Strange transistor driver circuit

[Philip Pemberton]

Hi,
I've just been reverse engineering a printer controller PCB and I've come
across a rather unusual (to me anyway) transistor driver circuit in the
printhead solenoid driver.
Forgive my naff ASCII art, but here's the schematic:

+V +V
--- ---
| |
| 3 Solenoid
| 3
e | 3
b | / PNP +--|>Z---*

------|< | | c
| \ | b | /

+-----*-----|< NPN
| \
| e
|
---
GND

To me, that looks like a pretty normal complementary Darlington circuit. What
I'm most interested in is the 20V Zener diode on the NPN - ZD.cathode to
NPN.collector, ZD.anode to NPN.base. AFAICT, when the PNP switches on, the ZD
will get put straight over the solenoid, with the anode more positive than
the cathode, causing current to flow A-K to ground. Surely that would blow
the Zener to bits (it's a BZX84C20 - 300mW SOT23)?
If it was oriented cathode to +V, anode to NPN.collector, I'd have guessed it
would have worked as a rather expensive spark-killer/back-EMF suppressor. In
this configuration I'm not so sure...

Thanks.
--
Phil. | Acorn RiscPC600 Mk3, SA202, 64MB, 6GB,
philpem@despammed.com (valid address)| ViewFinder, 10BaseT Ethernet, 2-slice,
http://www.philpem.me.uk/ | 48xCD, ARCINv6c IDE, SCSI
No to DRM, software patents and the EUCD! <http://www.ukcdr.org/>
.... Noah! Come quick! There's water in the basement!

 

[Larry Brasfield]

"Philip Pemberton" <philpem@despammed.com> wrote in
message news:a191b9734d.philpem@dsl.pipex.com...

Hi,
Hi.
I've just been reverse engineering a printer controller PCB and I've come
across a rather unusual (to me anyway) transistor driver circuit in the
printhead solenoid driver.
Forgive my naff ASCII art, but here's the schematic:

+V +V
--- ---
| |
| 3 Solenoid
| 3
e | 3
b | / PNP +--|>Z---*
-----[ 4k7 ]------|< | | c
| \ | b | /
+-----*-----|< NPN
| \
| e
|
---
GND

To me, that looks like a pretty normal complementary Darlington circuit.

Those transistors are not Darlington connected. (But I have never
heard of a "complementary Darlington", so maybe you're right.)

I suspect a current setting or limiting resistor is missing somewhere.
Either the emitter or collector of the PNP must have a resistor in
series, otherwise the NPN base current is poorly determined and
likely too high.

What
I'm most interested in is the 20V Zener diode on the NPN - ZD.cathode to
NPN.collector, ZD.anode to NPN.base. AFAICT, when the PNP switches on, the ZD
will get put straight over the solenoid, with the anode more positive than
the cathode, causing current to flow A-K to ground. Surely that would blow
the Zener to bits (it's a BZX84C20 - 300mW SOT23)?

If the circuit was really as you've drawn it, then there would be
a contest between the zener being over-driven and the NPN
base being over-driven. There must be a another resistor.

If it was oriented cathode to +V, anode to NPN.collector, I'd have guessed it
would have worked as a rather expensive spark-killer/back-EMF suppressor. In
this configuration I'm not so sure...

I think the zener orientation you've drawn is correct. I'm not
sure why the designer chooses to dissipate the stored energy
from the solenoid in the transistor rather than simply grounding
the zener and dissipating it there. Is it a small zener and a larger
NPN? Maybe the NPN had to be large anyway to carry the
necessary current, and that connection allowed a smaller zener
to be used.

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

 

[Joerg]

Hello Philip,

To me, that looks like a pretty normal complementary Darlington circuit. What
I'm most interested in is the 20V Zener diode on the NPN - ZD.cathode to
NPN.collector, ZD.anode to NPN.base. AFAICT, when the PNP switches on, the ZD
will get put straight over the solenoid, with the anode more positive than
the cathode, causing current to flow A-K to ground. Surely that would blow
the Zener to bits (it's a BZX84C20 - 300mW SOT23)?

The first transistor is unlikely able to pass enough current to blow the
zener. The 4.7k limits base current and beta will most likely be below
300. Anyways, the 2nd transistor's base won't go much above 700mV and
when that is the case its collector will reach saturation voltage.
Depending on the size of that transistor that could be around 150mV or
so. IOW there will be a bit less than a diode drop cross the zener while
it is "in reverse".

It's a bit unusual but probably the goal was to run the 2nd transistor a
bit into conduction when the voltage at its collector wants to rise
above the zener voltage upon switch-off. IOW, to protect the 2nd
transistor from exceeding that voltage. This is unusual because a strong
solenoid can generate enough of a spike to fry the base region or the
zener, or both. But maybe it's a small solenoid in your case.

Regards, Joerg

http://www.analogconsultants.com

 

[Philip Pemberton]

In message <cj%me.7$qF3.2814@news.uswest.net>
"Larry Brasfield" <donotspam_larry_brasfield@hotmail.com> wrote:

Those transistors are not Darlington connected. (But I have never
heard of a "complementary Darlington", so maybe you're right.)

ISTR it's also known as a "Sziklai" pair. You use an NPN and a PNP (or vice
versa) instead of a pair of NPNs or a pair of PNPs. In this case, the pair
acts like a PNP. There's another variant that acts like an NPN (like
Darlingtons - you can have one made of PNPs that acts like a PNP, or one made
of NPNs that acts like an NPN).

I suspect a current setting or limiting resistor is missing somewhere.

[scans board again]
Yep. A 150-ohm 1206 SMD between the PNP's collector and +V. The solenoid is
limited only by its own resistance.
The 150R is shared between six solenoid drivers, but only two can ever be
energised at once.

I think the zener orientation you've drawn is correct. I'm not
sure why the designer chooses to dissipate the stored energy
from the solenoid in the transistor rather than simply grounding
the zener and dissipating it there. Is it a small zener and a larger
NPN?

A huge NPN - an FZT649 (3A). The PNP is a BCW70.

Later.
--
Phil. | Acorn RiscPC600 Mk3, SA202, 64MB, 6GB,
philpem@despammed.com (valid address)| ViewFinder, 10BaseT Ethernet, 2-slice,
http://www.philpem.me.uk/ | 48xCD, ARCINv6c IDE, SCSI
No to DRM, software patents and the EUCD! <http://www.ukcdr.org/>
.... File not found. Should I fake it? (Y/N)

 

[Philip Pemberton]

In message <It%me.595$IE7.334@newssvr21.news.prodigy.com>
Joerg <notthisjoergsch@removethispacbell.net> wrote:

It's a bit unusual but probably the goal was to run the 2nd transistor a
bit into conduction when the voltage at its collector wants to rise
above the zener voltage upon switch-off. IOW, to protect the 2nd
transistor from exceeding that voltage.

So it makes the transistor ground the coil when the back-EMF hits it? Ick.
I'd have used a 1N4001 over the solenoid, but that's just me.

This is unusual because a strong
solenoid can generate enough of a spike to fry the base region or the
zener, or both. But maybe it's a small solenoid in your case.

It's a printhead solenoid in a dot-matrix impact printer. Not a very big
printer either - an Epson M-183 42-column receipt printer.
Epson's recommended circuit (from the datasheet) is a Darlington NPN pair
with a spark-killer diode over the solenoid (K to +V, A to drive transistor
collector).

Later.
--
Phil. | Acorn RiscPC600 Mk3, SA202, 64MB, 6GB,
philpem@despammed.com (valid address)| ViewFinder, 10BaseT Ethernet, 2-slice,
http://www.philpem.me.uk/ | 48xCD, ARCINv6c IDE, SCSI
No to DRM, software patents and the EUCD! <http://www.ukcdr.org/>
.... Beware of quantum ducks. Quark! Quark!

 

[Spehro Pefhany]

On Tue, 31 May 2005 08:22:55 -0700, the renowned "Larry Brasfield"
<donotspam_larry_brasfield@hotmail.com> wrote:

"Philip Pemberton" <philpem@despammed.com> wrote in
message news:a191b9734d.philpem@dsl.pipex.com...
Hi,
Hi.
I've just been reverse engineering a printer controller PCB and I've come
across a rather unusual (to me anyway) transistor driver circuit in the
printhead solenoid driver.
Forgive my naff ASCII art, but here's the schematic:

+V +V
--- ---
| |
| 3 Solenoid
| 3
e | 3
b | / PNP +--|>Z---*
-----[ 4k7 ]------|< | | c
| \ | b | /
+-----*-----|< NPN
| \
| e
|
---
GND

To me, that looks like a pretty normal complementary Darlington circuit.

Those transistors are not Darlington connected. (But I have never
heard of a "complementary Darlington", so maybe you're right.)

Probably thinking of Sziklai connection, which is a bit different. The
base current in this case comes from the supply rather than through
the load, which means it uses more current from the supply and
(potentially) has a far lower "on" voltage.

I suspect a current setting or limiting resistor is missing somewhere.
Either the emitter or collector of the PNP must have a resistor in
series, otherwise the NPN base current is poorly determined and
likely too high.

Definitely. A resistor is required.

The zener connection is not uncommon, especially in chips. The SOA of
the BJT has to be about good enough for this service to safely switch
the inductive load in the first place, so it's trading off a small
3-cent glass zener diode against a larger power zener.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

 

[Larry Brasfield]

"Philip Pemberton" <philpem@despammed.com> wrote in
message news:8ad0bd734d.philpem@dsl.pipex.com...

In message <cj%me.7$qF3.2814@news.uswest.net
"Larry Brasfield" <donotspam_larry_brasfield@hotmail.com> wrote:

Those transistors are not Darlington connected. (But I have never
heard of a "complementary Darlington", so maybe you're right.)

ISTR it's also known as a "Sziklai" pair. You use an NPN and a PNP (or vice
versa) instead of a pair of NPNs or a pair of PNPs. In this case, the pair
acts like a PNP. There's another variant that acts like an NPN (like
Darlingtons - you can have one made of PNPs that acts like a PNP, or one made
of NPNs that acts like an NPN).

Thanks for that bit of education. I've used that configuration
without knowing it had a name. One nit is left to pick here,
however. The pair would have to form a three terminal cutset
to be properly named a "complementary Darlington" or "Sziklai
pair". Your circuit's pair still has four distinct terminals.

I suspect a current setting or limiting resistor is missing somewhere.

[scans board again]
Yep. A 150-ohm 1206 SMD between the PNP's collector and +V. The solenoid is
limited only by its own resistance.
The 150R is shared between six solenoid drivers, but only two can ever be
energised at once.

Strange. I don't see how the current sharing is effected, unless
the NPNs are expected to split the current equally enough by
virtue of limited mismatch or their own series resistance. And
if they share the resistor, they must be connected together.

I think the zener orientation you've drawn is correct. I'm not
sure why the designer chooses to dissipate the stored energy
from the solenoid in the transistor rather than simply grounding
the zener and dissipating it there. Is it a small zener and a larger
NPN?

A huge NPN - an FZT649 (3A). The PNP is a BCW70.

It looks like a smaller zener was the motivation. I wonder how the
NPN bases that share the 150 Ohm limiting resistor manage to
split the feedback thru the zeners during solenoid shutoff. That
alone makes me suspect the sharing is not quite as claimed.

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

 

[Larry Brasfield]

"Philip Pemberton" <philpem@despammed.com> wrote in
message news:3992be734d.philpem@dsl.pipex.com...

In message <It%me.595$IE7.334@newssvr21.news.prodigy.com
Joerg <notthisjoergsch@removethispacbell.net> wrote:

It's a bit unusual but probably the goal was to run the 2nd transistor a
bit into conduction when the voltage at its collector wants to rise
above the zener voltage upon switch-off. IOW, to protect the 2nd
transistor from exceeding that voltage.

So it makes the transistor ground the coil when the back-EMF hits it? Ick.

No, the NPN collector is regulated, via feedback thru the
zener, at the zener breakdown voltage.

I'd have used a 1N4001 over the solenoid, but that's just me.

That method causes decay of the solenoid field to occur
much more slowly. For many print head applications,
the additional decay time would hinder performance.

This is unusual because a strong
solenoid can generate enough of a spike to fry the base region or the
zener, or both. But maybe it's a small solenoid in your case.

It's just a safe operating area consideration.

It's a printhead solenoid in a dot-matrix impact printer. Not a very big
printer either - an Epson M-183 42-column receipt printer.
Epson's recommended circuit (from the datasheet) is a Darlington NPN pair
with a spark-killer diode over the solenoid (K to +V, A to drive transistor
collector).

Interesting that the designer chose a faster driver than was
recommended in the datasheet. Either faster action was
sought, or the datasheet is a bit misleading.

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

 

[Philip Pemberton]

In message <c90ne.15$qF3.3549@news.uswest.net>
"Larry Brasfield" <donotspam_larry_brasfield@hotmail.com> wrote:

Interesting that the designer chose a faster driver than was
recommended in the datasheet. Either faster action was
sought, or the datasheet is a bit misleading.

The datasheet is in some form of Engrish/Jengrish hybrid. Basically, English
with the sentence structure of Japanese. Great fun to decipher. I looked at a
Seiko thermal printer datasheet and the Epson M-183 datasheet. The Seiko
datasheet is written in nice, clear, understandable English; the Epson
datasheet makes you try and guess what you need to do to make the damn thing
work.

I've looked up the solenoid driver ckt - it uses a 1S2075K small-signal
diode (DO35 pacakge). I haven't run it through Quickcross yet, but going by
the datasheet, it looks like the 2075 is similar to the 1N4148.

The tacho-generator circuit in the datasheet is even stranger. Two
transistors, two resistors, two diodes, a capacitor and the tach coil. The
one on the board I reverse-engineered is similar, only with one less
transistor and one less diode.

Later.
--
Phil. | Acorn RiscPC600 Mk3, SA202, 64MB, 6GB,
philpem@despammed.com (valid address)| ViewFinder, 10BaseT Ethernet, 2-slice,
http://www.philpem.me.uk/ | 48xCD, ARCINv6c IDE, SCSI
No to DRM, software patents and the EUCD! <http://www.ukcdr.org/>
.... Procrastination means never having to say you're sorry.

 

[Joerg]

Hello Spehro,

I suspect a current setting or limiting resistor is missing somewhere.
Either the emitter or collector of the PNP must have a resistor in
series, otherwise the NPN base current is poorly determined and
likely too high.

Definitely. A resistor is required.

I have seen "designs" where they relied on the beta to be in a certain
range to save a resistor. Mostly in disposable stuff though.

Regards, Joerg

http://www.analogconsultants.com

 

[Spehro Pefhany]

On Tue, 31 May 2005 17:19:39 GMT, the renowned Joerg
<notthisjoergsch@removethispacbell.net> wrote:

Hello Spehro,

I suspect a current setting or limiting resistor is missing somewhere.
Either the emitter or collector of the PNP must have a resistor in
series, otherwise the NPN base current is poorly determined and
likely too high.

Definitely. A resistor is required.

I have seen "designs" where they relied on the beta to be in a certain
range to save a resistor. Mostly in disposable stuff though.

Regards, Joerg

http://www.analogconsultants.com

Ha, I had a feeling that was coming. ;-)

Sure, and the specs can be tighter on some high-volume parts- for
example the ubiquitous 2SA1015(Y) part has hFE in the range of 120-240
(there are 3 beta bins for the PNP part and 4 for the NPN complement).

So, with 100uA base drive, you might expect 12-24mA at the base of
driver (18mA +/-33%), which is not too bad. Of course that's at 25°C,
it will drop at lower temperatures, just when the output drive
transistor needs more juice, and increase with increasing temperature,
just when you don't need it to drive the output transistor, and when
the extra heating is undesired. So, maybe 10-40mA over a wide
temperature range and from unit-to-unit, sloppy, but not crazy bad if
you can afford to burn the extra current. IC designers had to deal
with *resistors* just about that bad in the seventies, IIRC.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

 

[Lasse Langwadt Christense]

Larry Brasfield wrote:

"Philip Pemberton" <philpem@despammed.com> wrote in
message news:a191b9734d.philpem@dsl.pipex.com...

Hi,

Hi.

I've just been reverse engineering a printer controller PCB and I've come
across a rather unusual (to me anyway) transistor driver circuit in the
printhead solenoid driver.
Forgive my naff ASCII art, but here's the schematic:

+V +V
--- ---
| |
| 3 Solenoid
| 3
e | 3
b | / PNP +--|>Z---*

-----[ 4k7 ]------|< | | c

| \ | b | /
+-----*-----|< NPN
| \
| e
|
---
GND

To me, that looks like a pretty normal complementary Darlington circuit.


Those transistors are not Darlington connected. (But I have never
heard of a "complementary Darlington", so maybe you're right.)

I suspect a current setting or limiting resistor is missing somewhere.
Either the emitter or collector of the PNP must have a resistor in
series, otherwise the NPN base current is poorly determined and
likely too high.


What
I'm most interested in is the 20V Zener diode on the NPN - ZD.cathode to
NPN.collector, ZD.anode to NPN.base. AFAICT, when the PNP switches on, the ZD
will get put straight over the solenoid, with the anode more positive than
the cathode, causing current to flow A-K to ground. Surely that would blow
the Zener to bits (it's a BZX84C20 - 300mW SOT23)?


If the circuit was really as you've drawn it, then there would be
a contest between the zener being over-driven and the NPN
base being over-driven. There must be a another resistor.

won't the zener in series with the NPNs Vce be in parallel with the
NPNs Vbe, i.e. The zener will never be on in that direction
unless the NPN is driven really hard?

If it was oriented cathode to +V, anode to NPN.collector, I'd have guessed it
would have worked as a rather expensive spark-killer/back-EMF suppressor. In
this configuration I'm not so sure...


I think the zener orientation you've drawn is correct. I'm not
sure why the designer chooses to dissipate the stored energy
from the solenoid in the transistor rather than simply grounding
the zener and dissipating it there. Is it a small zener and a larger
NPN? Maybe the NPN had to be large anyway to carry the
necessary current, and that connection allowed a smaller zener
to be used.

Its needs to dissipate the power somewhere, its either that or the zener...

On lots protected mosfets it's done with two back to back zeners, the mosfet
can handle more energy "self clamped" like that than just letting it go in
to avalance.

-Lasse

 

[Larry Brasfield]

"Lasse Langwadt Christensen" <langwadt@ieee.org> wrote in
message news:429CF045.5010408@ieee.org...

Larry Brasfield wrote:
"Philip Pemberton" <philpem@despammed.com> wrote in
message news:a191b9734d.philpem@dsl.pipex.com...
I've just been reverse engineering a printer controller PCB and I've come
across a rather unusual (to me anyway) transistor driver circuit in the
printhead solenoid driver.
Forgive my naff ASCII art, but here's the schematic:

+V +V
--- ---
| |
| 3 Solenoid
| 3
e | 3
b | / PNP +--|>Z---*

-----[ 4k7 ]------|< | | c

| \ | b | /
+-----*-----|< NPN
| \
| e
|
---
GND
....
I suspect a current setting or limiting resistor is missing somewhere.
Either the emitter or collector of the PNP must have a resistor in
series, otherwise the NPN base current is poorly determined and
likely too high.

What
I'm most interested in is the 20V Zener diode on the NPN - ZD.cathode to
NPN.collector, ZD.anode to NPN.base. AFAICT, when the PNP switches on, the ZD
will get put straight over the solenoid, with the anode more positive than
the cathode, causing current to flow A-K to ground. Surely that would blow
the Zener to bits (it's a BZX84C20 - 300mW SOT23)?

If the circuit was really as you've drawn it, then there would be
a contest between the zener being over-driven and the NPN
base being over-driven. There must be a another resistor.


won't the zener in series with the NPNs Vce be in parallel with the
NPNs Vbe, i.e. The zener will never be on in that direction
unless the NPN is driven really hard?

True. My supposition was that the NPN would be driven
hard. As others have pointed out, the current from the
PNP is limited enough that such concern is unwarranted.
Of course, with the OP's later statement that there is a
limiting resistor, the concern is irrelevant.

If it was oriented cathode to +V, anode to NPN.collector, I'd have guessed it
would have worked as a rather expensive spark-killer/back-EMF suppressor. In
this configuration I'm not so sure...


I think the zener orientation you've drawn is correct. I'm not
sure why the designer chooses to dissipate the stored energy
from the solenoid in the transistor rather than simply grounding
the zener and dissipating it there. Is it a small zener and a larger
NPN? Maybe the NPN had to be large anyway to carry the
necessary current, and that connection allowed a smaller zener
to be used.

Its needs to dissipate the power somewhere, its either that or the zener...

That's more or less implied in what I stated.

On lots protected mosfets it's done with two back to back zeners, the mosfet
can handle more energy "self clamped" like that than just letting it go in
to avalance.

Maybe things have changed, but I once learned, (very
expensively), to not avalanche MOSFETs. So I agree.

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

 

[Winfield Hill]

Larry Brasfield wrote...

... the mosfet can handle more energy "self clamped" like
that than just letting it go in to avalance.

Maybe things have changed, but I once learned, (very
expensively), to not avalanche MOSFETs. So I agree.

Actually, for most modern MOSFET designs (made in the last
15 years), that's not true.


--
Thanks,
- Win

 

[Frithiof Andreas Jensen]

"Larry Brasfield" <donotspam_larry_brasfield@hotmail.com> wrote in message
news:c90ne.15$qF3.3549@news.uswest.net...

That method causes decay of the solenoid field to occur
much more slowly. For many print head applications,
the additional decay time would hinder performance.

Not only that, but the Zener doubles as an anti-saturation diode by
absorbing the excess base current from the sloppily driven first transistor,
where the designer relies on current gain to "get enough" current. Thus no
need for base resistor on 2nd device.

All-in-All quite clever and saves some USD 0.0001's worth of components per
printer;

 

[Larry Brasfield]

Winfield Hill" <hill_a@t_rowland-dotties-harvard-dot.s-edu>
wrote in message news:d7j5uh0152l@drn.newsguy.com...

Larry Brasfield wrote...

[quoting somebody else]
... the mosfet can handle more energy "self clamped" like
that than just letting it go in to avalance.

Maybe things have changed, but I once learned, (very
expensively), to not avalanche MOSFETs. So I agree.

Actually, for most modern MOSFET designs (made in the last
15 years), that's not true.

Are you stating that most modern MOSFETs can handle
the same pulse power in avalanche as when biased to keep
vds just below breakdown? I don't recall seeing this in the
datasheets of the MOSFETs I've used recently. I would
love to see some data on this because it is often otherwise
necessary to use higher voltage parts just to preclude the
possibility of brief breakdown.

If you are questioning my learning experience, that was
when small power MOSFETs were $100 a pop and
many scopes too slow to fully capture the turn-off spikes
made possible by their switching speed. (1980 or so)

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

 

[Ken Smith]

In article <8ad0bd734d.philpem@dsl.pipex.com>,
Philip Pemberton <philpem@despammed.com> wrote:
[....]

ISTR it's also known as a "Sziklai" pair. You use an NPN and a PNP (or vice
versa) instead of a pair of NPNs or a pair of PNPs. In this case, the pair
acts like a PNP.

I've always known that circuit as a "White Darlington". I suspect that
more than one person invented it.

I submit for your consideration, the "Smith Darlington"

!
--------------+
!/e !
-----! PNP !/e
!\ ---!
! !/ !\ PNP
------! !
NPN !\e !
------+
!


I invented this many years ago when I needed a PNP with a huge HFE.

--
--
kensmith@rahul.net forging knowledge

 

[Ken Smith]

In article <ysjne.484$Wk4.2227@news.uswest.net>,
Larry Brasfield <donotspam_larry_brasfield@hotmail.com> wrote:
[....]

Are you stating that most modern MOSFETs can handle
the same pulse power in avalanche as when biased to keep
vds just below breakdown?

With IRF ones, they can withstand the same power but less energy. Look at
the IRFPE40's datasheet. The Avalanche Current is 5.4A just like the
normal drain current. By using the MOSFET to hold the voltage down a bit,
you can spread out the energy from the inductive spike over more time so
the temperature rise is less.

--
--
kensmith@rahul.net forging knowledge

 

[Fred Bartoli]

"Ken Smith" <kensmith@green.rahul.net> a écrit dans le message de
news:d7poco$2b6$1@blue.rahul.net...

In article <8ad0bd734d.philpem@dsl.pipex.com>,
Philip Pemberton <philpem@despammed.com> wrote:
[....]
ISTR it's also known as a "Sziklai" pair. You use an NPN and a PNP (or
vice
versa) instead of a pair of NPNs or a pair of PNPs. In this case, the
pair
acts like a PNP.

I've always known that circuit as a "White Darlington". I suspect that
more than one person invented it.

I submit for your consideration, the "Smith Darlington"

!
--------------+
!/e !
-----! PNP !/e
!\ ---!
! !/ !\ PNP
------! !
NPN !\e !
------+
!


I invented this many years ago when I needed a PNP with a huge HFE.

--
--
kensmith@rahul.net forging knowledge

Then I invent this one :

.-----------+-----+-----
| | |
|< |< |
----| .---| |
|\ | |\ |
| | | |
| |/ | |/
'---| '---|
|> |>
| |
'-----------+-----

which has an even more huge HFE, uses an NPN as the power element and allow
a lower VCEsat (NPN power element).

It is very pompously named a smartlington.

--
Thanks,
Fred.

 

[Jim Thompson]

On Fri, 3 Jun 2005 16:35:41 +0200, "Fred Bartoli"
<fred._canxxxel_this_bartoli@RemoveThatAlso_free.fr_AndThisToo> wrote:

[snip]

Then I invent this one :

.-----------+-----+-----
| | |
|< |< |
----| .---| |
|\ | |\ |
| | | |
| |/ | |/
'---| '---|
|> |
| |
'-----------+-----

which has an even more huge HFE, uses an NPN as the power element and allow
a lower VCEsat (NPN power element).

It is very pompously named a smartlington.

And VCEsat=??

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| E-mail Address at Website Fax480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.