Finding the value of an unknown capacitor

[AMyatt]

I have a device I am trying to repair and have discovered that three of the capacitors (2 electrolytic and 1 ceramic) have burned out.
The resistance across each electrolytic is 2.8 - 3 meg ohm (out of circuit) and the ceramic is 80 ohm (in circuit).

I have been able to order replacement electrolytics as the value was written on them, the problem is the smt ceramic cap has nothing written on it, so I don't know what to replace it with.

It is part of a power supply circuit which has five identical sections (two electrolytics and a ceramic in parallel, followed by an inductor), so I could remove one of the working ceramics from the neighbouring circuits and test it, but I don't have a capacitor tester.

Is there another way I can test the value of one of the working caps?


- Anthony

[fcb]

If you've got a 'scope you could charge the unknown capacitor via a 'known resistor' from a 'known voltage' and measure the time it takes to rise a 'known amount'.

There are plenty of online calculators that will help you with the math.

[uwezi]

Is there another way I can test the value of one of the working caps?

Take a 555 and build an astable multivibrator using the capacitor as timing element. If you have an oscilloscope then you can use any combination of timing resistors R_A and  R_B, measure the ON- and/or OFF-time and do the math. Otherwise just choose a large enough value for the resistors so that you can time the ON- and/or OFF-time of an LED attached to the output with a watch or a stop watch.

For the circuit and math see the datasheet and e.g. http://www.instructables.com/id/555-Timer/step6/555-Timer-Astable-Mode-Circuit/

[fluxcapacitor]

What is the device you are trying to repair,do you have make/model number ,there might be a schematic for it .

[grumpydoc]

Most decent multimeters have capacitor measurement so it sounds like it's time to upgrade your DMM.

Otherwise I think fluxcapacitor's idea will be the most reliable.

[w2aew]

I generally don't trust in-circuit measurements - there are too many variables that can affect the results.

Here's a video I did that shows a simple way to test caps with a scope and some basic parts.

[AMyatt]

Thanks everyone, I ended up using my oscilloscope, a 1k resistor and a 5v bench supply.

Assuming a measurement to 60% charge, this yielded a time of 10ms.
Which according to this online calculator it is ~11?F.

[fcb]

More likely 11uF (or 10uF value) than 11F

http://mustcalculate.com/electronics/capacitorchargeanddischarge.php?vfrom=0&vto=3&vs=5&c=&r=1000&time=.011

[AMyatt]

Yes. Whoever set up the forum, or perhaps the software devs who wrote the forum platform appear to have goofed on the MYSQL character set and therefore my greek mu character isn't displaying properly.
My last post should have read; "it is ~11uF". Which as fcb pointed out is most likely a 10uf which would make sense.

I have ordered a 10uF 25v 1206 capacitor. Fingers crossed

I think it might be time to order a decent multimeter when I can afford one... my Jaytech is slowly becoming useless as I delve deeper into electronics.

[Swake]

the smt ceramic cap has nothing written on it, so I don't know what to replace it with.

I have ordered a 10uF 25v 1206 capacitor.

I might be wrong, but in my eyes the word 'ceramic' and the above capacitor values do not fit together.
Could you post a pic of the cap to be replaced please?

[Psi]

na, a 10uf 25v SMD ceramic is common and in the 'a few cents' price range.
But it does start to get expensive pretty soon after that though.

You can get up to ~100uf SMD ceramic caps but they're usually under 16V
Anything more than that and you start getting into super expensive 'stacked' ceramic arrays.