Bypass Cap question

Michael @ Afineol

How many is too many Bypass Cap's? Does every IC need one? Should I use them on long wires? What is considered a long wire?

I have attached my Schematic as Project1.pdf.

Mike Green

Yes, you should have a 0.1uF bypass capacitor across the Vdd / Vss supply for every digital IC (Prop, 74HC165, 24LC256). The regulators should have their own capacitors as specified by the manufacturer's datasheet. Long signal wires are different. You don't want additional capacitance normally there. Long power wires generally need something like a 10uF capacitor plus the usual 0.1uF bypass capacitors at the far end. Typically you have a 10uF capacitor where power enters another board (near the connector).

The Propeller, since it has several Vdd / Vss pins, should have a 0.1uF bypass capacitor for each pair of supply pins close to the IC.

Cluso99

A lot of the newer more powerful ICs (particularly with mixed digital and analog) are requiring tantalums (the prop really needs a 10uF tantalum near it as it can switch power fast - specifically we need it for overclocking which is an indication of the problem).

As Mike said, all ICs require a 100nF bypass and should now be X7R ceramic. In days gone by we used cheaper Z5U ceramic, but higher speeds and switching noise (because a lot of ICs now turn off internal circuits when not being used to conserve power). Gone are the days when we just had simple 74xxxx gate chips.

You should examine the manufacturers specifications for all chips you use to see what they recommend.

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Links to other interesting threads:

· Home of the MultiBladeProps: TriBlade,·RamBlade,·SixBlade, website
· Single Board Computer:·3 Propeller ICs·and a·TriBladeProp board (ZiCog Z80 Emulator)
· Prop Tools under Development or Completed (Index)
· Emulators: CPUs Z80 etc; Micros Altair etc;· Terminals·VT100 etc; (Index) ZiCog (Z80) , MoCog (6809)·
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My cruising website is: ·www.bluemagic.biz·· MultiBlade Props: www.cluso.bluemagic.biz

Leon

I had a quick look at the schematic; you need to follow the manufacturer's recommendations for the capacitors on the regulators. You have got them completely wrong.

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Leon Heller
Amateur radio callsign: G1HSM

Michael @ Afineol

Leon I will certainly defer to you, but I setup the Schematic based on the Prop Education Kit I recently purchased. I used (I think I did) the same parts that I received in the kit. I even followed the diagram on the box (this makes me sound lilke a little kid). When you say they are all wrong, what do you mean? How or what would you recommend? My question about Bypass Cap shows that· I am still learning how all the components snap together.

I did read the datasheets (as much as I can understand) and each of the regulators stated Cout must be atleast x.

Phil Pilgrim (PhiPi)

Leon is right. There's more to a capacitor than capacitance. There's also ESR (equivalent series resistance). The size of your filter caps implies that they're aluminum electrolytics, which typically have a high ESR. Those are okay, and you can leave them there if you want. But you also need ceramic or tantalum caps on the outputs that spec an ESR within the range specified in the VReg datasheets.

-Phil

william chan

All this while I thought ceramics have lower ESR than tantalums.
Recently there have been a lot of new high capacitance ceramics that costs much less than tantalums.

Like this 10uf 1206 part that cost only USD 1 cent each in 100 quantities.

my.farnell.com/multicomp/mc1206x106k6r3ct/cap-ceramic-10uf-6-3v-x5r-1206/dp/1785704

But they don't specify the ESR values.

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Sapieha

Hi Michael @ Afineol.

I we be write on forum some directions on Decoupling's CAP's - One recommendation are On at last one 10-47uF CAP near Propeller IC.
It has as function to Preserve some extra current for Propeller in states Propeller exit Low Power states AND that states in Propeller depends on many things.
Some on starting NEW COG some on exiting Waitxxx instructions in COG's

BUT one direction on building PCB's is to have maybe one CPA to much - BUT never one to little.

Regards
ChJ

Ps. For 10-47uF CAP - Smallest Power traces More near Propeller must them be. Wider Power Traces - CAP can be little more far from Propeller

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Sapieha

Post Edited (Sapieha) : 5/6/2010 4:12:35 AM GMT

Cluso99

10uF 10V Tantalums are only < 10c in 100's

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Links to other interesting threads:

· Home of the MultiBladeProps: TriBlade,·RamBlade,·SixBlade, website
· Single Board Computer:·3 Propeller ICs·and a·TriBladeProp board (ZiCog Z80 Emulator)
· Prop Tools under Development or Completed (Index)
· Emulators: CPUs Z80 etc; Micros Altair etc;· Terminals·VT100 etc; (Index) ZiCog (Z80) , MoCog (6809)·
· Prop OS: SphinxOS·, PropDos , PropCmd··· Search the Propeller forums·(uses advanced Google search)
My cruising website is: ·www.bluemagic.biz·· MultiBlade Props: www.cluso.bluemagic.biz

HollyMinkowski

I have a huge pile of surplus 10uf 10v tantalums.
They do not have a long lead and a short lead like
they usually do..with the longer lead designating + polarity.

The only marking is
+10
10V

Is the lead closest to the + mark always the positive
polarity lead???

Here is a picture of one of them.


Post Edited (HollyMinkowski) : 5/6/2010 4:01:39 PM GMT

Leon

Michael @ Afineol said...
Leon I will certainly defer to you, but I setup the Schematic based on the Prop Education Kit I recently purchased. I used (I think I did) the same parts that I received in the kit. I even followed the diagram on the box (this makes me sound lilke a little kid). When you say they are all wrong, what do you mean? How or what would you recommend? My question about Bypass Cap shows that I am still learning how all the components snap together.


I did read the datasheets (as much as I can understand) and each of the regulators stated Cout must be atleast x.

The data sheet for the LM2940 shows a 0.47 uF on the input (if it is some distance from the supply smoothing capacitor) and a 22 uF (minimum) on the output. The ESR for the latter is stated as critical (see page 13)! They should both be close to the regulator leads. There are probably similar requirements for the other regulator.

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Leon Heller
Amateur radio callsign: G1HSM

Post Edited (Leon) : 5/6/2010 4:23:25 PM GMT

yarisboy

On my latest layout I got up, got coffee, rubbed my eyes and looked at the screen from last night. I had automatically added the cap pads just like I was putting down a Prop. The spec sheet for my latest play toy chip doesn't call for them but I'm leaving the pads in, just in case. Thanks for the rules of thumb guys. I recently expanded my stock of capacitors from just the type of lessons learned as you have explained above. Good question/good responses.

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Dr_Acula

@Holly, yes the + is the +ve lead. It sounds like you have a lifetime's supply of tantalums there. Very handy.

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Toby Seckshund

When tants came out the convention was that with the writing towards you and the wires downwards, as in Holly's picture, the right hand side one was the positive.

To be safe put one onto a PSU and leave it to run, tants can "catch fire" when reversed.

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