I've come across a schematic with a way of labeling resistors that I haven't come across before. One resistor is labeled "5,7". Does anyone know what that means? Here's the project page:
and here's a direct link to the schematic:
Well I have not come across that in 40 years but I know what it means. The values are in ohms but the comma indicates a thousand so 5,7 is actually 5,700 ohms or 5.7K or as I would say 5k7.
Thanks, Mike! Imagine my frustration trying to google "resistor comma ...".
Or it could in fact mean 5.7 Ohms if that Swede just forgot that the decimal separator in English is . and not , as it is in Sweden.
I can't tell what would make most sense electrically in this case. Isn't it just there to drop voltage/limit current?
Can anyone explain why the backlight pins are connected as they are in the schematics?
Vdd connected to GND while Vss goes to +5V through that 5,7 resistor... shouldn't it be the other way around?
Edit:
Looked into it a bit but I'm no expert so please correct me if my assumptions are all wrong.
I don't know the exact LCD in use but many of these 16x2 HD44780 compatible LCDs seem to drop 4.2V across the backlight LEDs and the LED current is somewhere in the region of 120-160mA.
So let's say that you connect the backlight of that LCD to 5V and want to limit the current through the LEDs to 140mA.
To my understanding the formula for calculating current limiting resistors for LEDs gives us
R= (LED input voltage - LED voltage drop) / LED current
or (5 - 4.2)/ 0.140 = 5.7
So use a resistor of 5.7 ohms to limit your backlight LEDs to 140mA. 5.6 ohms should work pretty fine too.
However, to keep it at the recommended 120mA (if that's what your LCD manufacturer recommends) you should use 6.8 ohm instead.
And connect Vdd to 5V and Vss to GND instead.
Examples
Perhof looks exactly correct to me, complete with sensible analysis.
This looks like a simple case of the "," vs "." decimal separators in different countries.