Guy]

Hi,
does it matter which types of capacitors are used when building a circuit?

If I find a capacitor of any type but with the right value can I use it.

I want to replace some big round electrolytic capacitors with some smaller
ones. (tantalums or polyesters it think)(For a MAX232 circuit)

Does anyone have any advise for genaral capacitor selection?

Thankyou

Guy

 

[4electros]

selecting capacitor

the most important thing when you select a capcacitor is to pay attention of the voltage( maximum voltage) .... if the voltage applied on the capacitor is over its maximum voltage, it'll fuse and burn and may be affect some other elements in the circuit.

You have to consider polarization of the capacitor in the circuit, especially in rectifing circuits..

I wish I didn't forgot anything


good luck!

 

[Guy]

Ok i will remember that

I will check the direction/voltages and give them ago...

Thanks

Guy

 

[JimB]

Guy

For the MAX232, I think any capacitor of the correct value and voltage rating will be OK.

However, for other circuits, paricularly those with high frequencies, the construction of the capacitor becomes critical.

Some capacitors have inductance due to the wires connecting them to the circuit and the way they are made, they are rolled up like toilet paper! :lol:
These types are very poor at high frquencies.

What is a "high frequency"?
We could argue about this for pages on here, but I am going to say anything above 50kHz is high for the purpose of this discussion.

JimB

 

[Thunderchild]

just a little side note. it does not come up often but is worth looking out for: operating temperature. jusat make sure that u are ok there most stuff uses standard 85 °C (i think) but some do have 130°C (again i think) so just take a look at that factor u never know

 

[audioguru]

The MAX232 datasheet shows 1uF capacitors. You can use cheap electrolytic (new ones are tiny), expensive tantalum or ceramic, or huge and expensive plastic film caps.

I would use cheap electrolytic caps for the low-frequency circuit. I have some 1uF/35V caps that are 40 years old and are as big as my thumb (I should throw them out) and some new tiny ones about the size of a piece of tooth-pick. :lol:

 

[4electros]

just a little side note. it does not come up often but is worth looking out for: operating temperature. jusat make sure that u are ok there most stuff uses standard 85 °C (i think) but some do have 130°C (again i think) so just take a look at that factor u never know

really!! Is the operating temperature an important factor in selecting capacitors? I haven't heard about that before!!


could you explain more , if that's possible!! :wink:

 

[Thunderchild]

it's as plain as i said for a few applications a capacitor that will take a higher temperature than standard ones is used, i was just saying make sure this is not one of those before bunging in a standard one :wink:

 

[Styx]

just a little side note. it does not come up often but is worth looking out for: operating temperature. jusat make sure that u are ok there most stuff uses standard 85 °C (i think) but some do have 130°C (again i think) so just take a look at that factor u never know

really!! Is the operating temperature an important factor in selecting capacitors? I haven't heard about that before!!


could you explain more , if that's possible!! :wink:

Yes it is. The voltage capability of a capacitor decreases with temperature and when it exceeds its rated temp then the dialectric breaks down and POP.

I have some tants on one of my boards they are +125C rated. At 20C they are 35V, they then decrease linearly to 25V at +125C so yer watch the temp rating.

Equally IF you have high ripple current the ripple-current capability reduces with temperature (since ripple current heats the cap)

 

[Thunderchild]

oh thats news to me i only knew there was a max temp i didn't realise there was such a relationship :roll:

 

[Styx]

oh thats news to me i only knew there was a max temp i didn't realise there was such a relationship :roll:

Well it depends on the dialectric

COG,NPO... ie low-capacitiance high precision dont really have such an effect

BUT high capacitance elctrolytics (Tant based,aluminium,poly based) do

 

[4electros]

max232

excuse me friends... what is MAX232

 

[DigiTan]

It's Maxim-Dallas's **broken link removed**. 'Does data in both directions, and is extremely useful if you ask me. There's also a Sipex SP232 that is pin compatible and has (mostly) the same specs. I had a lot of luck with both monolithic and tantalum capacitors for the charge pump--and it didn't seem too sensitive to the swap.

 

[Nigel Goodwin]

really!! Is the operating temperature an important factor in selecting capacitors? I haven't heard about that before!!

Modern electrolytics are really rubbish!, we no longer stock 85 degree ones at work, we only stock 105 degree ones instead. These seem a little more reliable, for very little extra cost.

A lot though depends on the application, and the modern trend for small size, low cost, and using less suitable materials (on either cost or environmental grounds!), has dropped quality massively.