I understand that an NPN base-emitter junction needs a voltage drop of
c. 0.6V so won't start turning on until Vbe exceeds that. Can I get the
transistor to start conducting when the input voltage (i.e. between the
circuit input and ground, common emitter connection) is around 3V.

I don't think I can use something like a potential divider or apply a
bias voltage because the source may be of high impedance - and I guess
either would have too much effect on the input. Or I may be talking
nonsense. Feel free to tell me if I am!

--
James

James Harris wrote:
I understand that an NPN base-emitter junction needs a voltage drop of
c. 0.6V so won't start turning on until Vbe exceeds that. Can I get the
transistor to start conducting when the input voltage (i.e. between the
circuit input and ground, common emitter connection) is around 3V.

I don't think I can use something like a potential divider or apply a
bias voltage because the source may be of high impedance - and I guess
either would have too much effect on the input. Or I may be talking
nonsense. Feel free to tell me if I am!

There's may be an easier way, but you could put a 2.5V reference chip
(LM385-2.5) between the emitter and ground. The transistor would start
to turn on at about 3.1V wrt ground. You might need to have a resistor
from plus supply to the reference chip to get it stabilized.
--
John

John O'Flaherty wrote:
James Harris wrote:
I understand that an NPN base-emitter junction needs a voltage drop of
c. 0.6V so won't start turning on until Vbe exceeds that. Can I get the
transistor to start conducting when the input voltage (i.e. between the
circuit input and ground, common emitter connection) is around 3V.

I don't think I can use something like a potential divider or apply a
bias voltage because the source may be of high impedance - and I guess
either would have too much effect on the input. Or I may be talking
nonsense. Feel free to tell me if I am!

There's may be an easier way, but you could put a 2.5V reference chip
(LM385-2.5) between the emitter and ground. The transistor would start
to turn on at about 3.1V wrt ground. You might need to have a resistor
from plus supply to the reference chip to get it stabilized.

Interesting idea. I hadn't thought of anything between emitter and
ground. I wonder if I could use a diode chain there instead - 3 diodes
to drop around 2V, perhaps, or four to drop about 2.5V. I have the
sneaking suspicion this is not the right way to do it but it might work
and still be a low impedance path to ground. How does that sound?

James Harris wrote:
John O'Flaherty wrote:
James Harris wrote:
I understand that an NPN base-emitter junction needs a voltage drop of
c. 0.6V so won't start turning on until Vbe exceeds that. Can I get the
transistor to start conducting when the input voltage (i.e. between the
circuit input and ground, common emitter connection) is around 3V.

I don't think I can use something like a potential divider or apply a
bias voltage because the source may be of high impedance - and I guess
either would have too much effect on the input. Or I may be talking
nonsense. Feel free to tell me if I am!

There's may be an easier way, but you could put a 2.5V reference chip
(LM385-2.5) between the emitter and ground. The transistor would start
to turn on at about 3.1V wrt ground. You might need to have a resistor
from plus supply to the reference chip to get it stabilized.

Interesting idea. I hadn't thought of anything between emitter and
ground. I wonder if I could use a diode chain there instead - 3 diodes
to drop around 2V, perhaps, or four to drop about 2.5V. I have the
sneaking suspicion this is not the right way to do it but it might work
and still be a low impedance path to ground. How does that sound?

You could, but the turnon will be more gradual. For either method, the
impedance to ground will decrease as the input increases, so it may
still load your input some.
--
John

On 2006-09-16, James Harris wrote:
I understand that an NPN base-emitter junction needs a voltage drop of
c. 0.6V so won't start turning on until Vbe exceeds that. Can I get the
transistor to start conducting when the input voltage (i.e. between the
circuit input and ground, common emitter connection) is around 3V.

If it's "very high impedance" as the subject says, use a comparator.
Especially if you care about the precision of "around 3V".

--
Ben Jackson AD7GD

http://www.ben.com/

John O'Flaherty wrote:
James Harris wrote:
John O'Flaherty wrote:
James Harris wrote:
I understand that an NPN base-emitter junction needs a voltage drop of
c. 0.6V so won't start turning on until Vbe exceeds that. Can I get the
transistor to start conducting when the input voltage (i.e. between the
circuit input and ground, common emitter connection) is around 3V.

I don't think I can use something like a potential divider or apply a
bias voltage because the source may be of high impedance - and I guess
either would have too much effect on the input. Or I may be talking
nonsense. Feel free to tell me if I am!

There's may be an easier way, but you could put a 2.5V reference chip
(LM385-2.5) between the emitter and ground. The transistor would start
to turn on at about 3.1V wrt ground. You might need to have a resistor
from plus supply to the reference chip to get it stabilized.

Interesting idea. I hadn't thought of anything between emitter and
ground. I wonder if I could use a diode chain there instead - 3 diodes
to drop around 2V, perhaps, or four to drop about 2.5V. I have the
sneaking suspicion this is not the right way to do it but it might work
and still be a low impedance path to ground. How does that sound?

You could, but the turnon will be more gradual. For either method, the
impedance to ground will decrease as the input increases, so it may
still load your input some.
--
John

If you need to turn on a transistor at some arbitrary voltage, then a
comparator (with hysteresis in this case) would do the job. Comparators
are cheap (well under $1) and would do the task you need.

Cheers

PeteS

James Harris wrote:

I understand that an NPN base-emitter junction needs a voltage drop of
c. 0.6V so won't start turning on until Vbe exceeds that. Can I get the
transistor to start conducting when the input voltage (i.e. between the
circuit input and ground, common emitter connection) is around 3V.

I don't think I can use something like a potential divider or apply a
bias voltage because the source may be of high impedance - and I guess
either would have too much effect on the input. Or I may be talking
nonsense. Feel free to tell me if I am!

--
James

use a PNP transitor that pulls down a resistor from the + side
down to around 0.6 or so..
your 3 volt input can then reverse bias the PNP which will allow the
resistor to then drive the base with more voltage.

just a thought.



--
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

"Ben Jackson" wrote in message
...
On 2006-09-16, James Harris wrote:
I understand that an NPN base-emitter junction needs a voltage drop
of
c. 0.6V so won't start turning on until Vbe exceeds that. Can I get
the
transistor to start conducting when the input voltage (i.e. between
the
circuit input and ground, common emitter connection) is around 3V.

If it's "very high impedance" as the subject says, use a comparator.
Especially if you care about the precision of "around 3V".

[OP] Ok I have been looking at comparators which I was not aware of and
they seem ideal. For the positive voltage of c. +3v I think even I could
get it to work though I may have some issues with the potential input
range to +15v, possibly more, running with a 9v battery.

The negative comparator is more of an issue. It looks as though I need
a -3v reference unless a comparator will compare its non-inverting and
inverting inputs only with each other and not in reference to its own
ground. I can't quite make that out. Is that what they do? Or are there
some that work in that way?

BTW, a quad comparator looks good, especially as I've realised I will
need one for each of the 16 LEDs. Four chips is reasonable.

James Harris wrote:
"Ben Jackson" wrote in message
...
On 2006-09-16, James Harris wrote:
I understand that an NPN base-emitter junction needs a voltage drop
of
c. 0.6V so won't start turning on until Vbe exceeds that. Can I get
the
transistor to start conducting when the input voltage (i.e. between
the
circuit input and ground, common emitter connection) is around 3V.

If it's "very high impedance" as the subject says, use a comparator.
Especially if you care about the precision of "around 3V".

[OP] Ok I have been looking at comparators which I was not aware of and
they seem ideal. For the positive voltage of c. +3v I think even I could
get it to work though I may have some issues with the potential input
range to +15v, possibly more, running with a 9v battery.

If you mean the inputs can go to 15V while the comparator is running on
9V, that could be an issue. You can get around it by including a
largish resistor (say, 1 megohm) in series with the input, and using
diodes to clamp the input to a safe voltage. So if your reference input
is at a stiff 3V, you could connect back to back diodes from the other
input to it, and it would work for comparison to 3V. The diodes and
resistor would have no effect until the input went over 3.6 V or less
then -3.6V.

The negative comparator is more of an issue. It looks as though I need
a -3v reference unless a comparator will compare its non-inverting and
inverting inputs only with each other and not in reference to its own
ground. I can't quite make that out. Is that what they do? Or are there
some that work in that way?

The two inputs are compared with each other. If the + input is higher
(more positive) than the - input, the output goes all the way plus. If
the - input is higher than the + input, the output goes all the way
minus. Its just a differential amplifier with very high gain.
--
John

Jamie wrote:
snip

use a PNP transitor that pulls down a resistor from the + side
down to around 0.6 or so..
your 3 volt input can then reverse bias the PNP which will allow the
resistor to then drive the base with more voltage.

just a thought.

I've tried a few times to understand this but I don't follow? I would
still need some way to generate the 3v so the NPN doesn't turn on until
that has been exceeded. Not a major problem now, if using an op-amp or
a comparator rather than just an NPN.

On 2006-09-16, James Harris wrote:
I understand that an NPN base-emitter junction needs a voltage drop of
c. 0.6V so won't start turning on until Vbe exceeds that. Can I get the
transistor to start conducting when the input voltage (i.e. between the
circuit input and ground, common emitter connection) is around 3V.

yes, put a 2.4V drop between the input and the base.

I don't think I can use something like a potential divider or apply a
bias voltage because the source may be of high impedance - and I guess
either would have too much effect on the input. Or I may be talking
nonsense. Feel free to tell me if I am!

for high impedance use a comparitor eg:

+12V
| ----o out
|\ 1K /
in----|+\ | |/
| ---+--|
+3V---|-/ |\|
|/ ~\
|
---+--- 0V

Bye.
Jasen

James Harris wrote:

Jamie wrote:
snip

use a PNP transitor that pulls down a resistor from the + side
down to around 0.6 or so..
your 3 volt input can then reverse bias the PNP which will allow the
resistor to then drive the base with more voltage.

just a thought.


I've tried a few times to understand this but I don't follow? I would
still need some way to generate the 3v so the NPN doesn't turn on until
that has been exceeded. Not a major problem now, if using an op-amp or
a comparator rather than just an NPN.

using an op-amp is also a good way,


--
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5