Quick Newbie Question

[stcogolin]

Hi, I need 30pF capacitators so I ripped apart an old appliance
and found very small ceramic caps (about 3mm diameter) whose
code is simply 30 (some are also inscribed with 27).
Is this value in pF or uF? How much voltage can they sustain?
I searched the web for cap codes but couldn't find anything.

 

[k7elp60]

Capacitors

I think they are in pf, that is 30pf and 27pf. :lol:

 

[Optikon]

Hi, I need 30pF capacitators so I ripped apart an old appliance
and found very small ceramic caps (about 3mm diameter) whose
code is simply 30 (some are also inscribed with 27).
Is this value in pF or uF? How much voltage can they sustain?
I searched the web for cap codes but couldn't find anything.

They sound like ceramic disk types. If so, you can safely assume they can withstand at least 1000V and more likely 3000V
Ceramic material is very good at withstanding high volts.

 

[mozikluv]

ceramic caps



you said those caps came from an old appliance, are they still good? :wink:

 

[stcogolin]

Thanks for the replies.
I will assume they are in pF.
I don't know if they are still any good, what is
their life expectancy? Is there a simple way to
measure capacitance without using ac and an
oscilloscope?

 

[Nigel Goodwin]

Thanks for the replies.
I will assume they are in pF.
I don't know if they are still any good, what is
their life expectancy? Is there a simple way to
measure capacitance without using ac and an
oscilloscope?

It's extremely doubtful that a 30pF ceramic capacitor would be faulty, they are very reliable components - if they aren't actually cracked or broken they will almost certainly be OK.

There's lots of ways to test capacitors, but a 30pF one is certainly going to require AC to do so.

 

[stcogolin]

There's lots of ways to test capacitors, but a 30pF one is certainly going to require AC to do so.

So how would I go about measuring larger capacitances without
a scope?

 

[Nigel Goodwin]

There's lots of ways to test capacitors, but a 30pF one is certainly going to require AC to do so.

So how would I go about measuring larger capacitances without
a scope?

The classic way is a bridge, you compare it to an existing capacitor and adjust the bridge until it's balanced - but you need an AC source, and some kind of AC indicator (a crystal earpiece will do).

You can use the capacitor in an oscillator circuit, measure the frequency produced, and calculate it's value.

You can also charge it with a known current and measure it's charging time, and calculate it's value - but this is difficult with small capacitors.

Probably the easiest for you to build is a simple bridge, with a 555 for the oscillator and a crystal earpiece for the indicator (adjusting for zero tone in the earpiece). The bridge itself consists of a linear potentiometer, a known capacitor value, and the test capacitor. You can also use it to measure inductors and resisitors, by simply replacing the known capacitor with an inductor or resistor.

I've got a nice simple 555 bridge circuit in an old 555 book at home somewhere!.

 

[stcogolin]

Thanks Nigel, i'll check that out.