4
\$\begingroup\$

AFAIK, Power symbols in Eeschema have global scope. What is the best way to achieve local scope power rails? The only one I can think of is using normal net labels or hierarchical label.

When does this problem arise?

When using nested/hierarchical sub-sheets: Say you make a 'single channel LED driver' schematic, but you want multiple instances/channels of the same 'single channel LED driver'. Then in Kicad you could design the 'single channel LED driver' schematic once, and create multiple 'hierarchical sheets'. If you wanted to run all the instances/channels off a single power source, then the standard approach of using a 'power symbol' would be perfect. Here is the schematic of the 'nested/sub sheet' e.g. the 'single channel LED driver':

http://i.stack.imgur.com/FY9Mo.png

But say if you wanted to run some of the channels of 'power supply A' and some off 'power supply B', then you would use hierarchical pins in the schematic of the 'nested/sub sheet' (single channel LED driver) - But it looks ugly :(

http://i.stack.imgur.com/b7Ukw.png

Is there some sort of symbol that could be used in the 'nested/sub sheet'?:

http://i.stack.imgur.com/MmJQD.png

\$\endgroup\$
3
\$\begingroup\$

I think what I would do here is simply have two global supply names, one for each supply e.g. +5V_A, and +5V_B. Then you can use the standard supply symbols.

OR

Have a master sheet with all the hierarchical sub sheets on it (power supply and driver boards) and wire things up directly using the hierarchical sheet inputs. Here is an example of this (only power net wired for clarity):

Master Sheet:

Master

Individual Sheet:

Individual

Individual Sheet with two symbols used (Sheet 2):

Sheet 2

PCB connection:

PCB

The same hierarchical label (3V) is used on each sheet, but on the master two separate regulators are used. One supplies sheet 1, and the other sheets 2 and 3. On sheet two another IC is also supplied with a separate 3V symbol - on the master sheet you can see two inputs are needed.
The hierarchical symbol does not appear to have automatic connection, so you either have to wire it up normally on that sheet, or add as many input of the same name to that sheet as separate symbols used.
You can on the PCB snapshot (the other net is a normal global ground symbol) everything is connected correctly.

In case this is relevant - if you want to split one supply into two nets, use a "jumper" component (e.g. 0Ω resistor) so the schematic doesn't complain, so then you can have e.g. main_supply, supply_1 and supply_2 all electrically connected, but split for PCB requirements (e.g. like you might have an analog and digital ground)

OR Possibly:

Make a power supply symbol, use the # symbol in the reference designator (IIRC) which tells Kicad it's not a real component. Don't use a power flag on it though - this may work for a local power symbol if the quote below is correct (documentation is not the best though, and is outdated in some places so you need to be careful)

To quote from the link below:

A Power Symbol (VCC, V3P3, etc.) gives the net its name and is used on each page to tie to the global power net. It is a special component not listed in the BOM. A Power Flag (PWR_FLAG) symbol which gives the net its global characteristics - connecting power nets between sheets.

There is info on creating power symbols at the bottom of this link.

\$\endgroup\$
  • 1
    \$\begingroup\$ I like option #2. Option #1 could have a large number of LED driver sheets and they each need to be modified with their own power symbol. With #2, if you had so many LED drivers that you needed two different power supplies or something, you could custom wire them as needed in the root of the schematic tree. \$\endgroup\$ – Joe Baker Feb 14 '13 at 23:23
  • 1
    \$\begingroup\$ +1 -- Option #2. Don't be a fool; don't trust your tool! ;-) \$\endgroup\$ – DrFriedParts Feb 15 '13 at 5:06
  • \$\begingroup\$ Doesn't #1 defeat the point of making a single nested schematic? As you would have to now make 2 very similar 'single LED driver' schematics, but that each would have a different version power supply symbol (+5V_A or +5V_B). Wrt #2 ... so the nested schematic/sheet would look like my #2 or similar? #3 is what I was looking for, but the symbols would have to be linked (same local net) if they are to be anything beyond decoration. Is this the case? \$\endgroup\$ – Senthil Seveelavanan Feb 15 '13 at 15:18
  • \$\begingroup\$ I wouldn't say it exactly defeats the point of the nested schematic as there are other advantages also, but I see your point. It's commonly done this way though, to have many supply rails with relevant names, and it does make things clear. #2 would be similar to your #2, yes, you would have to use the labels (these link to the sheet inputs) I will experiment with #3, see if I can figure something. By the way - kicad users group is probably the best place to ask, if it can be done, they will know. \$\endgroup\$ – Oli Glaser Feb 16 '13 at 4:01
  • \$\begingroup\$ I added an example of option 2. It's not a proper power symbol, but it's not a bad option. I'd normally use this for signals (and use option 1 for the power rails), but there's no reason you can't use it for power. I can't find a way to make Option 3 work, despite the quote which seems to indicate it should be possible. Anyway, ask over at the users group, it's well worth joining if you are using Kicad, as it's the main resource info-wise (you can put in a feature request if it's not possible to do it in a way you are happy with) \$\endgroup\$ – Oli Glaser Feb 16 '13 at 5:04

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.