Transistor as a switch

johnl69 · Mar 23, 2008

Can a Transistor be used as a N/C switch so that it allows current to pass when there is no current at the base?

Analog · Mar 23, 2008

johnl69 said:
Can a Transistor be used as a N/C switch so that it allows current to pass when there is no current at the base?

A MOSFET is a transistor that has no current at its gate whether on or off. You could setup a p-channel MOSFET as normally closed, as 0V on its gate would be considered "ON", and positive voltage at its gate would shut it off...

Hero999 · Mar 23, 2008

No, not a BJT.

A JFET acts like a normally on switch, when the junction is biased by a negative voltage it turns off.

johnl69 · Mar 23, 2008

Cheers guys

So in the circuit attached when the switch is pressed the bulb should go out and the LED should light up

killivolt · Mar 23, 2008

Does it need a resistor or does the Jfet provide the resistance ?

johnl69 · Mar 23, 2008

killivolt said:
Does it need a resistor or does the Jfet provide the resistance ?

I dont know the one ive picked is rated at 25v but I will be switching 12V (not 9v as it shows in the diagram)

**broken link removed**

Hero999 · Mar 23, 2008

That won't work.

Notice, that I said that JUGFETs reuquire a negative voltage to turn off so you need to reverse the battery connected to the gate.

Also, you haven't included a series resistor so the LED will be toast.

How much current does the bulb draw?

JUGFETs tend to be low power devices which will be overloaded by the bulb.

johnl69 · Mar 23, 2008

Hero999 said:
That won't work.

Notice, that I said that JUGFETs reuquire a negative voltage to turn off so you need to reverse the battery connected to the gate.

Also, you haven't included a series resistor so the LED will be toast.

How much current does the bulb draw?

JUGFETs tend to be low power devices which will be overloaded by the bulb.

If I turned the battery connected to the gate around wouldnt it short out?(I am planning on using 1 supply.

I am not turning a bulb on its a relay that I am switching I just did that simple diagram the one below is what im building,

what should happen is;

For the motor to turn sw1 and sw3 need to be open and sw2 to be closed, if any of the switch is in a different postion(open when it should be closed) the motor will stop and an LED will light turn on, except for D3 which turns on when the motor is running.

I am trying to make this with the minimum amount of component.

Roff · Mar 23, 2008

What are the voltage and current requirements of your relay coil?
In your schematic, which needs some help, it looks like D3 is on when the motor is on, not D1.
Do you really need all 3 LEDs?

johnl69 · Mar 23, 2008

Roff said:
What are the voltage and current requirements of your relay coil?
In your schematic, which needs some help, it looks like D3 is on when the motor is on, not D1.
Do you really need all 3 LEDs?

Sorry yes D3 turns on not D1. the Led's will be mounted away from the switches which are liquid level sensors (float switches) the LED's are to give a quick referrence of which switch is in what state.

The relay requires 12v 35mv
**broken link removed**

Roff · Mar 23, 2008

johnl69 said:
If I turned the battery connected to the gate around wouldnt it short out?(I am planning on using 1 supply.

I am not turning a bulb on its a relay that I am switching I just did that simple diagram the one below is what im building,

what should happen is;

For the motor to turn sw1 and sw3 need to be open and sw2 to be closed, if any of the switch is in a different postion(open when it should be closed) the motor will stop and an LED will light turn on, except for D3 which turns on when the motor is running.

I am trying to make this with the minimum amount of component.

If the relay and D3 are energized when the motor is running, then SW1 will be closed, not open as you said. Am I confused, or are you? If it's me, I apologize. If it's you, why don't you try to restate the problem?

johnl69 · Mar 24, 2008

Roff said:
If the relay and D3 are energized when the motor is running, then SW1 will be closed, not open as you said. Am I confused, or are you? If it's me, I apologize. If it's you, why don't you try to restate the problem?

Your right its me that got confused (it was late)

I'll start again;

for the motor to run SW1 and SW3 need to be closed and SW2 needs to be open,

when SW1 is open the motor stop and all lights are out,

when SW2 is closed the motor stops and D2 is lit,

when SW3 is open the motor stops and D1 is lit,

Edit; D1/D2 are removed fromm the drawing below any ideas of where to connect them?

killivolt · Mar 24, 2008

Doesn't he have to control the back emf so that the low voltage circuit will be uneffected if this is an a/c motor ?

johnl69 · Mar 24, 2008

killivolt said:
Doesn't he have to control the back emf so that the low voltage circuit will be uneffected if this is an a/c motor ?

The motor is a water pump which will be sited at least a meter away from the control unit, would the relay and the transformer isolate any feedback?

killivolt · Mar 24, 2008

Actually I'm new at this and it may not prove to be a problem I'm still a learning. But as is with speakers there is somewhat of a problem with control input when you have mains voltage and I'm just not sure about your circuit we need help.

killivolt · Mar 24, 2008

I'm sorry for confusing you don't pay any attention to me I'll remain quiet.

Roff · Mar 24, 2008

johnl69 said:
Your right its me that got confused (it was late)

I'll start again;

for the motor to run SW1 and SW3 need to be closed and SW2 needs to be open,

when SW1 is open the motor stop and all lights are out,

when SW2 is closed the motor stops and D2 is lit,

when SW3 is open the motor stops and D1 is lit,

Edit; D1/D2 are removed fromm the drawing below any ideas of where to connect them?

You can use the circuit below if you don't absolutely have to have D1 and D2.
If you need D1 and D2, take a look at the truth table below and correct it, if necessary.

Code:

SW1  SW2  SW3  D1     D2     D3    Motor
O    O    O    off    off    off    off
O    O    C    off    off    off    off
O    C    O    off    off    off    off
O    C    C    off    off    off    off
C    O    O    on     off    off    off
C    O    C    off    off    on     on
C    C    O    on     on     off    off
C    C    C    off    on     off    off

johnl69 · Mar 24, 2008

Thanks for that but I relly do need all three LED's, as these will be warning lights, I have rearanged the true table.

Code:

SW1  SW2  SW3  D1     D2     D3    Motor
O    O    O    on     off    off    off
O    O    C    off    off    off    off
O    C    O    on     on     off    off
O    C    C    off    on     off    off
C    O    O    on     off    off    off
C    O    C    off    off    on     on
C    C    O    on     on     off    off
C    C    C    off    on     off    off

johnl69 · Mar 24, 2008

If I put an LED in series with SW2(between SW2 and ground) would that then light up when the switch is in the close position.

Roff · Mar 24, 2008

johnl69 said:
If I put an LED in series with SW2(between SW2 and ground) would that then light up when the switch is in the close position.

The voltage drop of the LED would also allow the transistor to turn on, which is not what you want.
I think the simplest design for you is going to require a 14-pin quad gate package, a couple of transistors, and a few resistors. A microcontroller might require less hardware, but I don't think you have the capability to program one.
I'll work on a design as i have time.

Transistor as a switch

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