smt capacitor kits

[eccerr0r]

Anyone have a good suggestion for a "starter kit" for stocking up on a few common values/sizes of surface mount electrolytic capacitors? Say, 1uF to 220uFs or so. I'm just so mad that I have a pile of through-hole caps that I stock over the years, just no SMT's.

I guess the quality is not a big concern, I'd not be unhappy with a pile of CapXons but rather avoid a pile of Fuhjyyus... but I'm not sure I'm willing to pay for a set of Rubycons.

Perhaps my volume needed is low enough that I should bite the bullet and buy on demand but I hate the wait time, higher chance of having yet another opened, unfinished project sitting on my floor waiting for me to trip over it and permanently destroying it. Actually maybe that would be a good thing.

[stj]

Re: smt capacitor kits

i dont use them, i replace them wih ceramics and tantalums.

[eccerr0r]

Re: smt capacitor kits

Those 47uF ceramic and tantalums get quite pricey if replacing all caps in a device, though if it were a device I really want to last a while it might be worth it...alas most aren't...

[stj]

Re: smt capacitor kits

i never buy them, just strip them from scrap pcb's

[eccerr0r]

Re: smt capacitor kits

I need to collect more scrap PCBs I guess
I sacrificed a CDROM drive unfortunately, but that doesn't have enough caps...

[stj]

Re: smt capacitor kits

dead motherboards are great for ceramics - specially around the cpu socket

[eccerr0r]

Re: smt capacitor kits

Hmm... know what, I've never wondered what kind of capacitances you can get from those ceramics near the CPU sockets... Of course they aren't marked but what capacities are there typically?

[mariushm]

Re: smt capacitor kits

0.01uF , 0.047uF , 0.1uF , 0.47uF, typical values for decoupling.
1uF , 10uF max (the larger ceramics) for filtering some of what goes through polymers caps.

with newer motherboards it makes no sense to use those ceramics, because they're usually very small sizes and rated for small voltages (10v or 16v at best, since the cpu runs at under 2v so voltage bias and temperature is less of issue there) but you shouldn't reuse such capacitors for decoupling 5v you have on microcontrollers for example.

[eccerr0r]

Re: smt capacitor kits

Hmm... I need to replace aluminum electrolytic SMT 2.2uF to 100uF and yeah, need 6V to 25V for audio/video/PSU use, so 4V caps are no good... I came across a few 1.2uF, 4.7uF, and 10uF ceramics but no idea of their voltage ratings, and would be tricky to replace the 22uF and 47uF aluminums...

Any other suggestions for cheap sources for decent quantities of various SMT electrolytic caps?

[mariushm]

Re: smt capacitor kits

Aluminum electrolytic capacitors in that range.. 2.2uF - 100uF typically have normal to high ESR, more than 0.5-1 ohm ... for 2.2uF it wouldn't suprise me to measure an esr of 3-5 ohm.

Ceramic or tantalum capacitors are usually very low esr, the esr would be under 0.2 ohm even at such low capacities. Tantalum capacitors have a bit higher ESR but still, very low compared to electrolytic capacitors.
Depending on how the circuit is designed, this may complicate things - if the circuit depends on that internal resistance of the capacitor to prevent oscillations or other things, replacing the electrolytics with ceramic or tantalum can cause problems.
For audio stuff, it can matter a lot.

In power supplies (or the part of circuit in an audio circuit that generates voltages for opamps and other things), such electrolytic capacitors aren't really that carefully selected.. i mean they're not usually selected to have a specific ESR, the designer knows the ESR will be lousy from the start and will change with time (aging, heat etc)

Because of this, you can usually replace an electrolytic of certain capacitance and voltage rating with one rated for a voltage rating higher than the original voltage rating, and that's why for example you see capacitors like 10uF / 50v in a power supply that works with 5-12v .. a 10uF 25v or a 10uF 35v or a 10uF 50v would have about the same diameter and height and about the same ESR so you go straight for 10uF/50v

Basically, if the space is not an issue, just use 16v or 25v or 35v rated capacitors in that 2.2uF-100uF range, regardless of the actual voltage going through the circuit.

You can't just replace an electrolytic with ceramic of the same value due to how a ceramic capacitor's capacitance varies with the voltage - simply put a 10uF 16v rated ceramic capacitor will have an actual 9-10uF capacitance at voltages of 1-3v .. the closer you go towards the voltage rating, the lower the actual capacitance. It's not like with polymer or electrolytic or tantalum, ceramics are a special case.
Read the article in the link above, and also remember that the capacitance of ceramic capacitors also varies with temperature.

In addition, it's often not a smart choice to use ceramic capacitors in some parts of audio circuits (in audio path, not the power supply area), because of microphonics - vibrations and general noise can affect the total capacitance of the ceramic capacitor, and when you put this... let's say "unstable"... component in the circuit, you can get unexpected results.

Tantalums have their own quirks, a very small percentage will simply blow up after months or years of use, no matter how much you care and how well you design everything. In general, they're also sensitive to overvoltage, everyone recommends to use tantalums rated for at least 2-2.5x the maximum voltage they'll ever see (so for 5v you'd use 12-15v, for 12v you'd use 35v and so on)

Unless you want it to look really professional, my advice would be to use leaded electrolytics to replace smd electrolytics.

I like how norcal715 does it, see for example one of his videos: https://www.youtube.com/watch?v=p1zhfYHUIIY - jump to 10m 30s to see his technique of removing old smt electrolytics and soldering leaded ones in their place.

[eccerr0r]

Re: smt capacitor kits

Yeah I wish I could use through hole capacitors. The current project at hand is that camcorder and clearance is a huge issue (along with having parts packed next to each other)...

Dang I have too many through hole...