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Purpose of cap from power to ground

There are small, local stores of charge, so that when a chip or transistor needs a pulse of current from the supply, it doesn't have to travel through all the inductance of the path back to the supply main storage capacitor or regulator. You don't need one at every load in the system, but the higher the rate of change of current needed by some circuit, the more important it is to have a storage cap nearby.

Remember that such capacitors do not hold the voltage constant, since the only way to draw charge from them is to have their voltage change. But they reduce the voltage sag caused by a sudden current increase or the rise in voltage caused by a sudden current decrease.

Reply to
John Popelish
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  1. To absorb transients.
  2. No.
Reply to
Don Foreman

I am curious... On many schematics, there is a cap shown from power to ground for many chips.

Two questions.

  1. Why is it necessary

  1. Can I use one cap from power to ground, for the entire circuit.

Thanks i

Reply to
Ignoramus26744

To bypass electrical noise to ground.

You want to use a cap at each IC. Ed

Reply to
ehsjr

Thank you John, that's a perfect explanation!

i
Reply to
Ignoramus8862

Many caps should be used, typically one 0.05 uf per three chips.

Their purpose is to stomp glitches when outputs switch. A very high current spike results when both high and low drivers conduct together.

The caps most definitely can NOT be eliminated.

--
Many thanks,

Don Lancaster                          voice phone: (928)428-4073
Synergetics   3860 West First Street   Box 809 Thatcher, AZ 85552
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Reply to
Don Lancaster

The answers you've gotten mostly focus on the "eliminate glitches" aspect of the problem. The other thing that caps can do for you is prevent amplifier circuits from becoming unstable.

In the amplifier case (or really in the case of anything that has to drive substantial current into a load), you need to think hard about what the "ground" is. Generally, you have to think about the entire circuit, from supply to IC to load to ground to supply. You are trying to reduce the size of that loop.

Reply to
Walter Harley

Thanks, that was interesting.

i
Reply to
Ignoramus4939

Thanks. Looks like 1 uF is what is recommended by XR2206 datasheet. Any suggestions for small caps, maybe axial, that fit a breadboard nicely. There are many choices. Thanks.

i
Reply to
Ignoramus4939

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Have Fun! Rich

Reply to
Rich Grise

--- Since EXAR didn't publish what their outputs look like when they're switching, I don't believe there's an easy way to calculate the value of the cap. But, since they specify 1µF I'd use a cheap radial 1µF aluminum electrolytic and [maybe] parallel it with a

0.1µF ceramic.

If you're concerned about the electrolytic's height, just spread its leads, bend them 90° where they're 0.1" away from each other, cut them so they're long enough to stick into the breadboard's sockets, lay the cap on its side, and stick 'em in there.

-- John Fields Professional Circuit Designer

Reply to
John Fields

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--
Digi-Key\'s got a $25 minimum, so unless you\'re building more than a
few of your widgets, that and shipping is gonna take it out of the
\'inexpensive\' range. 

Technically, I dunno.

If you\'re looking at something other than just getting a breadboard
to work, you need to look at the cap\'s loss tangent at the frequency
of operation of the XR2206 with a load across it (across the XR2206)
in order to determine whether it\'ll be a suitably low-cost
reservoir.  Certainly there\'s nothing holy about 1µF,  and if you
put more capacitance in there at your vorking voltage that might
even be better.  And cheaper.

And, if you can locate the XR2206 close to the supply (or the output
of the regulator, if that\'s what\'s you\'ll be using) you might be
able to blow off the cap(s) altogether.

Try it.
Reply to
John Fields

My favorite high performance (low series inductance and resistance and small) bypass caps are the V series of stacked film mylars from Panasonic.

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But the minimum quantity is 10 for $3.81 for the 1uF @50 volts.

The FM series of electrolytic capacitors also have good ESR and current rating for their size when you need something larger.

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Reply to
John Popelish

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Thank you. These 1 uF caps are kind of pricey. Do you have any idea how much capacitance do I actually need? Sorry for this ignorant question, I am just not sure how to calculate it.

i
Reply to
Ignoramus4939

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Thank you. How about Digikey item P993-ND.

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Picture:

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It is inexpensive and seems to "fit the bill".

i
Reply to
Ignoramus4939

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I will have to order quite a few things from them soon.

I see.

I see. Well, at $0.10 or so, I can as well afford to buy some caps.

i
Reply to
Ignoramus4939

Thanks, that looks good too!

I am going to place a decent sized order soon...

i
Reply to
Ignoramus4939

For common components, Mouser is often cheaper than Digikey, and they have no minimum order.

Jameco is also often cheaper; they have a smaller selection and most of their components are generic rather than coming from a specific manufacturer, but for hobby stuff they're a good choice.

Reply to
Walter Harley

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Have Fun! Rich

Reply to
Rich Grise

(snip)

Ask them for a paper catalog. Much easier to browse through than using the on line search engine.

Reply to
John Popelish

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