# Transistor as switch

Discussion in 'General Electronics Chat' started by sureshparanjape, Jul 14, 2013.

1. ### sureshparanjape Thread Starter Member

Feb 10, 2012
64
2
As a beginner in electronics I am fascinated by the ingenuity of transforming theoretical concepts in physical world. A simple physical action of on/off gate is achieved by a transistor on board, which led to the architecture of modern computers! I have a simple question - when a transistor is "closed", does the voltage at collector become practically zero, that is, a positive measurable by a multimeter but for all practical purposes zero? The reason, I am asking this question, is that on my board I do have a three transistor switches connected to perform a certain task and the voltage at the collector, which should be zero, is about 0.20 V.I tried various combinations of resistances at bases but this is the minimum voltage. The combination of three transistors gives the desired result.(The details of the circuit is not included only for not crowding the question).
sureshparanjape

2. ### WBahn Moderator

Mar 31, 2012
18,088
4,917
The lowest collector-emitter voltage (and note the distinction between "the voltage at the collector" and "the voltage difference between the collector and emitter") you can get when the transistor switch is "closed" is called the "saturation voltage". It is typically in the 50mV to 250mV range. Like everything, it is a function of several parameters including device design, temperature, and base current. For most purposes, using 0.2V for silicon BJT transistors is good enough.

3. ### crutschow Expert

Mar 14, 2008
13,505
3,383
If you need a very low ON (closed) voltage, you can use a MOSFET. They exhibit a pure ohmic resistance when ON and thus the ON voltage can be made arbitrarily low by selecting a suitably large MOSFET with low ON resistance.

sureshparanjape likes this.
4. ### sureshparanjape Thread Starter Member

Feb 10, 2012
64
2
Many Thanks,My level of electronics knowledge can't grasp implication of your comment- that is my drawback! I was thinking on using CD 4093 for the transistor gates that I am using- one NOT Gate and one AND gate. I could configure three of NAND gates of CD 4093 to perform the activities that I wish to carry out. However my (non)knowledge doesn't give me confidence in using it.
sureshparanjape

5. ### Ramussons Active Member

May 3, 2013
562
92
Can you describe what is that you wish to do? You will get a lot more info that way.

Ramesh

6. ### BobTPH Active Member

Jun 5, 2013
807
121
That is a logic gate, is can only deliver 2.25mA of current and it drops 0.5V at that. It is not appropriate to switch a load. How much current do you need to switch?

Bob

7. ### WBahn Moderator

Mar 31, 2012
18,088
4,917
Well, we aren't mind readers and are crystal balls aren't in very good shape. We can't help you too much unless you tell us what it is you are trying to do.

You are mixing a couple of concepts -- you originally were talking about using a transistor as a switch and now you are talking about using logic gates from a CD4093. These are logic gates, not transistors. They may or may not be suitable for what you are trying to accomplish. Again, we have no way to even guess at that unless you tell us what you are trying to accomplish.

8. ### sureshparanjape Thread Starter Member

Feb 10, 2012
64
2
Answer(s) to all kind people who responded,
Many many thanks.
I am sorry if my question raised more questions that I didn't anticipate.I had, in an earlier thread,talked about my little project, semi automatic switch. I have tried my hand in using three transistors as switches to get what I need. I will not repeat the whole story again; however I do owe transparency to all these supporters. I am attaching a file of my project which,I think, is working on board as desired.My practically knowledgeable friend helping to put it on board with relevant supporting smps etc of 12 V. Relays are working properly. Only check is to make sure the whole circuit becomes off for most of time and work only for the duration of pumping.
Briefly,at the cost of repeating what my goal is, I expect (i) to start pumping manually,(ii) to shut off completely automatically,(iii)pumping to continue till upper tank is full OR level of water in lower tank is below a certain level - this also requires that pumping should not start if level in lower tank is below the level.
I would like to add for Mr.WBahn the following. I came across CD4093 in my random search. What I see in my circuit is that in effect first I am converting input of U-tank probe like NOT gate and later using this output and input from the lower tank as inputs to AND gate. So the curiosity of checking whether I can use CD 4093 for the same purpose led me to study its functioning. However, as a after thought,I think, which may be wrong, using only three transistors is better.
I have attached the file.
sureshparanjape

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9. ### ananthapriya New Member

Jul 18, 2013
10
3
Can you say which transistor you have to choose? power mosfet or BJT , or IGBT which one is act as switch Say clearly?I think u have to choose may be Mosfet because of this one is voltage controlled device.