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Help about transister biasing

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Altafhal

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24v emitter follower

Hello friends
i have a problem. I am interfacing a proximity switch with plc. The switch is NPN. When it detects an object it gives me 0V at output and when there is no object it gives 24V or 12V depending on its input voltage. This sensor takes 24V and 12V at its input. I used an inverter to take 12V or 24V at its output. I used 2SA684 PNP transistor and biased it as followed.
I select Beta 250 and by calculation Rb is 100k. This trasistor operates a 12V, 400 ohm relay.
I want that the signal at base should be 5V. How i should acquire this five volts from 24 or 12V.
I have to use 7805, Zener volt diode(if any) or to use 12V at sensor input and use the voltage divider bias to operate the relay.
Please friends reply me soon.
Thanks
 

xaccto

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transister biasing

it would help if you did a schematic, but from what I understand of your problem
see attached pic.

I wouldn't go trying to assume a beta of 250 either, 100 is a good rule of thumb,
especially as I didn't see a relevant diagram in a datasheet of the said transistor.

I might be wrong though, i'm a bit rusty on these things now.
 

Audioguru

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The beta of a transistor is used when it has plenty of collector to emitter voltage so it is linear. Beta is not used for this saturated transistor.

The base current must be 1/10th to 1/20th of the collector current to guarantee it saturates well.
 

Altafhal

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can i apply 12V or 24V to the base of the transistor in any bias?
 

Audioguru

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Altafhal said:
can i apply 12V or 24V to the base of the transistor in any bias?
It depends on the circuit. An emitter-follower is fine. A common-emitter transistior with the emitter grounded will blow up.
 

Altafhal

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The same circuit as voltage divivder if it is not fine then plz can you upload the emitter follower for 12 V 400 ohm relay driver.
Thanks audioguru
 

Audioguru

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Altafhal said:
The same circuit as voltage divivder if it is not fine then plz can you upload the emitter follower for 12 V 400 ohm relay driver.
Thanks audioguru
But I don't know why you want to put 12V or 24V to the base of a transistor. To turn it on or to turn it off? Is the power supply 12V or is it 24V?
 

Altafhal

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no i am using a proximity sensor with plz which operates on 12-24 volts and give 12-24 volts at its output.
 

Audioguru

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A 12V relay will smoke and burn if you feed it 24V.
A 24V relay will not work if you feed it 12V.

What do you have?
 

siongboon

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As mention by Audioguru,
I think you are trying to use a transistor as a switch.

Transistor can be classified into 2 type of operation.
- transistor as a switch
- transistor as an amplifier

When you are using it as a switch,
parameter such as beta is of little or no use.
Often I will look into the current "Ic" it can handle as a switch.
It is the most important parameter when used as a switch.
Same as the property of a push button switch,
it determine the current capacity of the switch.
Ib, Vbe or gate voltage helps to compute the resistor Rb on the base.
Most of the time, I don't compute at all, and typical use 1kohm as Rb.
Unless the application requires power saving or design is complicated.

Perhaps you can provide us a schematic,
I am sure friends here are able to solve you problem precisely.
Friends here are very friendly.

Using transistor as a switch is actually very simple.
I have attached a article on this topic that I have for your reference.
http://www.siongboon.com/projects/2006-06-19_switch/#Transistor as a switch
I believe it should be what you are looking for.
And hopes the article can provide you a new perspective
of looking at a transistor.

Best Regards,
Siong Boon

www.siongboon.com
 

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