When the positive current flow through the diode, then the Vac will come out when you measure it, different current flow through the diode has different Vac, it is not a fixed voltage.

The transistor similar with diode, so when the Ib is different then the Ie and Ic will be different.

 

Yes, you do this in-circuit while it's powered on and put one probe to the emitter and one to the base in DC mode.

I don't get 0.7 volts , I get DC offset voltages , it's not 0.7 volts

 

What voltage reading do you get?

 

I can't measure the .7 volts across the base to emitter because
1.) The Transistor is a PNP , so you don't get .7 volts from base to emitter
2.) The Transistor is "energized" way pass .7 volts
3.) When the transistor is biased and has biasing resistors, you can't measure the .7 volts , it's way past the .7 volts to turn on and energize the transistor

 

Is there any way to measure the transistors .7 volts across the base to emitter , when the transistor is in circuit?

Because mostly the transistor is "energized" and past the turn on/forward biased voltage

So there is no way for me to measure the .7 volts right across the base to emitter?

Is energized a good work to use when saying it's been forward biased and turned on?

You can't measure the .7 volts when a transistor is PNP because it's only .7 volts from base to emitter using NPN transistors, is that right?

 

Well, ok, PNP. The same idea still applies though just with a reverse voltage. Touch the red probe to the emitter and the black probe to the base. It should still be around 0.7V. If you reverse the probes it will read around -0.7. Even if you have emitter resistors and the transistor is floating somewhere in the middle of the circuit supply voltage the distance between the base and emitter should still be around 0.7V. If you touch one probe to ground then that's a different story, you'll get a reading of where the transistor is floating at relative to ground and that would be useless.

 

Is energized a good work to use when saying it's been forward biased and turned on?

Not really. If you use the transistor as a switch it's usually referred to as either being in the "saturation region" (fully ON) or the "cutoff region" (fully OFF). If you're using it as an amplifier or anywhere in the middle, it's usually said to be in the "active" or "linear" region.

 

Do you mean I should turn the circuit OFF no power on and use the DVM diode mode when testing a transistor base to emitter in circuit?

PNP. The same idea still applies though just with a reverse voltage.

If you didn't reverse the voltage, how would the PNP transistor be used?

When do you use a PNP transistor ? and what for?

 

No, you should measure it with the circuit on for this test. Let's say you have a circuit that is powered with 10 volts. One point in the circuit is at 4V and one is at 5V. Say there are just a few resistors in the circuit. If you test the voltage with the black probe on ground (0V), they'll read 4V and 5V but if you put the black probe on the 4V point and the red probe on the 5V point, the reading will be 1V. So that's what I was suggesting.

 

Ya i know, the base to emitter for an npn transistor is not. 7 volts in circuit
You mean it will be around 1 volt in circuit?

For pnp transistors u have to reverse the probes to get the. 7 volts

 

What type of voltmeter are you using?
If you have a digital voltmeter, you do not have to reverse the leads.
You will get a negative reading on the meter.

 

Fluke 87 dvm meter

 

A PNP transistor is used for the same reasons as an NPN transistor except all polarities are reversed. (You cannot simply substitute one for the other in the same circuit unless you change the polarity of the power supply.)

Let's start from square one.

You have a Fluke 87 DVM.
You can measure the base-emitter voltage by connecting the positive lead (red) to the base and the negative lead (black) to the emitter. The DVM must be on DC voltage range. The circuit must be powered. (You cannot use the diode function of the DVM when the transistor is in-circuit and DEFINITELY NOT WHEN THE CIRCUIT IS POWERED.)

What is the reading on the DVM?
What is the sign of the reading?
Do you have a circuit schematic?
What is the function of the circuit?
Are all inputs to the circuit DC?
Do you have an oscilloscope?
Did you replace the transistor and subsequently obtained a stable 0.7V reading?

 

Transistor controlling the current?
http://s1348.photobucket.com/user/BillyMayo1988/media/Transistor1_zps275e0df8.jpg.html?sort=3&o=2


Transistor as a comparator? or a switch
http://s1348.photobucket.com/user/BillyMayo1988/media/NPNpic1_zpsda3bf1e1.jpg.html?sort=3&o=1

Transistor as a comparator? or a switch
http://s1348.photobucket.com/user/BillyMayo1988/media/NPNpic2_zpsa1075c0a.jpg.html?sort=3&o=0

Why did the designer use transistors instead of a FET?

Using a transistor as a switch is different than using a FET as a switch? what is the difference?

 

I think you missed part of the questions.

What is the reading on the DVM?
What is the sign of the reading?
Do you have a circuit schematic?
What is the function of the circuit?
Are all inputs to the circuit DC?
Do you have an oscilloscope?
Did you replace the transistor and subsequently obtained a stable 0.7V reading?

 

Oscilloscope used:
http://www.testequipmentdepot.com/tektronix/oscilloscope/tds3032c-tds3034c.htm

FLuke 87 meter

 

I also see Diodes from the base of a transistor , going to ground

The Diodes on the base going to ground protects it from negative voltage? what does that mean, because a diode is only 0.5 volts so it any negative voltage will be much more than 0.5 volts but it will conductor the transistor if the diode wasn't there.

They put the diodes on the base going to ground , so negative voltage won't turn on the transistor?

 

I think you missed some of the questions.

What is the reading on the DVM?
What is the sign of the reading?
Do you have a circuit schematic?
What is the function of the circuit?
Are all inputs to the circuit DC?
Do you have an oscilloscope?
Did you replace the transistor and subsequently obtained a stable 0.7V reading?

 

I think looking at the pictures tells u enough for a circuit analysis

You don't need anymore info.

 

Thanks. I was wondering how much you are willing to help us help you. Now I know.