biasing npn transistor without a resistor

[bananasiong]

FET, I think I have to spend some time to learn this.

Thanks

 

[eng1]

Just know that they are voltage-controlled devices, no gate current is required

 

[audioguru]

The datasheet for most transistors shows a max saturation voltage with a very high base current of 1/10th the collector current, even if the transistor has a current gain of 800!
The minimum current gain of a 2N3904 with a 40mA load is about 65 and it is guaranteed to have no more than 1.0V saturation voltage with a base current of only 0.62mA. So you can gamble a little and use a base current that is 1/20th of the load current and most transistors will saturate fine.

 

[Top Cap]

My old Mentor always told me to take a gain figure of 10 as a rule of thumb and then one was more or less guaranteed a working circuit.
Attached is a simple scheme of the transistor with protection diode in the base to stop blowing up your PIC if the transistor developed a short. The Darlington has no advantage here except to reduce base current requirement.
As Audioguru has stated, a better solution would be to use a Mosfet such as the 2N6660. Notice how the high input impedance allows us to increase the resistor values to a point where the input protection diode could probably be deleted.

 

[audioguru]

You don't need a protection diode. One transistor out of one billion of them will short.
The Mosfet needs a low impedance driving its gate or it will oscillate at a high frequency.

 

[Top Cap]

First my most sincere apologies for not deleting the diode in the FET circuit , it was a combination of a hasty Copy/Paste operation and tiredness. It should not be there as there can be another problem due to noise or RF pick up being rectified by the diode and turning on the FET! In case of FET failure, the higher values of resistors should provide enough current limiting to protect the lower voltage driving circuits from the 24 volts. Audioguru is correct in saying that the chance of a collector to base short in the transistor is extremely rare but it is a greater possibility should the relay coil be accidentally shorted during operation.
I only suggest the use of a diode where a typical PC port pin or equivalent is driving a transistor that is operating at a much higher voltage, in this case 24V. A cheap 10p diode is better than losing an interface device on a PC motherboard. In my profession, the signal source is likely to come from an Avionics unit costing many 100's of £k and the hardest part of my job is trying to stop even the impossible from happening to ensure safety of a multi £Million aircraft and of course the pilot. I must train my brain to accept that in the more down to earth world (no pun intended), such design paranoia should not be so necessary.

 

[audioguru]

Don't worry about the pilot. He just jumps out before a crash.
Don't stop all plane crashes, allow a few military ones because they are fun to watch. A site on the web has a collection of plane crashes all in one movie. Awesome!

 

[Top Cap]

Yep I like watching those, crazy sights
Trouble is I work on the stuff to jam missiles, gunnery radar and blind IR seekers.
If my lot fails then the poor ol' pilot don't get a chance to pop the canopy, the missile will do it for him and then it is too late!

 

[audioguru]

Has the 3rd World War already begun?? Who're the players?? What channel??

 

[Top Cap]

It may arrive sooner than we all think.
Heaven help us all if we are not ready this time!

 

[audioguru]

There are too many weirdo groups in the world today.