Įdomesnį sprendimą siūlo vokiečiai: RS485 -> USB (isoliert; via FT232R)
Input diodui įeinanti srovė, ir ribojančio rezistoriaus skaičiavimas:
Vokiečių schemoje stovi įeinančio diodo grandinėje 470 om ribojantis srovę rezistorius. Srovė prie 5V gausis apie 7,5 mA. Turint omeny CTR (Current transfer ratio), kuris LTV817 opto poroms nepriklausomai nuo modifikacijos (A, B, C, D), yra 50…600, vadinasi 7,5 mA padauginus iš minimalaus 50 CTR, gausim: 7,5 * 50 = 375 mA galimą maksimalią kolektoriaus srovę. Tranzistoriaus grandinėje stovi 1K srovę ribojantis rezistorius, kuris riboja srovę ties 5mA prie 5V.
The current range of LTV847 4-channel optocoupler and one channel LTV817 is 20mA~50mA for your 3.3V application.
If you using it on the static circuit then the current into led could be use as 10~20mA, calculate the resistor as
R = (3.3V-Vf(led))/I_led
= (3.3V-1.4V)/10~20mA
= 1.9V/10~20mA
= 190Ω~95Ω, choosing the practical resistor range as 180Ω~100Ω and then the current will be as 10.5mA~19mA.
If you using it on the pulse circuit then the current into led could be use as 20~40mA, calculate the resistor as
R = (3.3V-Vf(led))/I_led
= (3.3V-1.4V)/20~40mA
= 1.9V/20~40mA
= 95Ω~47.5Ω, choosing the practical resistor range as 100Ω~50Ω and then the current will be as 19mA~38mA, when the duty cycle is 50%/50% then average current as 9.5mA~19mA.
Įdomus variantas: SparkFun Opto-isolator Breakout
