Large BS170 drain current in off mode

[Koen Vermeer]

Hi,

I have some basic knowledge of electronics, but I'm having trouble trying
to get some real design to work. One of the things that do not work as
expected is the BS170 MOSFET I'm trying to use. I know, it's not a very
sophisticated part, but I have a few of them lying around and I thought
they would be able to assist my learning.

Anyway, I connect the gate and source of the BS170 both to the ground,
and the drain to a 330 ohm resistor, connected to 3.6 V. When I then
measure the potential of the drain, it's at 2.85 V, which means it draws
a current of (3.6-2.85)/330 = 2.3 mA. Thus, the drain-source resistance
is about 1.2 kohm. My expectation was a much larger 'off' resistance (say
100 kohm or more).

I cannot relate anything on the data sheet (http://www.fairchildsemi.com/
ds/BS%2FBS170.pdf) to my observation. I tried with another BS170, which
gave about the same results. Is my expectation wrong? Apparently, it is,
but what is the right explanation? And, how can I deduce this from the
data sheet? Then, how to solve this? Should I just use another
transistor, or is there some common way to handle this?

In my case, I connected a LED to the drain and it didn't shut off. I can,
of course, put a resistor in parallel, but that would increase power
dissipation and would reduce the LEDs brightness when turned on.

Thanks for any insight!

Koen

 

[John Larkin]

Hi,

I have some basic knowledge of electronics, but I'm having trouble trying
to get some real design to work. One of the things that do not work as
expected is the BS170 MOSFET I'm trying to use. I know, it's not a very
sophisticated part, but I have a few of them lying around and I thought
they would be able to assist my learning.

Anyway, I connect the gate and source of the BS170 both to the ground,
and the drain to a 330 ohm resistor, connected to 3.6 V. When I then
measure the potential of the drain, it's at 2.85 V, which means it draws
a current of (3.6-2.85)/330 = 2.3 mA. Thus, the drain-source resistance
is about 1.2 kohm. My expectation was a much larger 'off' resistance (say
100 kohm or more).

The drain current should be tiny in this setup. Idss is spec'd at 0.5
uA max and should actually be a lot less, nanoamps maybe; the fet
should be off, not conducting at all. So something is wrong. Maybe
it's connected wrong, maybe it's zapped,

I cannot relate anything on the data sheet (http://www.fairchildsemi.com/
ds/BS%2FBS170.pdf) to my observation.

Idss is the relevant parameter.

John

 

[Koen Vermeer]

The drain current should be tiny in this setup. Idss is spec'd at 0.5 uA
max and should actually be a lot less, nanoamps maybe; the fet should be
off, not conducting at all. So something is wrong. Maybe it's connected
wrong, maybe it's zapped,

My thoughts were that the current should be very small as well, but it
isn't. It's quite stupid. I just tried two others: One was about the
same, the other gives a current of about 350 uA. Still way too large. It
doesn't make sense to me. :-(

I can try changing the connections, but I guess that's a sure way to
break them...

Koen

 

[John Larkin]

My thoughts were that the current should be very small as well, but it
isn't. It's quite stupid. I just tried two others: One was about the
same, the other gives a current of about 350 uA. Still way too large. It
doesn't make sense to me. :-(

I can try changing the connections, but I guess that's a sure way to
break them...

Koen

Sounds like, from the original numbers, that you had the gate
connected to the drain.

John

 

[Koen Vermeer]

Sounds like, from the original numbers, that you had the gate connected
to the drain.

I was thinking about that, but while the data sheet isn't very clear
about the gate and source, the drain is rather obvious. Of course, I can
try to change the connection, and hope that I didn't kill the BS170
itself earlier...

Koen

 

[Koen Vermeer]

I was thinking about that, but while the data sheet isn't very clear
about the gate and source, the drain is rather obvious. Of course, I can
try to change the connection, and hope that I didn't kill the BS170
itself earlier...

OK, I bought some new BS170s, and the first one I tried worked as
expected. So I guess I had a bad batch or something like that. Anyway,
I'm happy to have regained my sanity

Thanks for the help!

Koen