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Old 25th December 2007, 08:29 PM   #1
mtl777 is offline mtl777  United States
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Default Capacitance Meter Questions

Sorry for this newbie question. Can I use an ordinary capacitance meter to measure the values of SMD capacitors in-circuit or do I need to buy Smart Tweezers...

What, if anything, makes the Smart Tweezers different that it is able to measure capacitance values in-circuit (assuming ordinary capacitance meters can't do that, as the Smart ads would lead me to believe)? The Smart is too pricey and I hope to be able to get by with an ordinary capacitance meter using its standard test leads.
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Old 26th December 2007, 02:47 AM   #2
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Default Supertweezers

Cute product but no..... these devices and all other meters cannot be used "in circuit".
Remember if measuring capacitance OR resistance values while still soldered onto a printed curcuit board you will get aggregate values.....which will be way off of the value you are expecting.
A circuit is made up of various values at various places...all these components work together for a final outcome. One must desolder at least one side of a capacitor or resistor to test it.....that being said surface mount devices are WAY small for human fingers and are virtually unserviceable.......I'm sure there are specialty tools for SMDs' but surely they are spendy and useless for any other application.
$299 USD is an insult as that amount will get you a slew of test gear.
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Old 26th December 2007, 03:48 AM   #3
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Location: Canandaigua, NY USA
No, you don't need any special tweezers for in-circuit measurements- I just use some sharp probes. If you have a schematic, lots of caps can be measured in-circuit, but as Richard says, you have to watch out. Some will give totally erroneous readings. You just have to know what they hook to, and estimate how badly that will throw off the readings. Remember, you're usually looking for serious problems in this mode, not trying to fine a .2dB error in your RIAA curve. Use the C meter. Use the ohm meter (not on diode check) and see how long it take to charge up. Use another cap of similar value in parallel and see if that makes the circuit start working. Heat it. Cool it. Vibrate it. Use everything in your bag of tricks, including the C meter, then add up all the results to come to some educated guess as to the condition of the cap.
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Old 26th December 2007, 06:23 AM   #4
mtl777 is offline mtl777  United States
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Thanks for the heads up, guys! You just saved me a bunch of money. I'm glad to find this forum!
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Old 26th December 2007, 06:53 AM   #5
Nordic is offline Nordic  South Africa
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Just on the side, I have never been able to get readings from SMD caps (ceramic), that were anywhere close to what it says on the package... in or out of circuit...
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Old 26th December 2007, 02:05 PM   #6
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Nordic, there are some really wide tolerance bypass caps that can look pretty awful, but in any case you should get a reading that's within the published tolerances. At work we routinely buy 1% SMT parts and they always measure within 1%. If you don't have a fixture, set the part on a completely non-conductive surface (not an esd mat) and keep your fingers off the probes. I don't like SMT parts because I can't see them very well, but they should perform just like their through-hole ancestors.
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