Capacitor Type question

Hi, I'm looking to build this **broken link removed** (schematic link right at the bottom) smart charger (unless you gurus suggest otherwise) Basically, how can you tell from the chematic which type of cap to purchase? they appear to not be polerised, does this mean any non polerised cap will do?

There are many types and I am never sure which I should get when purchasing for a project is there any rules I should stick to for cap purchases?

many thanks,

YOu usually can't tell from the schematic without background info on what the circuit has to do. You can use a non-polarized cap in place of a polarized cap (assuming sufficient capacitances, voltage ratings, frequency characteristics are available.). You cannot always use a polarized cap in place of a non-polarized cap however.

THe main kinds of capacitors and their comparisons with each other:

Ceramic
-best frequency characteristics of the three
-best voltage ratings
-smaller values than the others
-capacitance varies with temperature and DC-bias so not good for signalling like filtering
-most caps are used for bypassing or decoupling and of the three these should be used whenever possible
-X7R and X5R dielectrics are most economical for accuracy and stability. X7R is 15% stable up to 125C and x5R is 15% stable up to 85C.
-C0G dielectric used for timing and basically doesn't suffer from the stability issues mentioned. Very accurate and only comes in small values so these are used for high frequency timing
-the less accurate and stable the ceramic dielectric is, the larger the values that you can get them in as described here. This tends to be true between different types of capacitors too.
-non polarized so can be used in AC applications with zero DC-bias

Electrolytic
-largest values
-worst frequency characteristics
-low tolerances
-only used for low frquency bypassing where large capacitance is needed
-regular voltage ratings about as good as ceramic (there are really high voltage kinds though)
-polarized

Tantalum
-lowest voltage ratings
-more capacitance than ceramics but less than electrolytic
-better frequency characteristics than electrolytics but not as good as ceramics
-best for low-voltage bypassing where higher values than ceramics are needed
-capacitance independent of DC-bias so best for signalling (ie. filtering, timing where frequency is low enough that they can be used)
-polarized

Most caps you use are for bypassing, so go for ceramic when in doubt (they are the most popular kind for a reason). Anything where the frequency is too high for ceramics to be used, only very expensive, very accurate, very stable, and very small capacitors are used. You can ignore these- you'll know it when you need them.

That is very useful! Thank you so much.

When you refer to high frequency what would you call high?

I use metalized plastic film capacitors for medium values in audio circuits. 5% tolerance is common and inexpensive.

0.1uF and 0.47uF are medium values and the circuit is not very high speed so I would use film capacitors. A 0.1uF ceramic disc capacitor could be used.

I dunno...high frequency is just a descriptive term. But after searching a bunch of documents on typical impedance curves for ceramic capacitors...the maximum frequency a X7R surface mount capacitor (leaded is a possibility for larger capacitances at less than 1MHz) is good for bypassing (theoretically according to the measured impedance curves) is about...

300MHz for 0.001uF surface mount
60MHz for 0.01uF surface mount
10~11MHz for 0.1uF surface mount
4~6~8 MHz for 1uF surface mount
4MHz for 2.2uF surface mount
2MHz for 4.7uF surface mount
1-2MHz for 10uF surface mount
1Mhz for 22uF surface mount
500kHz for 100uF

References:
**broken link removed**
**broken link removed**
**broken link removed**
http://www.ttiinc.com/object/tech_seminars_052505muratapacket

Remember digital square waves contain harmonics that are 100x or 1000x greater than their frequency. Also be aware that just because a capacitor is better for bypassing at a higher frequency doesn't mean it's impedance will be as good (as low) for bypassing a lower frequency as a larger capacitor.

Neat stuff...if you're obsessive compulsive.

I can't think of any reason why a solar charger requires working frequencies in the MHz region, and I suspect for those uses (compensation, charge terminate) they're merely a low pass filter and even a low grade capacitor can be used for them. Suspicion that even the value isn't critical so an electrolytic can be used for those as well if needed. It doesn't hurt to use a higher quality capacitor if you're worried for this indeterminate case.

Typically a circuit should specify the type of the capacitor if it's known to be critical. But true, sometimes someone got something from the junk box and it just so happens to work and it's not mentioned that the type is critical...

The charger circuit was just a current example. It seems to me that I can use what I like if the capacitance and voltage are sufficient unless frequency is an issue which I doubt will be a problem for the simple circuits I build.

dknguyen said:

I dunno...high frequency is just a descriptive term. But after searching a bunch of documents on typical impedance curves for ceramic capacitors...the maximum frequency a X7R surface mount capacitor (leaded is a possibility for larger capacitances at less than 1MHz) is good for bypassing (theoretically according to the measured impedance curves) is about...

300MHz for 0.001uF surface mount
60MHz for 0.01uF surface mount
10~11MHz for 0.1uF surface mount
4~6~8 MHz for 1uF surface mount
4MHz for 2.2uF surface mount
2MHz for 4.7uF surface mount
1-2MHz for 10uF surface mount
1Mhz for 22uF surface mount
500kHz for 100uF

References:
**broken link removed**
**broken link removed**
**broken link removed**
http://www.ttiinc.com/object/tech_seminars_052505muratapacket

Remember digital square waves contain harmonics that are 100x or 1000x greater than their frequency. Also be aware that just because a capacitor is better for bypassing at a higher frequency doesn't mean it's impedance will be as good (as low) for bypassing a lower frequency as a larger capacitor.

Neat stuff...if you're obsessive compulsive.

Click to expand...

Nice work dknguyen on these capacitor listings but
I would like to add a disclaimer to this table..

This table is a general guide only. How well (how high frequency) a capacitor still acts like a capacitor generally is not dependent on the capacitance as this table might suggest. Instead, it is a stronger function of the parasitic inductance associated with the packaging and placement of these devices. It is quite possible to create capacitances that work much better And worse than what the table suggests.

It is intended to be a rough guide and certainly the particular manufacturers datasheets should be consulted in each case.

Don't worry, I'm not going to sue... :s

dannix said:

Don't worry, I'm not going to sue... :s

Click to expand...

Thats not the problem...

We just don't want you walking away thinking a 2.2uF capacitor is only good to 4MHz... maybe it is maybe it isnt.. the details will determine that.
I generally don't like seeing hard-fast rule tables like that for these reasons. It is much more satisfying to understand the fundamental issues with components.

Optikon said:

Thats not the problem...

We just don't want you walking away thinking a 2.2uF capacitor is only good to 4MHz... maybe it is maybe it isnt.. the details will determine that.
I generally don't like seeing hard-fast rule tables like that for these reasons. It is much more satisfying to understand the fundamental issues with components.

Click to expand...

I'm a bit obsessed with capacitors because so much if it seems like a black art. I know you don't trust the rules, but after digging and digging and digging to see where they are coming from, I always end back at the beginning just following the rules knowing they aren't always true just because it's so much easier.

Lately I've been getting really paranoid about using a few different sized caps in parallel to bypass due to impedance spikes. I know what a impedance curve for two caps looks like, but I'm paranoid about using 3 because I can't predict how they will interact with each other.

The chart seems to suggest 0.01uF for a 40MHz PIC (which in itself isn't accurate since it's a square wave and the harmonics are much higher suggesting an even smaller value!), but it seems to work just fine with 0.1uF.

And that reminds me, the reason you tend to use a smaller capacitor for higher frequencies isn't because small capacitances bypass better there (on the contrary, in theory where capacitors have no parasitic resistances or inductances, a larger capacitor is always better). But in real life, larger capacitors have higher inductances which completely nullifies the effectiveness of the capacitance at higher frequencies. Hence, the packaging and capacitor type also affect this heavily. So all that stuff really says more about the trend of parasitic inductances with capacitances rather than how capacitance is related to effectivebypass frequencies.

dknguyen said:

I'm a bit obsessed with capacitors because so much if it seems like a black art. I know you don't trust the rules, but after digging and digging and digging to see where they are coming from, I always end back at the beginning just following the rules knowing they aren't always true just because it's so much easier.

Lately I've been getting really paranoid about using a few different sized caps in parallel to bypass due to impedance spikes. I know what a impedance curve for two caps looks like, but I'm paranoid about using 3 because I can't predict how they will interact with each other.

The chart seems to suggest 0.01uF for a 40MHz PIC (which in itself isn't accurate since it's a square wave and the harmonics are much higher suggesting an even smaller value!), but it seems to work just fine with 0.1uF.

And that reminds me, the reason you tend to use a smaller capacitor for higher frequencies isn't because small capacitances bypass better there (on the contrary, in theory where capacitors have no parasitic resistances or inductances, a larger capacitor is always better). But in real life, larger capacitors have higher inductances which completely nullifies the effectiveness of the capacitance at higher frequencies. Hence, the packaging and capacitor type also affect this heavily. So all that stuff really says more about the trend of parasitic inductances with capacitances rather than how capacitance is related to effectivebypass frequencies.

Click to expand...

I agree with what you're saying... but when you say 0.1uF works fine, what are you saying works fine? The PIC still runs, yes - I'm sure that is true but there is an effect. It is all really a matter of what the design requires to be acceptable. Do you know that your 0.1uF case isnt radiating more electromagnetically? How about power supply noise? You can be sure there is an electrical difference somewhere. The question is whether or not it remains acceptable.

Bypassing is really a power supply impedance problem. It is especially important for low noise analog work and very high speed digital designs.

I have found that below a 0603 surface mount part, there is little to be gained in terms of reducing parasitic inductance (i.e. 0402, 0201 or similar)
power and ground plane capacitance become dominant at >100 MHz and often that only constitutes < 1000 pF

Optikon said:

I agree with what you're saying... but when you say 0.1uF works fine, what are you saying works fine? The PIC still runs, yes - I'm sure that is true but there is an effect. It is all really a matter of what the design requires to be acceptable. Do you know that your 0.1uF case isnt radiating more electromagnetically? How about power supply noise? You can be sure there is an electrical difference somewhere. The question is whether or not it remains acceptable.

Bypassing is really a power supply impedance problem. It is especially important for low noise analog work and very high speed digital designs.

I have found that below a 0603 surface mount part, there is little to be gained in terms of reducing parasitic inductance (i.e. 0402, 0201 or similar)
power and ground plane capacitance become dominant at >100 MHz and often that only constitutes < 1000 pF

Click to expand...

That's why it's a black art! You know it works, but you aren't sure whether it works better without it there. But it works, so you don't usually bother going any farther. Just like mathematical proofs. I try to use 805 because it's cheap.

Ok capacitor gurus, I need a smoothing cap for a project I'm working on, but I am not sure what value to go for, what im doing is a thermostat circuit will run a 12v fan, I want to use the signal wire of the fan to feed a mosfet (that turns the PTC heating element on) As the signal from the fan is pulsed (1 pulse per half rotation of the fan im told) I want to smooth this pulse out so the mosfet does not keep switching the heater on/off. Its all battery run so I would like to keep this efficient as possible.

I'm not sure what voltage a PC Fan signal pulses at. Im looking at a 40 or 50mm fan 12v which I have not purchased yet. I thought this was the simplest way to ensure the fan is working before the PTC Element is turned on.

Any sugestions?

What's so inefficient about switching the heater completely on and off to get variable heating? Your concept is flawed since the MOSFET would be operating in the linear region halfway between on and off most of the time due to the voltage probably never being low enough to turn it completely off or turning it completely on. You'd be spending your time in the most inefficient region for a switching MOSFET.

Also remember that if the cap does what you say you want it to do, it will slow down how fast the MOSFET switches when it actually does turn on and off which seems to not be what you want. But the previous paragraph makes this point completely moot.

I would presume you do need a MOSFET driver though since I doubt the drive current from the fan is sufficient to switch the MOSFET at the pulse rise times the fan is outputting, not to mention the voltage is probably too low to completely switch most MOSFETs in the first place.

The fan probably pulses 1.8V-5V. Just measure it...or something. Shouldn't be too hard to figure out.

you have lost me there. The fan has 3 wires, 2 for supply, one for signal

1) Can I not just connect that signal wire to the mosfet to switch it on?
2) If yes to (1) Will I need to do anything between the signal and the mosfet for good operation?

You seem to have completly edited your reply so my last post does not make much sense now, please ignore.

If it is pulsed can it still be measured with a standard voltmeter?
What do I need to "drive" the mosfet? it must be a mosfet or similar as the heater is 15w 12v

Yeah, you can delete your post by going edit and clicking delete. I misread your post so I had to edit it.

So you aren't going for the cap anymore? Wonderful. It wouldnt' have done anything but dig you into the hole you were trying to avoid.

10V will switch most MOSFETs. So the same 12V supply used for the fan should work (assuming it doesn't exceed the maximum gate voltage of the MOSFET). The gate driver you choose allows the control signal (ie. from the fan) to be a different voltage from the one actually switching the MOSFET gate. You could use the same 12V supply used for the fan to power the "gate drive side or high current output side" of the gate driver IC. Depending on the driver, it may also require a second voltage source for the logic control side, but not always. If not, it is likely that the control voltage thresholds are dependent on the single voltage supply for the IC (which will be dictated by the high current output side).

**broken link removed**

There are also low voltage 1.8V, 3.3V, switching MOSFETs that might work with your fan output signal, but like before the drive current is probably not enough anyways and you probably don't have easy access to another voltage source other than the 12V for the motor.

YOu need an oscilloscope to measure a pulse. I suppose you could connect an RC filter and measure that voltage when the fan is at a constant speed and calculate it out...

But you should start a new thread since this isn't a capacitor question anymore.

Driver circuits aside and sticking with the capacitor topic, can you explain why the cap would compound the problem, I always thought capacitors would "fill in the gap" so to speak.

WHen a MOSFET switches it goes:

OFF/ON->Linear Region->ON/OFF

THe rise time dictates how long it spends in the linear region, among other things.This is the region when the MOSFET is dissipating the most power since sizeable current is flowing through a high resistance since it is neither on nor off:

"Steady-State" Losses: I^2xR
ON = I is large, but R is 0, so no losses
OFF = R is large, but I is 0, so no losses
Linear = both R and I are non-zero so power dissipated

This switching time is dictated, among other things, by the gate capacitance. Add another capacitor, and it just takes longer to charge up the gate-source capacitance causing it to take longer to reach the threshold gate voltage, thereby increasing transition times forcing the MOSFET to spend more time in the linear region. THe inefficiency in MOSFET switching comes from this region, and to reduce these losses you either switch the MOSFET less often so you enter this region less, or switching faster (at the expense of radiating more noise) so you spend less time in this region.

Inefficiency doesn't come from being on where losses are minimal, or from being off where minimal power is being dissipated- your so called "gaps".

Of course, when I say zero ohms, and "no losses" etc, it's not absolute, but a relative comparison to the other operation regions.