Guys, I have a quick question about the circuit below: VbeO for Q3 is -4V.
In this design is this being exceeded when the device is ON or OFF?
Doesnt look like to me but a coworker disagrees. What do you think? Any insight would be appreciated. Thank you.
My component calculations are Below:
Parameters Used:
Worst Case Rail voltage = 26.3Volts
Q3 Saturation Vce = .3V
Q3 hFEmin = 100
Q3 VbeON = 1.2V
ULN28 Saturation VceULN = 1.1V
LED1 Vf ~ 2.2V
Current required through LED = 20mA
Calculations:
- - R27 = (26.3V VCEsat Vf)/20mA = (26.3 - 0.3 - 2.2V)/(20mA) = 1190 ohms.
Lets go with a 1.1K.
Power Check: (20mA)^2 * 1.1k = .44W.
We will need a 1/2W Resistor. Size 1210?
- Ib = IC/hFEmin = 20mA/100 = .2mA
- R29 = (26.3- VbeONQ3 VceULN)/Ib = (26.3-1.2-1.1)V/0.2mA = 120K.
But lets drive Q3 hard when ON and go with R29 = 10K. This will ensure Q3 is well within saturation.
- R25 = roughly 5 times R29 or 50K. We will pick a standard value and go with 51K.
A smaller resistor may allow too much current through the R29/(R25+R29) divider causing Vb to be too high to ever turn off.
I think 51K is a safe number.
In this design is this being exceeded when the device is ON or OFF?
Doesnt look like to me but a coworker disagrees. What do you think? Any insight would be appreciated. Thank you.
My component calculations are Below:
Parameters Used:
Worst Case Rail voltage = 26.3Volts
Q3 Saturation Vce = .3V
Q3 hFEmin = 100
Q3 VbeON = 1.2V
ULN28 Saturation VceULN = 1.1V
LED1 Vf ~ 2.2V
Current required through LED = 20mA
Calculations:
- - R27 = (26.3V VCEsat Vf)/20mA = (26.3 - 0.3 - 2.2V)/(20mA) = 1190 ohms.
Lets go with a 1.1K.
Power Check: (20mA)^2 * 1.1k = .44W.
We will need a 1/2W Resistor. Size 1210?
- Ib = IC/hFEmin = 20mA/100 = .2mA
- R29 = (26.3- VbeONQ3 VceULN)/Ib = (26.3-1.2-1.1)V/0.2mA = 120K.
But lets drive Q3 hard when ON and go with R29 = 10K. This will ensure Q3 is well within saturation.
- R25 = roughly 5 times R29 or 50K. We will pick a standard value and go with 51K.
A smaller resistor may allow too much current through the R29/(R25+R29) divider causing Vb to be too high to ever turn off.
I think 51K is a safe number.