reading SMD capacitors in a circuit - testing value

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reading SMD capacitors in a circuit - testing value

Postby Beedoola » 30 Nov 2014, 22:25

I'm getting ready to attempt a trace of a Red Witch Pentavocal tremolo. Its all SMD components. The resistors have the numbers so I'm able to get their values from their codes - I also tested them and got readings close, if not on to their written specs, but the capacitors have no values written on them.

I've measured a few with my DMM on there capacitor reading function. Is it possible to get accurate or close enough readings/values of capacitors when they are in the circuit?

If not, is there any way to determine SMD values of caps within a circuit without removing each and measuring that way?

Thanks.
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Re: reading SMD capacitors in a circuit - testing value

Postby Dirk_Hendrik » 01 Dec 2014, 08:32

- If you can measure in circuit depends on how the cap is used in the circuit. The short answer is no.
- Is there a way to determine w.o. desoldring? Yes, if you have the full circuit and can deduct how to "correct" your measurement. The short answer is... no.
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Re: reading SMD capacitors in a circuit - testing value

Postby bajaman » 01 Dec 2014, 09:44

As Dirk said in the previous post - the short answer is NO!
I have found the only reliable way is to carefully desolder them using soderwick brand desoldering braid and then measuring them out of the circuit. You will most like ly find they have been glued to the board but a careful tap on the side should loosen them, but be warned - they are easily damaged.
SMD capacitors are a real pain in the arse - I have yet to find any with codes on them :evil:
Another method is to carefully slice the PCB trace on one side - this effectively removes the capacitor from the circuit - a sharp craft knife should do the job - after you have measured it you can scrape the solder mask lacquer coating off the trace ends and usually resolder the trace ends back together - push the cut trace ends down with a small flat blade screwdriver helps. :wink:
If you try and measure capacitors in curcuit you will not even get close to their actual values - no easy way sorry :cry:
Good luck
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Re: reading SMD capacitors in a circuit - testing value

Postby Dirk_Hendrik » 01 Dec 2014, 19:33

For what it's worth,
I have found, especially for smaller (<10nF) capacitances, that many capacitance measuring devices are barely up to perform. in the <1nF range it's even (far) worse.
Building a small oscillator with a 4049 CMOS gate used to be an easier trick. Let it oscillate with a couple of "known" capacitances and make a list of those capacitances vs the frequency produced. After that use the same oscillator with the SMD caps with no code and conclude, based on the frequency, what the most likely capacitance is. Yes, with tolerance it may be difficult do differentiate, as an example, between a 1nF and a 1.2nF but that's still better as a Fluke calling all <1nF caps .. 3nF.... or 4 when you move the meter's leads...

When I traced the Ibanez AF2 flanger I used this trick a lot. Desolder, measure in oscillator, resolder.
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Re: reading SMD capacitors in a circuit - testing value

Postby tabbycat » 02 Dec 2014, 02:14

beware too if you go anywhere near those little glued down sm type diodes. the glass is so thin they crumble like biscuits on contact with anything intended to nudge them free. i think bajaman's idea re trace cutting sounds a better idea for them if you need to measure them out of the circuit.
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