Problem Using Bipolar Transistor as a Switch

In summary, the encoder works great but the user needs to use a bipolar transistor to convert the 5Vdc signal to a 24Vdc signal. The user is looking into a way to convert a 5Vdc signal to a 24Vdc signal with an open-collector stage.
  • #1
lindhaw
9
0
Im new here and I am currently working on a small IR encoder project. I have the encoder working great but now I need to use the 5vdc signal that comes from the encoder and make it 24vdc. I am currently trying to use a bipolar transistor but its not working. The basic diagram that I am going off of is attached along with the schematic of the optical encoder schematic. Am i way off track or am I missing something obvious.

BTW I am using 20k for R1 and 200K for R2 for the switch
 

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  • #2
lindhaw said:
Im new here and I am currently working on a small IR encoder project. I have the encoder working great but now I need to use the 5vdc signal that comes from the encoder and make it 24vdc. I am currently trying to use a bipolar transistor but its not working. The basic diagram that I am going off of is attached along with the schematic of the optical encoder schematic. Am i way off track or am I missing something obvious.

BTW I am using 20k for R1 and 200K for R2 for the switch

Welcome to the PF.

Can you run the first stage off of 24V? A 5V pullup stage will not shut off a 24V high-side switch stage...
 
  • #3
If you can't run the first stage at 24V, use an open-collector stage to convert from 5V to 24V.
 
  • #4
  • #5
lindhaw said:
I don't think I can do that because I think it would just burn up the emitter and possibly the receiver. I've also been looking into this http://www.goldmine-elec-products.com/prodinfo.asp?number=G18694 but I've never used anything like that so I am not sure if the response time will be fast enough.

Your link is just a DC-DC converter to convert a 5V power supply to a 24V voltage source.

What you want is a way to convert a 5V logic signal to a 24V logic signal, correct?

The way to do that is with an open-collector BJT stage. Are you familiar with those yet?
 
  • #6
Ya that's correct. I am not familiar with those yet. Is that something that I could make or is that something that I should just buy?
 
  • #7
ok dumb question sorry I just looked it up and I think i can manage one of those. Thanks for the advice! Ill let you know how it goes!
 

1. How does a bipolar transistor work as a switch?

A bipolar transistor can work as a switch by controlling the flow of current between its two terminals, called the collector and emitter. When a small current is applied to the base terminal, it allows a larger current to flow from the collector to the emitter, effectively switching the transistor "on" or "off".

2. What are the advantages of using a bipolar transistor as a switch?

One advantage of using a bipolar transistor as a switch is that it can handle relatively high currents and voltages, making it suitable for a variety of applications. Additionally, it can switch on and off rapidly, making it useful for digital circuits. It also has a low cost and is readily available.

3. What are some common problems when using a bipolar transistor as a switch?

Some common problems when using a bipolar transistor as a switch include thermal runaway, where the transistor overheats and fails due to excessive current, and saturation, where the transistor cannot fully turn off and causes a voltage drop. Other issues can arise from improper biasing or incorrect wiring.

4. How can I prevent thermal runaway when using a bipolar transistor as a switch?

To prevent thermal runaway, it is important to ensure that the transistor is not being overdriven with excessive current or voltage. This can be achieved by using appropriate resistors to limit the current and by properly selecting a transistor with a suitable maximum current and power rating.

5. Can a bipolar transistor be used as a switch in both digital and analog circuits?

Yes, a bipolar transistor can be used as a switch in both digital and analog circuits. In digital circuits, the transistor acts as a simple on/off switch, while in analog circuits, it can be used to control the level of a continuous signal. However, it is important to consider the specifications and limitations of the transistor when using it in different types of circuits.

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