- #1
StealthRay
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Can anyone help me to check and calculate the resistance required?I am confused on the calculation part.
For example,the specification of a phototransistor is given below:
Vceo = 45v
Veco = 6v
Vcbo = not in datasheet
Vebo = not in datasheet
Power dissipation = 75 mW
Ic = 10 mA (condition Vce = 5V) (typical)
Ic = 2.5mA min
Ic = not stated for max
Dark current, Iceo = 2 nA
Vce (sat) = 0.2 V (condition Ic = 1mA)
The relay coil resistance = 393 ohms
Calculation of the Rc:
This is the way which I calculated (most likely wrong);
When phototransistor is off (no light);
Since we need 12 V to energize the coil,
Ic = V coil/Resistance of relay
= 12/393
= 30.5mA
When Voltage across coil = 12 V,
then Vc = 12V
Hence Rc = Vc/Ic
= 12V/30.5mA
= 393 ohms
If I am to disconnect the relay, and when phototransistor on;
Given Ic = 10 mA and Vce = 5v in datasheet;
Vcc = IcRc + Vce
Rc = (Vcc - Vce)/Ic
= 1900 ohms
I am abit confused here on the calculation.If I am to select 1900 ohms for Rc,I will surely not get 12V dc to energize the relay when phototransistor is off since the relay coil resistance is so small.I think something is wrong the way I used the datasheet.Can anyone correct me please.