Transistor biasing problem

transistor biasing

Hi Freinds
I have a small problem, in the given circuit the transistor bias is a problem,, as the circuit itself defines that the transistor emitter is directly connected to 12V but the base is connected to uc port which varies b/w 0 ~ 5v but in the circuit's transistor base keeps the transistor on through 0v~5v and not turns off, although the circuit is designed in such a way that it shoud be off at 5V and should turn on at 0V. Can any one help in this regards.

transistor level shifter

For sure it will not work
The transistor is always conducting due the fact the emitter is tied at 12 V and the base is or at 0 or at +5 but not a + 12 to switch off the transistor

Also you risk to destroy the micrprocessor

A suggestion

Put in series with the 10K resistor a zener diode with about 8 v voltage cathode connected to the base of the transistor and reduce the value of the resitor to 2K2

This circuit will work only if the 12 v is stabilized

Mandi

level shifter transistor circuit

I have used this circuit many times

transistor biasing problem

FANT said:

For sure it will not work
The transistor is always conducting due the fact the emitter is tied at 12 V and the base is or at 0 or at +5 but not a + 12 to switch off the transistor

Also you risk to destroy the micrprocessor

A suggestion

Put in series with the 10K resistor a zener diode with about 8 v voltage cathode connected to the base of the transistor and reduce the value of the resitor to 2K2

This circuit will work only if the 12 v is stabilized

Mandi

Click to expand...

I tried what you have suggested but it still does not work??? Can you plz upload a diagram (roughly made in windows paint as i did). May be i m not getting what you are suggesting? plz

ads transistor biasing

sn_burki said:

FANT said:

For sure it will not work
The transistor is always conducting due the fact the emitter is tied at 12 V and the base is or at 0 or at +5 but not a + 12 to switch off the transistor

Also you risk to destroy the micrprocessor

A suggestion

Put in series with the 10K resistor a zener diode with about 8 v voltage cathode connected to the base of the transistor and reduce the value of the resitor to 2K2

This circuit will work only if the 12 v is stabilized

Mandi

Click to expand...

I tried what you have suggested but it still does not work??? Can you plz upload a diagram (roughly made in windows paint as i did). May be i m not getting what you are suggesting? plz

Click to expand...

if the output of mcu is 5v,so the voltage of Vbe is 12v-5v=9v>>.7v,the Transistor is still conduct.

transistor biasing problems

I have tried both the above ideas but still its not working, still fail to achieve the point. The transistor still conduct and does not shut off??????????????????

a1015 basing

With my circuit you can increase the zener to 6.8V if your control signal is not close enough to 5V. Make sure you have the zener and transistor connected the right way.

If it still does not work you need to take some voltage readings on all the important nodes so we can see what is causing the problem.

diode transistor biasing

E-design said:

With my circuit you can increase the zener to 6.8V if your control signal is not close enough to 5V. Make sure you have the zener and transistor connected the right way.

If it still does not work you need to take some voltage readings on all the important nodes so we can see what is causing the problem.

Click to expand...

yes I have tried the Zener up till 8V but the problem still exists, at 8.2V Zener it works but the display output (brightness) is very low I am connecting every thing in the right way. I am using A1015 pnp transistor.

transistor biased but not conduct

Your circuit is badly designed in several regards.

As a first point, you have a segment current of about 60 mA, so the total anode current can be as high as 420 mA with 7 segments on. A1015 has a maximal collector current of 150 mA and a rather low guaranteed current gain at high currents.

So you should start by choosing a suitable PNP transistor and determine the base current, that has to be provided with worst case transistor specs. You should expect e.g. 5 or 10 mA, depending on the transistor. A PNP darlington could be used to reduce the base current below 1 mA.

Then, design a level converter, that is able to drive this current, possibly with varying supply voltages. If the supply known stable, the suggested Z-Diode level shift can work, but there are better ways with similar effort, I think. I suggest an auxilary NPN transistor driven from the uP (through a base series resistor) as a level shifter.

use transtor for level shifter

FvM said:

Your circuit is badly designed in several regards.

As a first point, you have a segment current of about 60 mA, so the total anode current can be as high as 420 mA with 7 segments on. A1015 has a maximal collector current of 150 mA and a rather low guaranteed current gain at high currents.

Click to expand...

Thanks for kind answer,,, Actually there are four 7segment display in the circuit but i have kept 4 X A1015 each display have sepret transistor and the display is multiplexed. Each 7 segment display consume 60 mA current and A1015 can pass 150 mA so it is ok. My problem is that I want to drive this display from 12V rather than 5V and badly stuck what to do????

choosing a suitable transistor

If your above circuit is correct, the current would be 60 mA for each segment, summing up to 420 mA.
I also suggested to use a NPN transistor as a level shifter.

biasing of transistor to work as switch

FvM said:

If your above circuit is correct, the current would be 60 mA for each segment, summing up to 420 mA.
I also suggested to use a NPN transistor as a level shifter.

Click to expand...

What would you say about it?

biasing in transistor

Yes, that's exactly the level shifter I thought of. Depending on the required A1015 collector current (I had a doubt about total anode current, you'll remember) and individual transistors current gain, the A1015 4K7 base resistor can be further reduced (e.g. 1K), but you can start as shown.