Probably a stupid question, but...

[Dave]

Is there a type of resistor that senses the potential applied to it, and
sets it's value accordingly? I know there's a part that does this, but I
can't for the life of me think of what it's called. Sorry, I'm getting old
and the gray cells aren't doing what they used to for me. Wanting
something that raises it's value as the potential drops, and it would need
to be "programmable" so I can "tune" it to my particular needs. Seems like
a standard transistor could be made to do this, but again I can't figure out
how. Thanks.

Dave

 

[Jamie]

Dave wrote:

Is there a type of resistor that senses the potential applied to it, and
sets it's value accordingly? I know there's a part that does this, but I
can't for the life of me think of what it's called. Sorry, I'm getting old
and the gray cells aren't doing what they used to for me. Wanting
something that raises it's value as the potential drops, and it would need
to be "programmable" so I can "tune" it to my particular needs. Seems like
a standard transistor could be made to do this, but again I can't figure out
how. Thanks.

Dave






Yes, it's called a transistor and depending on how accurrate you need

the transfer and how stable it should be, depends on additional design
to the over all circuit.

A basic NPN common emitter could be used as a current sink type of
output where you know the supply voltage on the load side and simply
calculate the needed current which would translate to voltage across the
load on the circuit.. It's all about gain ratios.

You can add a diode in the base circuit to help with tempo problems as
the transistor is subjected to various levels of heat,which will alter
the gain setting..

Jamie

 

[Dave M]

Dave wrote:

Is there a type of resistor that senses the potential applied to it,
and sets it's value accordingly? I know there's a part that does
this, but I can't for the life of me think of what it's called. Sorry, I'm
getting old and the gray cells aren't doing what they
used to for me. Wanting something that raises it's value as the
potential drops, and it would need to be "programmable" so I can
"tune" it to my particular needs. Seems like a standard transistor
could be made to do this, but again I can't figure out how. Thanks.

Dave

There's no single component that I'm aware of that will do what you've asked
for. If you will elaborate a bit on what you're trying to accomplish,
someone will surely be able to assist in a solution. Likely, a circuit will
have to be designed to do it, but without more info, it's just a shot in the
dark.
Vague questions will get vague answers.
--
David
dgminala at mediacombb dot net

 

[Dave]

"Dave M" <dgminala4444@mediacombb.net> wrote in message
news:UrKdnVR1H_KF8UnQnZ2dnUVZ_oydnZ2d@giganews.com...

Dave wrote:
Is there a type of resistor that senses the potential applied to it,
and sets it's value accordingly? I know there's a part that does
this, but I can't for the life of me think of what it's called. Sorry,
I'm getting old and the gray cells aren't doing what they
used to for me. Wanting something that raises it's value as the
potential drops, and it would need to be "programmable" so I can
"tune" it to my particular needs. Seems like a standard transistor
could be made to do this, but again I can't figure out how. Thanks.

Dave


There's no single component that I'm aware of that will do what you've
asked for. If you will elaborate a bit on what you're trying to
accomplish, someone will surely be able to assist in a solution. Likely,
a circuit will have to be designed to do it, but without more info, it's
just a shot in the dark.
Vague questions will get vague answers.
--
David
dgminala at mediacombb dot net

Thanks for the reply. Wasn't trying to be deliberately vague, just didn't
know what to specifiy without diving into the whole thing. And didn't want
to look like I was trying to get someone else to do my homework. What I am
trying to do is bias a JFET for optimum performance while keeping it very
near the pinch-off voltage. In this case I am using a 2N5486 (NTE451) to
feed signal to a BJT (SK3245 - NTE199). Problem is, it works fine while the
batteries are very fresh, but when they start to lag Vgs becomes greater
than pinch-off. Please see my post to Jamie for more details. And thanks
agian.

Dave

 

[Dave]

"Jamie" <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote in message
news:BsZAp.8851$oq.3889@newsfe17.iad...

Dave wrote:

Is there a type of resistor that senses the potential applied to it, and
sets it's value accordingly? I know there's a part that does this, but I
can't for the life of me think of what it's called. Sorry, I'm getting
old
and the gray cells aren't doing what they used to for me. Wanting
something that raises it's value as the potential drops, and it would
need
to be "programmable" so I can "tune" it to my particular needs. Seems
like
a standard transistor could be made to do this, but again I can't figure
out
how. Thanks.

Dave






Yes, it's called a transistor and depending on how accurrate you need the
transfer and how stable it should be, depends on additional design to the
over all circuit.

A basic NPN common emitter could be used as a current sink type of
output where you know the supply voltage on the load side and simply
calculate the needed current which would translate to voltage across the
load on the circuit.. It's all about gain ratios.

You can add a diode in the base circuit to help with tempo problems as
the transistor is subjected to various levels of heat,which will alter the
gain setting..

Jamie

Man, I knew that I was going to end up using a transistor to do that job, I
just didn't know how. So anyway, as I mentioned in my post to Dave M. I am
trying to use a JFET (2N5486 - NTE451) to feed signal to a BJT (SK3245 -
NTE199) while keeping Vgs very near Pinch-Off. Problem is, as the 12V worth
of batteries begins to wane, Vs remains the same while Vg falls, resulting
in Vgs surpassing Pinch-Off. Please see my schematic posted to abse for
particulars. And please understand that I know next to nothing about RF, I
am learning this as I go along. That much will become obvious very shortly,
if it hasn't already.

Thanks for the informative post.

Dave