SMD capacitor

[Bahmed5]

I have a PCB that had faulty smd capacitors and I'm not able to get a schematic however I have another good working board whereby I'm able to get the uf measurements from the caps, how do I determine what value voltage smd capacitors I should get it just get highest possible.

[martin1454]

No, dont just put any caps on it.

Use a LCR meter to measure the cap from the good device.

Or if you cannout figure out the value (If you have no LCR meter):

I would look into the circuit, and try to estimate what it does - And if I find that it is a decoupling cap, I would go for 100nF, but if it is a series capacitor or a filter cap, I would look into what frequencies we are working with. If it is a load capacitor for a crystal oscillator, I would look into the crystal.

Look what it is connected to and try to draw a schematic for the nearby components


EDIT: As below was noted, it is just the voltage, not the capitance you are looking for -
If you can, power on the working device, and measure the voltage across the working cap - you rated voltage should be 1.5X the voltage you measure at least.

[Kjelt]

Most SMD capacitors still have their capacitance and voltage printed on the device.
Although sometimes a bit cryptic. From the datasheet you can often tell more, the codes on the cap will give clues to the series.
Can you post a picture , we might be able to tell you.

[wraper]

Most SMD capacitors still have their capacitance and voltage printed on the device.

If it was tantalum/electrolytic/polymer, I doubt this thread would be here to begin with. Most of SMD capacitors are MLCC with no marking.

[PKTKS]

Most SMD capacitors still have their capacitance and voltage printed on the device.
Although sometimes a bit cryptic. From the datasheet you can often tell more, the codes on the cap will give clues to the series.
Can you post a picture , we might be able to tell you.

geezzz i have a bunch failed crappy MLCC without any marks...

pure guessing .. in particular if they are cracked or burned badly..
they are just tiny little crappy things without marks ...

dangerous ones.

Paul

[PA0PBZ]

Guys, OP is asking about the voltage rating, not the capacitance. Read his post again...

[wraper]

Power it without a cap (or power up another working board) and measure voltage or select accordingly to maximum voltage available in the circuit. Knowing what kind of circuit you are talking about would help a lot.

[Kjelt]

Most SMD capacitors still have their capacitance and voltage printed on the device.

If it was tantalum/electrolytic/polymer, I doubt this thread would be here to begin with. Most of SMD capacitors are MLCC with no marking.

Yup my bad, I never had those failing in my products, only electrolytics by the hundreds that failed.
A picture would have helped 

[Bahmed5]

Hi,
Im working on a sonos soundbar pcb board I have attached pictures of the board and removed a few caps to make it easier to view, I know the UF value of the tanatalum capacitors but dont know what voltage value I need to get, i need to know the value of c24, c26 and c27, Im a beginner at this stuff so basic explanation would be helpful guys,
Appreciate any help you can give.

[Kjelt]

Ok so you have a good working board and determined the capacitance of the capacitors from that. 
There is no way of knowing the exact voltage rating without measuring it in circuit.

So what I would do is solder two very thin teflon or other insulated wires across the capacitor to a voltmeter/multimeter and power it up.
For a good rule of thumb derating multiply the voltage by two and get the nearest voltage you can get in the same component size, usually they are 10V / 16V / 25V / 35V / 50V / ..... / 100V

[mzzj]

No, dont just put any caps on it.

Use a LCR meter to measure the cap from the good device.

Or if you cannout figure out the value (If you have no LCR meter):

I would look into the circuit, and try to estimate what it does - And if I find that it is a decoupling cap, I would go for 100nF, but if it is a series capacitor or a filter cap, I would look into what frequencies we are working with. If it is a load capacitor for a crystal oscillator, I would look into the crystal.

Look what it is connected to and try to draw a schematic for the nearby components


EDIT: As below was noted, it is just the voltage, not the capitance you are looking for -
If you can, power on the working device, and measure the voltage across the working cap - you rated voltage should be 1.5X the voltage you measure at least.

Selection based on measured capacitance and measured voltage is a good start but the headache starts when you consider the different MLCC materials and voltage coefficient.
If you guess your replacement wrong you might end up with 80% less capacitance under bias voltage compared to the  original even if voltage rating is "sufficient" and measured capacitance matches the original one.