One issue I see frequently in the forums that Ignorance is belong to correctly calculate the base resistance of transistor drivers.
I must assume that the reader knows the basics of a transistor, such as its parameters Β and hFE, so I will not dwell on it but specifically in the calculation of this resistance which depends on the supply voltage of the microcontroller port or device related.
For practical purposes we will assume that the output of a PIC that we want to control a relay has a coil resistance of 400Ω and is powered with 12V, making a quick count, we see that the current flowing through the coil to be energized which will exceed 30mA 20mA (maximum) that can handle the output of a PIC.
Clearly, we must resort to a driver to handle this current and voltage transistor which will select a chord, which could be a BC547B that supports a 45V with 100mA Vce Ic as parameters maximum according to the data sheet shown below.
Now for the circuit to use
The formula for calculating the base resistance is as follows:
Rbase = ((VDD - VBE) * hFE * Rload) / Vcarga Where
:
- = V VDD = 5V PIC
- VBE = base emitter saturation voltage = 700mV = 0.7V hFE
- = DC Current Gain (If the transistor is to be used as switch should take the minimum value so ensuring its saturation). In this case can be seen that although hFE values \u200b\u200branging from 100 to 800, as we have selected this B BC547B end indicates that the range of hFE is 200 to 400 as can be seen in the last table of the features. Therefore we take the value of 200
- Rload relay = R = 400Ω
- Vcarga the relay = V = 12V
Then passing the Rbase calculation becomes:
Rbase = ((5V-0.7V ) * 200 * 400Ω) / 12V = 344000 / 12 = 28.666Ω closest commercial value is 27KΩ
505 KB Author: Gabriel Web
of origin: www.geglab.com.ar
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