I'm creating a repro of a small pcb. The original design (from the 80s) has 3 .1uF capacitors in parallel. To save space on the board, I want to replace the 3 .1uF with a single .33uF. Am I correct in thinking the new version is equivalent? I've attach snippets of the original design and mine. Thanks

 

Not necessarily. The schematic sometimes does not represent the original designers thought process. Depending on the application and the layout there may be valid reasons for doing what she did. It could be as simple as the 0.1 parts were in the manufacturer's inventory and adding a new and different part would cost more than using an existing part already in stock. We really don't know without more information and investigation.

 

Three capacitors right next to each other on the circuit diagram doesn't indicate three capacitors next to each other on the board. The designer has simply grouped all the decoupling capacitors together on the diagram. There will be one per IC on the board (at a guess), and that's how they should stay.

 

If the caps were
physically side by side on the pcb then the .33 would suffice.

 

I'm creating a repro of a small pcb. The original design (from the 80s) has 3 .1uF capacitors in parallel. To save space on the board, I want to replace the 3 .1uF with a single .33uF. Am I correct in thinking the new version is equivalent? I've attach snippets of the original design and mine. Thanks

Hi,

I think the suggestions that the caps are not really physically located right next to each other is a good one. When several parts on placed on a PC board there are usually bypass caps distributed around the board so that each component gets a reasonable bypass on the power lines. This is especially true with IC chips. The 7400 series TTL is very typical for using a technique like this but of course almost any set of IC's could need that.

There is another reason this could be done. That is to reduce the impedance of what would otherwise be a 0.33uf cap. All caps have some ESR and by using three in parallel you can reduce the effective ESR of the combination. It may not go down as much as 3 times with 3 caps, but it will go down.
However, due to the 100 Ohm resistors it is very doubtful that they used 3 caps for this purpose the better idea is still the idea that the distributed capacitors was the real reason why they show 3 caps like that.

It's also interesting that the lettering for the values read "MF" instead of "uf". "MF" is a fairly old way to indicate the value of a cap. I have to wonder what this circuit goes to. That would also reveal more of the story as to why there are 3 caps shown like that. I was thinking "tube" amplifier but plus and minus 12v hints more at an op amp type project. Just some guesses.

 

Guess the designer's intent...I love this game.

Mine is, they wanted to create a single low ESR cap by paralleling 3 of them.

(And I have no idea if that would even work)

 

Guess the designer's intent...I love this game.

Mine is, they wanted to create a single low ESR cap by paralleling 3 of them.

(And I have no idea if that would even work)

Hi,

Yes i mentioned that possibility but it may not be true because of the 100 Ohm resistors on the outputs of both supply lines. Of course maybe they figured it would still be a problem at RF frequencies, so yes we can start a game show, "Guess the reason for the extra parts", ha ha. If you win you get a free kit of fifty 0.1uf, 35v capacitors (ha ha).

 

Guess the intent??!! You guys need to get a hobby. Oh wait, you already have one. My bad.

 

The original design (from the 80s) has 3 .1uF capacitors in parallel.

Do you mean "THREE" 0.1µF caps? Obviously finding a 3.1µF cap would be difficult, so you probably mean Three 0.1µF caps.
Not an expert on caps, but when you parallel their capacitance you ADD them together. Two caps of 0.1µF parallel would be 0.2µF. The total voltage will still be the same - or which ever cap has the lower value.

Seems you may already understand that fact.

Here's my best GUESS at why there are three parallel caps as opposed to a single equivalent cap: The thinking is (and is probably flawed) that the first cap will buffer ripple leaving a smaller ripple. The second cap will be seeing smaller ripple, and the third even less. But that's a guess and is probably wrong unless each cap were isolated from each other so that there is no reverse current between them. With all three caps shorted (positive to positive to positive) and the same for the negative, you've effectively built a 0.3µF cap.

As

Depending on the application and the layout there may be valid reasons for doing what she did. It could be as simple as the 0.1 parts were in the manufacturer's inventory and adding a new and different part would cost more than using an existing part already in stock.

said - it may be a matter of convenience or profit margin. I'm thinking it could also have something to do with space.

 

Here's my best GUESS at why there are three parallel caps as opposed to a single equivalent cap: The thinking is (and is probably flawed) that the first cap will buffer ripple leaving a smaller ripple. The second cap will be seeing smaller ripple, and the third even less.

Others are suggesting that the draftsperson put them all in the same location on the schematic but on the PCB they're distributed about. Looking at the drawings: S1, S2 and S3 suggest a connector, so the caps likely reside on a single board that a load of some sort connects to. In the other drawing it's depicted as JP1. If my understanding of plugs are concerned, J1 - the JAY indicates a male pin whereas if it were P1 that would indicate a female pin. Perhaps the draftsperson chose JP1 to indicate that J1 connects to P1. But from my computer I'm not able to see the actual part, nor can I discern for certain exactly what JP1 really means.

 

From the proceeding discussion, your best chance of success lies with just going with the original design. Many times we think we can improve upon the original design but without know what the original engineer went through to get the board ready for production this second-guessing game is full of hazards.