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Basic Electronics » Anything wrong with adding a large bypass capacitor?

February 06, 2012
by ThomasH
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Hello fellow nerds!

I've had my nerdkit for a couple of months now and really enjoying it.

When setting up the nerdkit on page 27 of the guide, you need to put a 0.1uF capacitor which plays the role of a bypass capacitor. I understand that capacitors store energy so that when there are fluctuations in demand for current, the capacitor can give up some of its stored energy.

My questions are: - do I have to use a 0.1uF capacitor in this role with the nerdkit? Or can I put in a larger capacitor? - What's the largest capacitor I can put in before damaging the microcontroller? The reason I ask is that I have a bunch of capacitors lying around greater than 0.1uF and it would great if I can use them instead of buying 0.1uF capacitors. - Also, does it matter if they are electrolytic or ceramic?

Thanks!

Cheers, Tom.

February 07, 2012
by Rick_S
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My understanding is that the purpose of the bypass capacitor isn't so much as to filter power supply ripple but to act as a sort of 'band pass' to prevent certain frequencies from effecting the micro-controller. .1uf is a very common size for this. You could put any capacitor you want there, without damaging the micro, but it may not do what is desired in the job of a bypass capacitor.

There are others here much more educated on the purpose of the bypass capacitor than I and they'll probably chime in here.

Rick

February 07, 2012
by JimFrederickson
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First off I don't think that using larger values would cause any 'fatal problems for the chip'.

Modern circuitry is much better that the circuits of old. (Long gone are the days where 'flipping a chip on it's back and blowing across the pins' created a significant risk of CHIP DEATH!)

The Atmel AVR is also very forgiving in general, but 'noise/variances/ripples' on the power can still cause problems.

Probably, in most cases, for the simple circuits most people create here it would not make much of a difference. (Unless you are talking about larger values. Maybe < 100uf that you intend to be replacements...)

There are specific formulas that are used to determine what values to use under what circumstances as well. (Easily findable on the Internet through searches.)

Generally...

1 - Larger capacitors can smooth out larger 'variances' in the voltage, but they do not work very well with higher frequency 'variances'.

2 - Smaller capacitors can not handle the larger 'variances' in the voltage, but they do work very well with higher frequencies.

So you can end up with what does initially seem like strange situations where you have several capacitors in parallel...

Usually ones initial reaction to seeing that is to 'total the 3 small capacitors and put in a single capacitor with a larger value'.

Yes that will give you a 'greater overall capacitance', but you will 'loose frequency response' on the smaller variances.

In the end using the bypass capacitors are there to avoid 'hardware related issues'.

If you use 'large values' and don't have any issues running the 'example software' then you are probably good to go.

Program away...

For me, i tend to try to avoid issues, and stick with what seems to work for most people...

(I still remember, way back, when I wrote my first real Windows Program. I noticed there were tons of 'Windows Event Messages' coming through that didn't seem to affect my program.

I worked through every Windows Event Message that didn't affect my program, and 'killed it'! Just to make my program more efficient.

Then the second two of three computers in the wild I put the program on wouldn't run it...

I looked at my code? It looked good...

Played around for hours...

Nothing...

Finally I put all those 'Extraneous Windows Events' back in.

Viola! No problems...

The point being...

Also remember what works in 'Your Test Environment' may have 'issues in the wild'...

Mostly what has been 'common', 'accepted practice', or 'capacitor values' is such for a reason...)

June 30, 2012
by dlhylton
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If you have a 1uf capacitor, that shouldn't cause a major difference, but I wouldn't use a capacitor any higher. The point of this capacitor is not to act as a power balancer, but as a frequency filter. It needs to be around the 0.1-1uf range in order to do this job. It cleans up AC noise from the DC signal.

To learn more about the role of bypass capacitors, read this article: [http://www.learningaboutelectronics.com/Articles/What-is-a-bypass-capacitor

It should help to clear up this issue.

June 30, 2012
by dlhylton
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Sorry, the link above should have been:

http://www.learningaboutelectronics.com/Articles/What-is-a-bypass-capacitor

July 25, 2012
by bretm
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That article is misleading. It's diagrams don't show a bypass capacitor for passing power supply ripple, it shows a capacitor bypassing an emitter resistor to boost the gain for higher-frequency signals in a common-emitter amplifier. That's a completely different purpose than what the text talks about.

The bypass capacitors you're talking about go between power supply rails e.g. GND and V+, not across a single resistor within a circuit.

It serves two functions. One is to filter out noise from the power supply, but the primary function is to provide a local energy source for current surges that happen when a digital circuit switches state. CMOS gates, for example, draw little current when they're not switching. Most of the current draw happens when the gates change state. That's why, for example, the Atmega uses more current at higher clock frequencies. In a chip with a lot of gates, such as a microcontroller, there is a big current draw for a very brief period of time on every clock tick.

The power supply can't supply this quickly enough because of the length of the traces (or wires) and their natural inductance. Without the bypass capacitor, the voltage would droop on each of these clock ticks, and this would also pollute the supply for the rest of the circuit.

Using a large value as a bypass capacitor, e.g. 100uF, is a bad idea because when power is first turned on it would cause a huge "in-rush" current that might exceed the power supply's current rating. Using a just-big-enough capacitor avoids this.

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