[sdiy] Voltage vs current, was Low voltage synthesis?

usenet at teply.info usenet at teply.info
Tue Jan 18 10:29:36 CET 2022

On 17.01.22 18:06, Ingo Debus via Synth-diy wrote:

> Yes, probably because it’s easier to measure voltages than currents. For 
> current measurements usually a wire or trace has to be cut. That’s often 
> difficult or even impossible (think multilayer PCBs).
> Thinking in terms of voltage is also reflected in the traditional way 
> schematics are drawn (i. e. the way we all usually draw them). If three 
> points (components) A, B and C are to be connected, it’s perfectly legal 
> to draw either A—B—C or A—C—B, even if it is important that, say, B is 
> in the middle (think rectifier - filter cap - load in a power supply for 
> instance). Since both ways to draw these connections are equivalent, PCB 
> design software cannot tell the difference (at least I don’t know of any 
> software that does), so doing a good PCB layout is so much more than 
> just translating the schematics. Maybe it wasn’t if there was a more 
> appropriate way to draw schematics?
In cases where it actually matters (mostly RF/microwave stuff as far as 
I can see), it is pretty common to place PCB traces explicitly as 
elements in the schematic and possibly also the layout. This tends to be 
a bit more work as these extra elements need to have a schematic symbol 
and a footprint in order to be recognized by the layout software, but 
can help a lot when it matters.
In the end it's a tradeoff. You trade flexibility for reproducibility. 
You can either describe an PCB element as "quarter wavelength at 
f=12.96GHz, Z=25R" or as "trace, 1.2 mm wide, 14.46 mm long". Both cases 
have their use, and they can be transformed into the respective other 
*IF* the PCB stackup and materials are known. But in Audio synths I 
don't see any benefit in doing that extra effort.

Even the mentioned arrangement of rectifier, filter cap and load hardly 
maters at the frequencies in question for reasonable dimensions. Sure, 
placing the load directly at the rectifier with the filter cap two 
metres away probably doesn't serve the purpose, but at a few dozen 
kilohertz it probably wouldn't make a huge difference.


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