SMT question....

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eyevancsu
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SMT question....

Post by eyevancsu » Thu Mar 27, 2003 3:33 pm

I was wondering how you can determine the values of smt capacitors, the ones i have have no code or markings. Any help will be appreciated.<p>thx,...ivan

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Externet
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Re: SMT question....

Post by Externet » Thu Mar 27, 2003 4:40 pm

Hello Ivan<p>Surface mount components with not much room to print have the markings on the reel or package where they are supplied.
They were not meant to be serviceable, they are aimed for manufacturing.
Capacitors are the ones mostly with no markings; you should get a capacitance meter to discern their value.
Transistors and diodes are marked per manufacturer, you need to know the manufacturer to find out the characteristics and data sheet.<p>Sad, but that is reality. Surface mount components are not hobbyst friendly; are designed to be handled and mounted by robots.<p>Miguel
- Abolish the deciBel ! -

Dimbulb
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Re: SMT question....

Post by Dimbulb » Thu Mar 27, 2003 4:44 pm

Please give some other details:<p>What is the physical size ?<p>Are there any other markings or labels given ?<p>Do you have a capacitance meter ?<p>Are you able to hold the parts securely by it's contacts ?

eyevancsu
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Re: SMT question....

Post by eyevancsu » Thu Mar 27, 2003 6:39 pm

they are approximately 1mm and under, no markings, no meter.<p>thx

Dean Huster
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Re: SMT question....

Post by Dean Huster » Thu Mar 27, 2003 6:39 pm

About the only SMT caps that are marked are the black tantalum jobbies with the chamfer on the positive end. The other are ceramic types and I have yet to see one marked. They mark resistors, and they could mark the caps if they were the same size as the resistors, but they're usually a heck of a lot smaller.<p>The best way around this problem is to use a digital capacitor meter to measure their value as mentioned previously. But don't use the actual reading as the "marked" value. If it comes up as 0.013µF, figure that it's probably inserted as a 0.01µF cap and store it accordingly. If you don't, you'll end up with billions of "weird" values to deal with.<p>I made up a couple of special clips for my digital cap meter for this exact problem. I modified an alligator clip by soldering some thin double-sided PCB material onto one of the jaws and then soldered a very flexible wire (Litz wire is good for this) to the side of the PCB directly opposing the other clip jaw. A banana plug was soldered onto the wire end of the clip and plugs into the common or negative terminal of the meter. The wire goes out to a short "flying" lead with a banana plug on the end for connection to the positive terminal of the meter. This makes for a nice low-capacitance holder for the chip caps. Use tweezers to insert the ends of a chip cap between the jaws and make a reading.<p>With this clip, zero the meter by inserting a thick piece of plastic about the same thickness as a typical chip cap between the jaws. That'll adjust out most of the small stray capacitance that remains from the clip.<p>Another handy tool I made was doing something similar with a pair of tweezers, except I used some RG-174/U coax cable for the wiring. Both jaws had a little 0.1" square piece of double-sided PCB material soldered to the ends (make sure the tweezers are of a solderable metal and not stainless steel) with either side of the coax soldered to the opposing PCB pads. the other end of the coax has standard banana plugs attached, black to the shield, red to the center conductor. With these tweezers, I can pick up a chip resistor and immediately read the value on a DMM and then deposit it in a sorted pile. No hassles with trying to use probes and handle the little SMT parts. This tweezer has a lot more stray capacitance than the previous clip modification, so can't be used with small values of capacitance without error. It can still used to sort the larger values and values that measure less than 0.001µF can be put in a separate pile for later measurement with the better clip.<p>It's little tricks like this that help to make SMT a bit more manageable for hobbyists.<p>Dean
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

eyevancsu
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Re: SMT question....

Post by eyevancsu » Thu Mar 27, 2003 6:41 pm

thx alot guys for the help!!<p>ivan

Dimbulb
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Re: SMT question....

Post by Dimbulb » Sun Mar 30, 2003 1:42 pm

The meter makes it more difficult.
I think the size is refered to as 805
very small.<p>alot of those that size are in the pF range.<p>I built an fm transitter and pluged several known fixed value pico Farad capacitors in place of the variable capacitor and using formula found the inductor value. <p>Using formula I made a table with pico Farad and frequency. I carefully marked the dial on a transistor radio in pico Farad instead of frequency. It tells me to very precisely what value the capacitor is when it is tuned to the transmitter. It beats my good cap meter for pF if I read it with a magnifying glass as my eyes aint what they used to be.<p>If it were me I would solder the 805 to a small copper board or across traces on piece of scraped board.

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