Base resistor

[bananasiong]

Hi,
As shown in the attachment, this counter counts up to 5 seconds and stop, and the LED is turned off after that.
This is one of the experiment done in my college. Since there's no NAND gate in the lab of the college, most of them used NOT gate, but I've chosen a BJT as inverter. During presentation, the lecturer asking me about the BJT, but not digital electronics
And he asked what's the purpose of the 1 khm: resistor, I said to limit the current flowing through the LED, he accepted; he asked how about the 10 khm: resistor, I said to limit the current flow into the base to control the collector current with the gain, but he didn't accept it. He said that it has nothing to do with the gain, and the 10 khm: resistor can be removed. And he told me that the best transistor for switching purpose is 2N2222.
Am I wrong?

 

[Hero999]

No you are correct, the gain is important, the 1k resistor is to ensure the transistor saturates properly.

I wouldn't run this circuit from batteries though because it uses more power when the LED is turned off than it does when it's turned on!

 

[bananasiong]

Without the base resistor, the BE junction is in break over region right? Then the transistor is fully turned on with Icmax. Will this reduce the life span of the transistor?

Thanks

 

[Hero999]

It will either kill the transistor or the device driving it. Sometimes it's alright to do it with CMOS but only when they're running from a very low power supply voltage like 3V.

 

[bananasiong]

Last time my friend used BJT's to build H-bridge controlled by a microcontroller, and didn't include the base resistor. I didn't know whether it was working or not during the experiment, but during the presentation, the car ran very very slow, but I'm not sure, maybe the load is too heavy?
And my friend was using opto coupler with the BJT's as darlington pair, but the resistors across the base and emitter wasn't there, maybe this is the reason that the car not working.

 

[Hero999]

That might be because the microcontroller can't supply enough base current to drive the transistor properly.

What transistors were you using?

How much current did the motor draw?

 

[ericgibbs]

Hi,
; he asked how about the 10 khm: resistor, I said to limit the current flow into the base to control the collector current with the gain, but he didn't accept it. He said that it has nothing to do with the gain, and the 10 khm: resistor can be removed. And he told me that the best transistor for switching purpose is 2N2222.
Am I wrong?

Its possible that your tutor was referring the 'gain' of the transistor rather than the gain of the circuit.
As you say the 1K0 is a current limiting resistor for the LED/Transistor.

As already explained, its good practice to choose a base drive resistor than drives the transistor hard on [ ref: switching circuits] that suits the current gain of the transistor and does not overload the driving source.

The 2N2222 has been in production since 1968, so I expect its one that your tutor 'knows'.

Eric

 

[audioguru]

It depends on how much current the AND gate can supply to the base if a base resistor is required.

The max collector current for the transistor is 5V/1k= 5mA.
The transistor saturates well if its base current is 5mA/10= only 0.5mA.
If the output of the AND gate goes up to 4V then the 10k resistor provides a base current of (4V - 0.7V)/10k= 0.33mA which is not enough to fully saturate all 2N2222 transistors but it is perfectly OK to turn off the LED.

You should have told the tutor that the transistor has current gain so the base current needs to be only 1/10th of the collector current for any transistor to switch well.

 

[bananasiong]

That might be because the microcontroller can't supply enough base current to drive the transistor properly.

What transistors were you using?

How much current did the motor draw?

I'm not sure, it is almost 9 months ago. I know the motor rated 12 Vdc. And The transistor is power transistor, not darlington. They used opto coupler with the power transistor to form darlington, but miss out 2 resistors (attachment), and they did not use any base resistor as well.

The 2N2222 has been in production since 1968, so I expect its one that your tutor 'knows'.

But I think any general purpose transistor works well with this application right? Any NEW transistor that is good for switching? BTW, good means can perform switching fast?

It depends on how much current the AND gate can supply to the base if a base resistor is required.

The max collector current for the transistor is 5V/1k= 5mA.
The transistor saturates well if its base current is 5mA/10= only 0.5mA.
If the output of the AND gate goes up to 4V then the 10k resistor provides a base current of (4V - 0.7V)/10k= 0.33mA which is not enough to fully saturate all 2N2222 transistors but it is perfectly OK to turn off the LED.

You should have told the tutor that the transistor has current gain so the base current needs to be only 1/10th of the collector current for any transistor to switch well.

Yea, I've told him that the collector current is determined by the base current with the current gain, but just wasn't accepted.
If the 10k resistor cannot fully saturate the transistor when the output voltage of the AND gate drops a bit, the base resistor should be reduced, and it is still needed to be there, am I right?

 

[audioguru]

If the 10k resistor cannot fully saturate the transistor when the output voltage of the AND gate drops a bit, the base resistor should be reduced, and it is still needed to be there, am I right?

No.
An ordinary Cmos gate with a 5V supply has an output current of only about 3mA. It could directly drive the base of the transistor without a resistor.

 

[bananasiong]

No, I'm not using CMOS, but TTL gate. The output current is 40 mA max right? So a base resistor is needed, I think. But the collector resistor is limiting the Ic max to 5mA.

 

[audioguru]

No, I'm not using CMOS, but TTL gate. The output current is 40 mA max right? So a base resistor is needed, I think. But the collector resistor is limiting the Ic max to 5mA.

An ordinary 74xx TTL gate goes low with at least 16mA. It goes high to about 3.5V with a current of only 0.4mA minimum.

A 74LSxx output goes low with 8mA minimum. It goes high the same as a TTL gate.

 

[bananasiong]

There is still current flowing out from the output even it is logic low?

 

[audioguru]

Current doesn't flow in or out of an output. The output either goes high or it goes low.

The input of a TTL gate is the emitter of an NPN transistor. It needs a max low current of 1.6mA. A TTL output can drive 10 of them so its minimum low current is 16mA. When the TTL input goes high then very little current flows. So a TTL high output drives an LED poorly.

 

[ericgibbs]

hi,

Quote: audioguru
Current doesn't flow in or out of an output. The output either goes high or it goes low.

The input of a TTL gate is the emitter of an NPN transistor. It needs a max low current of 1.6mA. A TTL output can drive 10 of them so its minimum low current is 16mA. When the TTL input goes high then very little current flows. So a TTL high output drives an LED poorly.

How can both these statements be true???

If a TTL output is high and its connected to a TTL input which requires 1.6mA, where is the current coming from, if not the driving source.???

As the nominal value of the TTL's internal input pullup resistor is 4K0
If the TTL output is 'high' it sources approx 40uA for the TTL input its connected to.
If its 'low' it sinks approx -1.6mA from the TTL input its connected to.

TTL ic's are current controlled devices, not voltage, the outputs are designed to source and sink current.

Eric

 

[AllVol]

As the discussion progresses, I have a working circuit on the bench using a 74LS90 counter. I took the two circuits shown in the attachment from the 12 pin of the counter. The ammmeter shows the current drawn.
I used no resistor to the base of the 2N2222. Both circuits work as they should.

 

[ericgibbs]

hi AllVol,
Is it possible with your setup to measure the base current into the transistor from pin 12 and the pin 12 voltage, without a resistor?.
Thanks,

Eric

 

[AllVol]

hi AllVol,
Is it possible with your setup to measure the base current into the transistor from pin 12 and the pin 12 voltage, without a resistor?.
Thanks,

Eric

GM, Eric

Referring to the circuit using the 2N2222 transistor, Vout at pin 12 was measured at 3.92 volts wrt ground. Ibase with transistor on (LED glowing) was 27.63 mA. Ibase w/ transistor off: 0ma.



EDIT: When I returned to the bench after reporting the above results, I noticed that the 7-segment displays were showing "00" although the LED was blinking properly. Inserting a resistor in the base line allowed the display to return to normal as well as the LED operating. My guess is all the current produced by pin 12 was going to the LED, with nothing left for the counter chain to recognize. AV

 

[ericgibbs]

hi,

Many thanks, the base input current is higher than I would have expected??

Eric

 

[AllVol]

hi,

Many thanks, the base input current is higher than I would have expected??

Eric

You're welcome. Notice the edit to my results post, though.