I am using a 2N3904 transistor to switch a 12v relay. The relay has a 25mA coil. The transistor is connected to a PIC.
The data sheet for the 2N3904 says the current gain at Ic=50mA is 60. To be conservative, I assumed 30 as the current gain. I calculated Ib as 25mA / 30 = 0.83mA. I calculated the resistor needed as (5.0v - 0.7v)/0.83mA = 5.16K.
Later I learned about collector-emitter saturation. I want to make sure that I am using the correct resistor at the base of the transistor to saturate the transistor. On a breadboard, with a 5.6K resistor, Vce=0.111v. The collector-emitter saturation graph in the data sheet looks like Vce(sat) = 0.06v for Ic=25mA @25 celsius.
Again on the breadboard, I replaced the 5.6K resistor with a 1K resistor and measured Vce=0.061v.
The table of the data sheet says Vce(sat) max for Ic=10mA and Ib=1.0mA is 0.2V. For Ic=50mA and Ib=5.0mA, Vce(sat) has a max value of 0.3V.
With a 5.6K resistor Vce is 0.111V, which is higher than 0.06V point (25mA at 25 celsius) for saturation according to the graph in the data sheet (fairchild). Switching to a 1K resistor, gives Vce of 0.061v. According to the table in the data sheet, as long as Vce is under 0.2V the transistor should be saturated. I am confused as to what value to expect for Vce when saturation occurs.
The table in the data sheet says the max Vce for saturation is 0.2V. The graph indicates the Vce = 0.06V for saturation. Will saturation occur at any value of Vce under 0.2V? In the graph, is the Vce shown the absolute minium that will be achieved or is a target that should be strived for? Practically, will there be any noticeable decline in transistor performance/life with a Vce of 0.111V versus Vce of 0.061V? Would Vc=0.111V still be considered saturated? During saturation, is there a range of Vce values that are expected when the the transistor is saturated or should the goal always be to minimize Vce?
A bunch of questions, but ultimately just trying to get a clearer understanding of when saturation is achieved and the Vce values I should expect when saturated
Thank you.
JG
The data sheet for the 2N3904 says the current gain at Ic=50mA is 60. To be conservative, I assumed 30 as the current gain. I calculated Ib as 25mA / 30 = 0.83mA. I calculated the resistor needed as (5.0v - 0.7v)/0.83mA = 5.16K.
Later I learned about collector-emitter saturation. I want to make sure that I am using the correct resistor at the base of the transistor to saturate the transistor. On a breadboard, with a 5.6K resistor, Vce=0.111v. The collector-emitter saturation graph in the data sheet looks like Vce(sat) = 0.06v for Ic=25mA @25 celsius.
Again on the breadboard, I replaced the 5.6K resistor with a 1K resistor and measured Vce=0.061v.
The table of the data sheet says Vce(sat) max for Ic=10mA and Ib=1.0mA is 0.2V. For Ic=50mA and Ib=5.0mA, Vce(sat) has a max value of 0.3V.
With a 5.6K resistor Vce is 0.111V, which is higher than 0.06V point (25mA at 25 celsius) for saturation according to the graph in the data sheet (fairchild). Switching to a 1K resistor, gives Vce of 0.061v. According to the table in the data sheet, as long as Vce is under 0.2V the transistor should be saturated. I am confused as to what value to expect for Vce when saturation occurs.
The table in the data sheet says the max Vce for saturation is 0.2V. The graph indicates the Vce = 0.06V for saturation. Will saturation occur at any value of Vce under 0.2V? In the graph, is the Vce shown the absolute minium that will be achieved or is a target that should be strived for? Practically, will there be any noticeable decline in transistor performance/life with a Vce of 0.111V versus Vce of 0.061V? Would Vc=0.111V still be considered saturated? During saturation, is there a range of Vce values that are expected when the the transistor is saturated or should the goal always be to minimize Vce?
A bunch of questions, but ultimately just trying to get a clearer understanding of when saturation is achieved and the Vce values I should expect when saturated
Thank you.
JG