transistor switch

J

Jenalee K.

Guest
To improve my design skills I like to look at the schematics of
commercial stuff. In those designs I often see transistor switches
connected as below.

.. VCC
.. +
.. |
.. .-.
.. | | RL
.. | |
.. '-'
.. R1 |
.. ___ |/
.. Vi--|___|---o----|
.. | |>
.. .-. |
.. | |R2 |
.. | | |
.. '-' |
.. | |
.. '------o
.. |
.. ===
.. GND
.. (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


I usually only use R1, why would I need R2?

I calculate my R1 this way: R1 = (Vi-Vbe)/(0.1*iLoad)
How would R2 affect this?

Thanks,
Jenalee K.
 
In article <1145884933.039163.140300@t31g2000cwb.googlegroups.com>, "Jenalee K." <jenaleek@yahoo.com> wrote:
To improve my design skills I like to look at the schematics of
commercial stuff. In those designs I often see transistor switches
connected as below.

.. VCC
.. +
.. |
.. .-.
.. | | RL
.. | |
.. '-'
.. R1 |
.. ___ |/
.. Vi--|___|---o----|
.. | |
.. .-. |
.. | |R2 |
.. | | |
.. '-' |
.. | |
.. '------o
.. |
.. ===
.. GND
.. (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


I usually only use R1, why would I need R2?

I calculate my R1 this way: R1 = (Vi-Vbe)/(0.1*iLoad)
How would R2 affect this?
Click to expand...
To shut the transistor off if the driver can't. You can also vary the switch point.


greg
 
Jenalee K. wrote:
To improve my design skills I like to look at the schematics of
commercial stuff. In those designs I often see transistor switches
connected as below.

. VCC
. +
. |
. .-.
. | | RL
. | |
. '-'
. R1 |
. ___ |/
. Vi--|___|---o----|
. | |
. .-. |
. | |R2 |
. | | |
. '-' |
. | |
. '------o
. |
. ===
. GND
. (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


I usually only use R1, why would I need R2?

I calculate my R1 this way: R1 = (Vi-Vbe)/(0.1*iLoad)
How would R2 affect this?
Click to expand...
R2 has two purposes that I know of. It helps discharge the stored
base charge and speed the turn off process (dump the charge delivered
by the base to collector capacitance, as the collector voltage rises
during turn off), if Vi does not go all the way to the emitter voltage
and cannot be relied upon to do this. R2 also provides a path for
collector to base leakage current that would otherwise be amplified by
the transistor current gain and show up as high off state collector
current. Again, this is more important if V1 does not go all the way
to (or below) the emitter voltage.
 
"Jenalee K." <jenaleek@yahoo.com> wrote in message
news:1145884933.039163.140300@t31g2000cwb.googlegroups.com...
To improve my design skills I like to look at the schematics of
commercial stuff. In those designs I often see transistor switches
connected as below.

. VCC
. +
. |
. .-.
. | | RL
. | |
. '-'
. R1 |
. ___ |/
. Vi--|___|---o----|
. | |
. .-. |
. | |R2 |
. | | |
. '-' |
. | |
. '------o
. |
. ===
. GND
. (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


I usually only use R1, why would I need R2?

I calculate my R1 this way: R1 = (Vi-Vbe)/(0.1*iLoad)
How would R2 affect this?

Thanks,
Jenalee K.
Click to expand...
R2 will make the transistor have a faster shut off time. Use a scope and
look at the shut off time, with and without R2. I'm not sure what the
preferred way of selecting R2 is, but I usually select R2 to have 10% of the
current as the base current of the transistor.
Brian
 
Jenalee K. wrote:
To improve my design skills I like to look at the schematics of
commercial stuff. In those designs I often see transistor switches
connected as below.

. VCC
. +
. |
. .-.
. | | RL
. | |
. '-'
. R1 |
. ___ |/
. Vi--|___|---o----|
. | |
. .-. |
. | |R2 |
. | | |
. '-' |
. | |
. '------o
. |
. ===
. GND
. (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


I usually only use R1, why would I need R2?
Click to expand...
Two cases:

1) If the driving circuit goes from low to high but never floats, you
dont need R2.

2) If the driving circuit floats or is not always connected, R2 will
keep the transistor completely off in case of stray signals or
transistor base-collector leakage.

Luhan
 
On 24 Apr 2006 06:22:13 -0700, "Jenalee K." <jenaleek@yahoo.com>
wrote:

To improve my design skills I like to look at the schematics of
commercial stuff. In those designs I often see transistor switches
connected as below.

. VCC
. +
. |
. .-.
. | | RL
. | |
. '-'
. R1 |
. ___ |/
. Vi--|___|---o----|
. | |
. .-. |
. | |R2 |
. | | |
. '-' |
. | |
. '------o
. |
. ===
. GND
. (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


I usually only use R1, why would I need R2?
Click to expand...
Usually, you don't. Sometimes you don't need R1, either. I used to
drive transistor bases from TTL gates directly just to annoy nearby
engineers.

What matters is how much base current you get when the input is high,
and how much when it's low, and whether these numbers are safe.

John
 
? ... I used to
drive transistor bases from TTL gates directly just to annoy nearby
engineers.
Click to expand...
Hmm, this can have been slippy - one misjudgement of the currnt limit
and we have smoke.... But I like the idea, I am sure it has worked
(on the nearby engineers, engineers, that is :).

BTW, I almost have not used bipolar switches of this type since
1990... I just use a 2N7002 or something, logic output goes directly
into the gate, works fine with 5 and 3V logic. I guess this must
be comon practice all around he place, of course.

Dimiter

------------------------------------------------------
Dimiter Popoff Transgalactic Instruments

http://www.tgi-sci.com
------------------------------------------------------


John Larkin wrote:
On 24 Apr 2006 06:22:13 -0700, "Jenalee K." <jenaleek@yahoo.com
wrote:

To improve my design skills I like to look at the schematics of
commercial stuff. In those designs I often see transistor switches
connected as below.

. VCC
. +
. |
. .-.
. | | RL
. | |
. '-'
. R1 |
. ___ |/
. Vi--|___|---o----|
. | |
. .-. |
. | |R2 |
. | | |
. '-' |
. | |
. '------o
. |
. ===
. GND
. (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


I usually only use R1, why would I need R2?


Usually, you don't. Sometimes you don't need R1, either. I used to
drive transistor bases from TTL gates directly just to annoy nearby
engineers.

What matters is how much base current you get when the input is high,
and how much when it's low, and whether these numbers are safe.

John
Click to expand...
 
On 24 Apr 2006 08:22:11 -0700, "Didi" <dp@tgi-sci.com> wrote:

? ... I used to
drive transistor bases from TTL gates directly just to annoy nearby
engineers.

Hmm, this can have been slippy - one misjudgement of the currnt limit
and we have smoke.... But I like the idea, I am sure it has worked
(on the nearby engineers, engineers, that is :).

BTW, I almost have not used bipolar switches of this type since
1990... I just use a 2N7002 or something, logic output goes directly
into the gate, works fine with 5 and 3V logic. I guess this must
be comon practice all around he place, of course.

Dimiter
Click to expand...

A 2N7002 is great with 5 volts on the gate, but really marginal at
3.3. They'll also switch 50 volts into 50 ohms in a ns or two, if you
whack the gate hard.

Is there an equivalent dirt-cheap SOT-23 low-threshold nfet out there?
2N7002's cost something like 3 cents by the reel.

John
 
John Larkin scrobe on the papyrus:

On 24 Apr 2006 08:22:11 -0700, "Didi" <dp@tgi-sci.com> wrote:

? ... I used to
drive transistor bases from TTL gates directly just to annoy nearby
engineers.

Hmm, this can have been slippy - one misjudgement of the currnt
limit and we have smoke.... But I like the idea, I am sure it has
worked (on the nearby engineers, engineers, that is :).

BTW, I almost have not used bipolar switches of this type since
1990... I just use a 2N7002 or something, logic output goes directly
into the gate, works fine with 5 and 3V logic. I guess this must
be comon practice all around he place, of course.

Dimiter


A 2N7002 is great with 5 volts on the gate, but really marginal at
3.3. They'll also switch 50 volts into 50 ohms in a ns or two, if you
whack the gate hard.

Is there an equivalent dirt-cheap SOT-23 low-threshold nfet out there?
2N7002's cost something like 3 cents by the reel.

John
Click to expand...
Bipolar equivalent is BCR148.

--
John B
 
A 2N7002 is great with 5 volts on the gate, but really marginal at
3.3.
Click to expand...
Well strictly speaking they are marginal, but in many cases when
you need not the highest current they can handle, but something
within several mA they are quite adequate. They typically
open to well below 50 Ohm just by the charge you give them
with a DMM ohmmeter - which won't open a junction...

Another of my widely employed switches is an SST176 (P-JFET)
when I have to switch a negative voltage powered load to GND
and drive the switch by a +5V logic signal (4V high min.). These
are good for analog signal switching, if on resistance about
100 Ohm is OK.Fast and fairly low capacitance to the driving
signal. Anyway, it is again a single component switch :).

Dimiter

------------------------------------------------------
Dimiter Popoff Transgalactic Instruments

http://www.tgi-sci.com
------------------------------------------------------

John Larkin wrote:
On 24 Apr 2006 08:22:11 -0700, "Didi" <dp@tgi-sci.com> wrote:

? ... I used to
drive transistor bases from TTL gates directly just to annoy nearby
engineers.

Hmm, this can have been slippy - one misjudgement of the currnt limit
and we have smoke.... But I like the idea, I am sure it has worked
(on the nearby engineers, engineers, that is :).

BTW, I almost have not used bipolar switches of this type since
1990... I just use a 2N7002 or something, logic output goes directly
into the gate, works fine with 5 and 3V logic. I guess this must
be comon practice all around he place, of course.

Dimiter


A 2N7002 is great with 5 volts on the gate, but really marginal at
3.3. They'll also switch 50 volts into 50 ohms in a ns or two, if you
whack the gate hard.

Is there an equivalent dirt-cheap SOT-23 low-threshold nfet out there?
2N7002's cost something like 3 cents by the reel.

John
Click to expand...
 
On 24 Apr 2006 11:38:57 -0700, "Didi" <dp@tgi-sci.com> wrote:

A 2N7002 is great with 5 volts on the gate, but really marginal at
3.3.

Well strictly speaking they are marginal, but in many cases when
you need not the highest current they can handle, but something
within several mA they are quite adequate. They typically
open to well below 50 Ohm just by the charge you give them
with a DMM ohmmeter - which won't open a junction...
Click to expand...

If you connect a 2N7002 drain to a 9v battery, an led, and maybe a
series resistor, then tease the gate voltage to turn on the led
halfway, and open the gate, it'll stay like that for days. I figure
the gate leakage current must be electrons per second.

John
 
Didi wrote:

? ... I used to
drive transistor bases from TTL gates directly just to annoy nearby
engineers.

Hmm, this can have been slippy - one misjudgement of the currnt limit
and we have smoke.... But I like the idea, I am sure it has worked
(on the nearby engineers, engineers, that is :).

BTW, I almost have not used bipolar switches of this type since
1990... I just use a 2N7002 or something, logic output goes directly
into the gate, works fine with 5 and 3V logic. I guess this must
be comon practice all around he place, of course.
Click to expand...
It might be common practice if you can get a 2N7002 for 2 cents !

I still use discrete bipolars for driving leds, relays and the like.

Graham
 
On Mon, 24 Apr 2006 09:48:37 -0700, John Larkin wrote:

On 24 Apr 2006 08:22:11 -0700, "Didi" <dp@tgi-sci.com> wrote:

? ... I used to
drive transistor bases from TTL gates directly just to annoy nearby
engineers.

Hmm, this can have been slippy - one misjudgement of the currnt limit
and we have smoke.... But I like the idea, I am sure it has worked
(on the nearby engineers, engineers, that is :).

BTW, I almost have not used bipolar switches of this type since
1990... I just use a 2N7002 or something, logic output goes directly
into the gate, works fine with 5 and 3V logic. I guess this must
be comon practice all around he place, of course.

Dimiter


A 2N7002 is great with 5 volts on the gate, but really marginal at
3.3. They'll also switch 50 volts into 50 ohms in a ns or two, if you
whack the gate hard.

Is there an equivalent dirt-cheap SOT-23 low-threshold nfet out there?
2N7002's cost something like 3 cents by the reel.
Click to expand...
I saw a design once where the guy had used VN2222s:
http://www.supertex.com/pdf/datasheets/VN2222.pdf

I don't know where to get them, and I'm not sure how to read the data
sheet, but it looks like 3.3V might turn it on - somebody who knows
that stuff should take a look at it. :)

Cheers!
Rich
 
John Larkin wrote:

On 24 Apr 2006 08:22:11 -0700, "Didi" <dp@tgi-sci.com> wrote:

? ... I used to
drive transistor bases from TTL gates directly just to annoy nearby
engineers.

Hmm, this can have been slippy - one misjudgement of the currnt limit
and we have smoke.... But I like the idea, I am sure it has worked
(on the nearby engineers, engineers, that is :).

BTW, I almost have not used bipolar switches of this type since
1990... I just use a 2N7002 or something, logic output goes directly
into the gate, works fine with 5 and 3V logic. I guess this must
be comon practice all around he place, of course.

Dimiter

A 2N7002 is great with 5 volts on the gate, but really marginal at
3.3. They'll also switch 50 volts into 50 ohms in a ns or two, if you
whack the gate hard.

Is there an equivalent dirt-cheap SOT-23 low-threshold nfet out there?
2N7002's cost something like 3 cents by the reel.
Click to expand...
You got me interested in this one.

SOT-23 is Fairchild's NDS7002A

http://www.fairchildsemi.com/ds/2N/2N7002.pdf

Is there a p channel complement ?

Ahhhh - mosfet not jfet. How does that handle ac signals ?


Graham
 
To improve my design skills I like to look at the schematics of
commercial stuff. In those designs I often see transistor switches
connected as below.

. VCC
. +
. |
. .-.
. | | RL
. | |
. '-'
. R1 |
. ___ |/
. Vi--|___|---o----|
. | |
. .-. |
. | |R2 |
. | | |
. '-' |
. | |
. '------o
. |
. ===
. GND
. (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


I usually only use R1, why would I need R2?

I calculate my R1 this way: R1 = (Vi-Vbe)/(0.1*iLoad)
How would R2 affect this?
Click to expand...
R2 parallels the base, so it has Vbe across it and Vbe/R2 current
through it. Therefore R1 should conduct Iload/10 + Vbe/R2 making
R1=(Vin-Vbe)/(0.1*Iload+Vbe/R2). There are various reasons for including
R2 such as speed-up, linearization, input voltage attenuation, and
shunting reverse CB current leakage. Most of the time it is included to
shunt the CB leakage current around the BE junction.
 
Fred Bloggs scrobe on the papyrus:

To improve my design skills I like to look at the schematics of
commercial stuff. In those designs I often see transistor switches
connected as below.

. VCC
. +
. |
. .-.
. | | RL
. | |
. '-'
. R1 |
. ___ |/
. Vi--|___|---o----|
. | |
. .-. |
. | |R2 |
. | | |
. '-' |
. | |
. '------o
. |
. ===
. GND
. (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


I usually only use R1, why would I need R2?

I calculate my R1 this way: R1 = (Vi-Vbe)/(0.1*iLoad)
How would R2 affect this?

R2 parallels the base, so it has Vbe across it and Vbe/R2 current
through it. Therefore R1 should conduct Iload/10 + Vbe/R2 making
R1=(Vin-Vbe)/(0.1*Iload+Vbe/R2). There are various reasons for
including R2 such as speed-up, linearization, input voltage
attenuation, and shunting reverse CB current leakage. Most of the
time it is included to shunt the CB leakage current around the BE
junction.
Click to expand...
Google for "digital transistor". The BCR148 is very useful.

--
John B
 
John B wrote:

Fred Bloggs scrobe on the papyrus:


To improve my design skills I like to look at the schematics of
commercial stuff. In those designs I often see transistor switches
connected as below.

. VCC
. +
. |
. .-.
. | | RL
. | |
. '-'
. R1 |
. ___ |/
. Vi--|___|---o----|
. | |
. .-. |
. | |R2 |
. | | |
. '-' |
. | |
. '------o
. |
. ===
. GND
. (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


I usually only use R1, why would I need R2?

I calculate my R1 this way: R1 = (Vi-Vbe)/(0.1*iLoad)
How would R2 affect this?

R2 parallels the base, so it has Vbe across it and Vbe/R2 current
through it. Therefore R1 should conduct Iload/10 + Vbe/R2 making
R1=(Vin-Vbe)/(0.1*Iload+Vbe/R2). There are various reasons for
including R2 such as speed-up, linearization, input voltage
attenuation, and shunting reverse CB current leakage. Most of the
time it is included to shunt the CB leakage current around the BE
junction.

Google for "digital transistor". The BCR148 is very useful.
Click to expand...
Rohm also do a nice line in digital transsitors. Used them once when tight
for space. They do various configurations of built in resistors.

http://www.chipdocs.com/datasheets/datasheet-pdf/ROHM-Co/DTB114.html

Graham
 
On Tue, 25 Apr 2006 12:10:26 GMT, Fred Bloggs <nospam@nospam.com>
wrote:

To improve my design skills I like to look at the schematics of
commercial stuff. In those designs I often see transistor switches
connected as below.

. VCC
. +
. |
. .-.
. | | RL
. | |
. '-'
. R1 |
. ___ |/
. Vi--|___|---o----|
. | |
. .-. |
. | |R2 |
. | | |
. '-' |
. | |
. '------o
. |
. ===
. GND
. (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


I usually only use R1, why would I need R2?

I calculate my R1 this way: R1 = (Vi-Vbe)/(0.1*iLoad)
How would R2 affect this?

R2 parallels the base, so it has Vbe across it and Vbe/R2 current
through it. Therefore R1 should conduct Iload/10 + Vbe/R2 making
R1=(Vin-Vbe)/(0.1*Iload+Vbe/R2). There are various reasons for including
R2 such as speed-up, linearization, input voltage attenuation, and
shunting reverse CB current leakage. Most of the time it is included to
shunt the CB leakage current around the BE junction.
Click to expand...
Is that habitual holdover from germanium days? Transistors don't
really leak any more, and R1 would slurp that up anyhow. Ironically,
big power mosfets can have serious d-s leakage nowadays.

John
 
On Tue, 25 Apr 2006 02:22:29 +0100, Pooh Bear
<rabbitsfriendsandrelations@hotmail.com> wrote:

John Larkin wrote:

On 24 Apr 2006 08:22:11 -0700, "Didi" <dp@tgi-sci.com> wrote:

? ... I used to
drive transistor bases from TTL gates directly just to annoy nearby
engineers.

Hmm, this can have been slippy - one misjudgement of the currnt limit
and we have smoke.... But I like the idea, I am sure it has worked
(on the nearby engineers, engineers, that is :).

BTW, I almost have not used bipolar switches of this type since
1990... I just use a 2N7002 or something, logic output goes directly
into the gate, works fine with 5 and 3V logic. I guess this must
be comon practice all around he place, of course.

Dimiter

A 2N7002 is great with 5 volts on the gate, but really marginal at
3.3. They'll also switch 50 volts into 50 ohms in a ns or two, if you
whack the gate hard.

Is there an equivalent dirt-cheap SOT-23 low-threshold nfet out there?
2N7002's cost something like 3 cents by the reel.

You got me interested in this one.

SOT-23 is Fairchild's NDS7002A

http://www.fairchildsemi.com/ds/2N/2N7002.pdf

Is there a p channel complement ?

Ahhhh - mosfet not jfet. How does that handle ac signals ?
Click to expand...
If you can drive the capacitance, they're fierce. I use them to switch
50 volts into 50 ohms in something like 2 ns. They have bigger
capacitances than a jfet, but jfets don't have transconductances
anywhere near these gadgets.

Next step is a gaasfet, then GaN.

John
 
In article <444D79D5.6BE4A36C@hotmail.com>,
Pooh Bear <rabbitsfriendsandrelations@hotmail.com> wrote:
[...]
You got me interested in this one.

SOT-23 is Fairchild's NDS7002A

http://www.fairchildsemi.com/ds/2N/2N7002.pdf

Is there a p channel complement ?
Click to expand...
Its not an exact P channel complement but you may want to look at
www.supertex.com for the TPxxxx MOSFETs. P channel ones usually have a
higher Rds(on) and Vth than N channel.


--
--
kensmith@rahul.net forging knowledge
 
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