Solve Open Collector Circuit Problem | R1, R2, R3 Values

[brad sue]

HI,
I have his problem on open collector and I don't know how to deal with it:

In the figure attached, the first transistor is driven by an output of the MCU, with the goal of turning the LEDs(several in parallel) on and off.
Compute the values of R1,R2,R3 required to make the circuit work properly, if it can.

Please can I have some suggestions meaning the step I need to do.
My idea for the first resistor R1: If the output is low, The transistor inside the output of the MCU will sink current, letting T1 off I am right?

However, I don't know how to solve the case where the output is 'high'. Please can someone help me? what the different currents do at the first node?

Thank you
B

 

[MATLABdude]

Open collector means that the output stage is an NPN BJT with an open collector. When the BJT is on, it (hopefully) saturates, and pulls the collector low to the V_ol voltage. However, when the BJT is off, the collector just floats. That's the reason you need an external pull-up resistor.

The value of this resistor needs to be low enough to allow enough current to pass through to turn on your next stage (and put it in the right mode--linear or saturation), and needs to be high enough that your collector doesn't burn out from excessive current.

 

[piercas]

ased on the information provided, it seems like you are trying to design an open collector circuit using a transistor to turn on and off multiple LEDs in parallel. In order to determine the values of R1, R2, and R3, you will need to consider the characteristics of the transistor and the desired operation of the circuit.

First, let's review the basic operation of an open collector circuit. In this type of circuit, the collector of the transistor is not connected to a power supply, but rather to a load (in this case, the LEDs). When the transistor is turned on, it allows current to flow from the power supply through the load and to ground. When the transistor is turned off, no current flows through the load.

Now, let's consider the values of R1, R2, and R3. R1 is used to limit the current flowing through the base of the transistor, while R2 and R3 are used to set the voltage levels at the base and collector, respectively.

To solve for the values of these resistors, you will need to consider the characteristics of the transistor being used, such as its current gain (hFE) and saturation voltage (VCEsat). You will also need to know the voltage level of the output of the MCU and the desired current through the LEDs.

Here are some steps you can follow to solve the problem:

1. Determine the required current through the LEDs. This will depend on the type of LEDs being used and the desired brightness. Let's say the desired current is 20mA.

2. Choose a transistor with appropriate characteristics for the circuit. For example, let's say you choose a transistor with an hFE of 100 and a VCEsat of 0.2V.

3. Calculate the base current required to turn on the transistor. This can be done using Ohm's Law, where Ib = (Vout - VBE)/R1. VBE is the base-emitter voltage of the transistor, which is typically around 0.7V.

4. Calculate the value of R1. Using the example values above, if the output of the MCU is 5V, the base current would be (5V - 0.7V)/R1 = 20mA. Therefore, R1 = 215Ω.

5. Calculate the voltage level at the base of the transistor. This can be done using a voltage divider equation, where Vb = V