The input to my ULN2003B is connected to a switch.
When the switch is OPEN, the input to the ULN2003B is therefore OPEN and float also.
Do I need to add a pulldown resistor on the ULN2003 input to assure it is OFF, or can I just leave it open and float?
Could you please give an example how to select the series resistor and Vb?
For example, Iout=100mA @25Celcious is needed.
How to understand Figure 5. hFE vs IOUT？ Iout =hFE*Ib, according to Figure 5, Iout is Monotone increasing with hFE.
IF Iou=100mA, hFE=1000, so the Ib=100uA, right?
then how to apply Vb and the external series resistor R to realize this.
Hi Ba,
Ba Dong said:How to understand Figure 5. hFE vs IOUT？ Iout =hFE*Ib, according to Figure 5, Iout is Monotone increasing with hFE.
You are correct, and it is measured at input pin to output pin. and is the LARGE signal current gain. For darlington pairs, you consider the hFE1*hFE2=hFE of the darlington pair.
Ba Dong said:IF Iou=100mA, hFE=1000, so the Ib=100uA, right?
Right.
Ba Dong said:Could you please give an example how to select the series resistor and Vb?
You don't really need an extra series resistor if you can select VB as the part has built-in base resistor. Selecting base resistor is helpful if you can't tune VB. In that case you can either estimate the current by considering the VBE drop (base of NPN1 is ~1.4V higher than emitter by rule of thumb).
A better option is you can use Figure 7 in the d/s. By knowing hFE at the output current need -> you get input current. From input current you need-> you get input voltage at the pin. From input current requirement + input voltage as a result of that current you can fine tune VB/RB_ext. If VB is fixed, then you can change RB os that VB-Vin(at pin) drop across your RB resistor gives you that input current you need.
If you can tune VB , even easier, you can directly get the voltage you should apply to the input based on the figure you shared from d/s + this figure.
Hopefully this helps, if you have any other questions on the design process please don't hesistate to reach out. And if this answers all your questions, please let me know by pressing the green button .
best
dimitri
