CE leakage current?

[vlad777]

If I connect collector to +50V Vcc and ground base, what will be the voltage
on unconnected emitter?
My multimer has input resistance of 1MOhm (when voltmeter) and it
measures 2-12 volts.
This is even more of a problem with darlingtons.
I'm guessing this is because of leakage from c to e so I put 10k between e and gnd
and it works.

Am I correct and what is the best way to solve this problem?

Thanks.

 

[crutschow]

Yes, all transistors and diodes have a small amount of leakage across a reverse-biased junction. A resistor of the correct size will absorb the leakage with only a small voltage drop across the resistor.

But why are you concerned about the voltage on an open emitter?

 

[vlad777]

But why are you concerned about the voltage on an open emitter?

Well if base is grounded I expected 0V on open emitter but I got 2-12 V.

 

[crutschow]

Well, you said this voltage was a problem and I'm trying to understand why it is. Or are you just concerned from an academic point of view?

 

[vlad777]

I (tried) to make most basic voltage regulator (zener+emitter follower) but I had to use darlington (for higher current),
so I had to solve this problem that it gives some voltage on it's output when it's meant to give 0V.
So I am asking where is best to put this grounding resistor(s) (and what would be the values) given I am using discrete darlington.

Edit:
Also now I am interested what would be the answer if it was academic concern.

 

[MrAl]

Hello,


It's a good idea to have some load on the output anyway. Even a 1 megohm resistor might do it. If it is a darlington, then a resistor from the second base to ground is also a good idea if you really (for some reason) need some very low output. Some darlingtons have a built in resistor from second base to second emitter though.
The only reason i can think of for needed this is when you disconnect the load entirely so that there is no load connected. With a fixed permanent resistor from emitter to ground this should no longer be a problem.

 

[crutschow]

I (tried) to make most basic voltage regulator (zener+emitter follower) but I had to use darlington (for higher current),
so I had to solve this problem that it gives some voltage on it's output when it's meant to give 0V.
So I am asking where is best to put this grounding resistor(s) (and what would be the values) given I am using discrete darlington.

Edit:
Also now I am interested what would be the answer if it was academic concern.

Now that you've told us your application, I understand your concern.

Yes a resistor to ground should solve you problem. A value of 1k-10kΩ from the emitter to ground should suffice. If that's not sufficient for a Darlington, then go to a lower resistor value, but no lower than needed.

By academic, I meant whether it was only of concern from a theoretical point of view, and not a practical one. But you have a practical concern.

 

[vlad777]

It seems to have been the soldering paste conducting from c to e. (TO-220 pin2-pin3)
When I cleaned the pcb I no longer get the phantom voltage.

What is funny is when I try to measure resistance of the paste I don't get any reading
(I can only measure up to 2MOhm).

Edit:
Since Vc=60V and Vphantom was 10V and input resistance of my multimer is 1MOhm,
that gives 5MOhm for the paste.

 

[vlad777]

Is it wrong to put an inductor between the bridge and caps on +rail ?