Question on smd parts

[Oscman]

How do you tell what the properties of a component are when they are smd and no information printed on them. An example would be a capacitor that is in a small package of yellow or whatever?

 

[Osvaldo de Banfield]

This is a very unfortunate thing. I have the same problem almost all days, repairing industrial electronics. The only way is to take one and measure it. The voltage ratings are almost always unknown, I usually use smd components.

 

[rayma]

How do you tell what the properties of a component are when they are smd
and no information printed on them.

It's best to discard them and use known, bagged parts from a reliable vendor, like Digikey.

 

[Oscman]

Just so I understand this:

So, When repairing a board you find a cap is bad (smd) and its just a plain yellow or orange packaging with nothing on it - your out of luck unless you have a schematic?

So its kinda like one step further than not letting you have a schematic for something in for repair. They now (or have been) hiding the value of the caps in smd parts.

Is that a fair assumption on my part?

 

[rayma]

When repairing a board you find a cap is bad (smd) and its just a plain yellow or orange packaging
with nothing on it - your out of luck unless you have a schematic? So its kinda like one step further
than not letting you have a schematic for something in for repair. They now (or have been) hiding
the value of the caps in smd parts.

Pretty much. If it's stereo, you can measure the corresponding cap in the other channel.

 

[Osvaldo de Banfield]

If you have other similar parts in the PCB, usually take one without doubts and measure it. It is common that, for example, a bypass capacitor, the circuit have many equal units around the PCB.

 

[BinaryMike]

It seems bizarre that SMD resistors generally have markings but capacitors do not. Is this because laser marking would destroy multilayer ceramic caps?

 

[rayma]

It seems bizarre that SMD resistors generally have markings but capacitors do not. Is this because laser marking would destroy multilayer ceramic caps?

It's mostly cost reasons. Some are marked. http://www.niccomp.com/catalog/storage.pdf

 

[nigelwright7557]

Some smd parts are too small to have much info on them.

I keep mine in the bags they came in that have the value marked on them.
All the smd bags are kept in a large project bag with the projects name on it.

 

[benb]

This is a very unfortunate thing. I have the same problem almost all days, repairing industrial electronics. The only way is to take one and measure it. The voltage ratings are almost always unknown, I usually use smd components.

You can make a guess on voltage (or decide the minimum it needs to be) by looking at the voltage across the cap in the circuit, or the voltage used to power the circuit. Assume it's at least the next voltage rating available above the actual circuit voltage.

It's best to discard them and use known, bagged parts from a reliable vendor, like Digikey.

He's talking about unknown, likely faulty parts pulled off a PCB, and trying to figure out a replacement.

Just so I understand this:

So, When repairing a board you find a cap is bad (smd) and its just a plain yellow or orange packaging with nothing on it - your out of luck unless you have a schematic?

I wouldn't say totally out of luck, but it's certainly a bigger challenge. Others have described how you might discern the part's value.

So its kinda like one step further than not letting you have a schematic for something in for repair. They now (or have been) hiding the value of the caps in smd parts.

Is that a fair assumption on my part?

They're not "hiding" the value (it's not part of a conspiracy to make things harder to repair even if it does have that effect), it's just too much trouble to print the value on them, especially when they're so small. The value is on the tube or reel the part comes in, the part gets automatically put on the PCB in the right place, so (at least in manufacturing) no one has to look at the part to determine what it is.

 

[Colin Benton]

If possible, you can take photos and search on alibaba, or get help from the suppliers in it.

 

[Osvaldo de Banfield]

You can make a guess on voltage (or decide the minimum it needs to be) by looking at the voltage across the cap in the circuit, or the voltage used to power the circuit. Assume it's at least the next voltage rating available above the actual circuit voltage.

Most of the times, this is true, but not always. Some time ago I had a corroded SMD cap because of capacitor plague, in a circuit that was an elementary blocking oscillator as a SMPS. This cap makes the circuit to work (Provides positive feedback and timing to the circuit), an is of a very special value. And if the cap is removed, the circuit changes dramatically the way of working, including power switch (MOSFET in this case) to blown, as the device becomes class A without switching. So, there is no option to measure the voltage across it. Cut and try is the only option.