question about temperature characteristics of transistors

[Tolstoy]

I have a project that requires current
limiting. The sensing has to be done on the
high side and I don't want to order
some special chip, so I dreamed up a circuit
that just uses jellybean parts.


Icharge-->

------+-----Rs-------+------batt, 12v
| |
\ PN2907 /

| x2 |<

|---+----|
/| | |\
/ 1M \
| | |
| gnd |
| |
+--------------|----
| | to comparator
| +---- or op amp
| |
R1 R2
| |
'------+-------'
|
gnd

Circuit operation:

the b/e junctions of the transistors will
experience a differential voltage Ich*Rs.
For a particular DeltaVbe=Ich*Rs,
the base currents will exhibit a certain ratio
and the collector currents will hew to the
same ratio, at least for matching transistors.
Choosing R1 and R2 in a ratio equal to the
ratio that the collector currents exhibit at
target charging voltage will cause equal
voltage across the comparator inputs
at that moment and trip it.

I haven't tried prototyping this circuit yet,
I just came up with the idea this morning,
so please criticize it and
point out what I may have missed.

This circuit will get bolted
somewhere on an internal combustion engine
operating outdoors, so temperature will vary
to extremes. The transistors' 2mV
tempco *should* cancel; but then the circuit
operation also relies on the exponential
voltage/current relationship, and
I haven't found any information about whether
the terms of this expression exhibit a
temperature dependence.
Does anybody know please?

 

[Jim Thompson]

I have a project that requires current
limiting. The sensing has to be done on the
high side and I don't want to order
some special chip, so I dreamed up a circuit
that just uses jellybean parts.


Icharge-->

------+-----Rs-------+------batt, 12v
| |
\ PN2907 /
|---+----|
/| | |\
/ 1M \
| | |
| gnd |
| |
+--------------|----
| | to comparator
| +---- or op amp
| |
R1 R2
| |
'------+-------'
|
gnd

Circuit operation:

the b/e junctions of the transistors will
experience a differential voltage Ich*Rs.
For a particular DeltaVbe=Ich*Rs,
the base currents will exhibit a certain ratio
and the collector currents will hew to the
same ratio, at least for matching transistors.
Choosing R1 and R2 in a ratio equal to the
ratio that the collector currents exhibit at
target charging voltage will cause equal
voltage across the comparator inputs
at that moment and trip it.

I haven't tried prototyping this circuit yet,
I just came up with the idea this morning,
so please criticize it and
point out what I may have missed.

This circuit will get bolted
somewhere on an internal combustion engine
operating outdoors, so temperature will vary
to extremes. The transistors' 2mV
tempco *should* cancel; but then the circuit
operation also relies on the exponential
voltage/current relationship, and
I haven't found any information about whether
the terms of this expression exhibit a
temperature dependence.
Does anybody know please?

Perhaps something along the lines of...

http://analog-innovations.com/SED/CurrentSense.pdf

You need to make-up for the drop in the sense resistor by biasing
(adding R in emitter of one of the transistors).

Temperature IS going to be an issue. Perhaps a dual transistor
package would help.

...Jim Thompson

 

[Jonathan Kirwan]

I have a project that requires current
limiting. The sensing has to be done on the
high side and I don't want to order
some special chip, so I dreamed up a circuit
that just uses jellybean parts.


Icharge-->

------+-----Rs-------+------batt, 12v
| |
\ PN2907 /
|---+----|
/| | |\
/ 1M \
| | |
| gnd |
| |
+--------------|----
| | to comparator
| +---- or op amp
| |
R1 R2
| |
'------+-------'
|
gnd

See:
http://www.edn.com/contents/images/30101di.pdf

as a start.

Jon

 

[Jonathan Kirwan]

The transistors' 2mV
tempco *should* cancel; but then the circuit
operation also relies on the exponential
voltage/current relationship, and
I haven't found any information about whether
the terms of this expression exhibit a
temperature dependence.

Is is also temperature dependent. And since the voltage at some
current depends on Is, that does change. Something like:

Is(T) = Is(Tnom)*(T/Tnom)^3*e^[-(Eg/k)*(1/T-1/Tnom)]

Jon

 

[John Larkin]

Perhaps something along the lines of...

http://analog-innovations.com/SED/CurrentSense.pdf

You need to make-up for the drop in the sense resistor by biasing
(adding R in emitter of one of the transistors).

Temperature IS going to be an issue. Perhaps a dual transistor
package would help.

...Jim Thompson

Why not add a resistor into both emitters? That will linearize the
transfer function and mostly swamp the temperature mismatch. That
would allow, say, 50 or 100 mV of peak drop in the sense resistor.



A little more base current wouldn't hurt. And maybe some base
resistors to keep the transistors from screaming.

Note that Icharge will typically be a very nasty waveform.

John

 

[D from BC]

I have a project that requires current
limiting. The sensing has to be done on the
high side and I don't want to order
some special chip, so I dreamed up a circuit
that just uses jellybean parts.


Icharge-->

------+-----Rs-------+------batt, 12v
| |
\ PN2907 /
|---+----|
/| | |\
/ 1M \
| | |
| gnd |
| |
+--------------|----
| | to comparator
| +---- or op amp
| |
R1 R2
| |
'------+-------'
|
gnd

Circuit operation:

the b/e junctions of the transistors will
experience a differential voltage Ich*Rs.
For a particular DeltaVbe=Ich*Rs,
the base currents will exhibit a certain ratio
and the collector currents will hew to the
same ratio, at least for matching transistors.
Choosing R1 and R2 in a ratio equal to the
ratio that the collector currents exhibit at
target charging voltage will cause equal
voltage across the comparator inputs
at that moment and trip it.

I haven't tried prototyping this circuit yet,
I just came up with the idea this morning,
so please criticize it and
point out what I may have missed.

This circuit will get bolted
somewhere on an internal combustion engine
operating outdoors, so temperature will vary
to extremes. The transistors' 2mV
tempco *should* cancel; but then the circuit
operation also relies on the exponential
voltage/current relationship, and
I haven't found any information about whether
the terms of this expression exhibit a
temperature dependence.
Does anybody know please?

This is starting to look like..
http://www.zetex.com/3.0/pdf/ZDS1009.pdf
D from BC

 

[Jim Thompson]

Why not add a resistor into both emitters? That will linearize the
transfer function and mostly swamp the temperature mismatch. That
would allow, say, 50 or 100 mV of peak drop in the sense resistor.




A little more base current wouldn't hurt. And maybe some base
resistors to keep the transistors from screaming.

Note that Icharge will typically be a very nasty waveform.

John

A resistor in both emitters cuts the gain... only needed on the left.
Run the math, you have to set a transition level somehow ;-)

The 1Meg should go bye-bye. Diode connect left-hand transistor (or
right-hand) depending on signal-sex desired.

...Jim Thompson

 

[Jim Thompson]

This is starting to look like..
http://www.zetex.com/3.0/pdf/ZDS1009.pdf
D from BC

Yep. Looks like useful "popcorn" circuits are returning, as people
discover that uP's have limited capabilities ;-)

...Jim Thompson

 

[Fred Bloggs]

I have a project that requires current
limiting. The sensing has to be done on the
high side and I don't want to order
some special chip, so I dreamed up a circuit
that just uses jellybean parts.


Icharge-->

------+-----Rs-------+------batt, 12v
| |
\ PN2907 /
|---+----|
/| | |\
/ 1M \
| | |
| gnd |
| |
+--------------|----
| | to comparator
| +---- or op amp
| |
R1 R2
| |
'------+-------'
|
gnd

Circuit operation:

the b/e junctions of the transistors will
experience a differential voltage Ich*Rs.
For a particular DeltaVbe=Ich*Rs,
the base currents will exhibit a certain ratio
and the collector currents will hew to the
same ratio, at least for matching transistors.
Choosing R1 and R2 in a ratio equal to the
ratio that the collector currents exhibit at
target charging voltage will cause equal
voltage across the comparator inputs
at that moment and trip it.

I haven't tried prototyping this circuit yet,
I just came up with the idea this morning,
so please criticize it and
point out what I may have missed.

This circuit will get bolted
somewhere on an internal combustion engine
operating outdoors, so temperature will vary
to extremes. The transistors' 2mV
tempco *should* cancel; but then the circuit
operation also relies on the exponential
voltage/current relationship, and
I haven't found any information about whether
the terms of this expression exhibit a
temperature dependence.
Does anybody know please?

The comparator does not trip on a ratio, it trips on a difference. This
difference voltage will be something like:
Is*exp(Veb/Vt)*(R1-R2*exp(-Ich*Rs/Vt)), assuming matched Is, and so you
want R1-R2*exp(-IchRs/Vt)=0 or R2=R1*exp(Ich*Rs/Vt). Then the trip
current at any other T will be Ich=(Vt/Rs)*LN(R2/R1) so that dIch/dT=
K/(Q*Rs)*LN(R2/R1). But you selected R2/R1 to be exp(Ich*Rs/Vt) at Tnom,
making dIch/dT=K/(Q*Rs)*(Ich*Rs/Vt)=Ich/Tnom or dIch/Ich=dT/Tnom. This
is not what you would call insensitive to temperature.

 

[John Larkin]

A resistor in both emitters cuts the gain...

Gosh, really?

only needed on the left.

If you don't care about DC errors.

Run the math, you have to set a transition level somehow ;-)

Transition level? What's that?

The 1Meg should go bye-bye. Diode connect left-hand transistor (or
right-hand) depending on signal-sex desired.

His original circuit, fully symmetric, is fine.

John

 

[Tolstoy]

[Snip ugly circuit]

Perhaps something along the lines of...

http://analog-innovations.com/SED/CurrentSense.pdf

Wow, talk about simple.

You need to make-up for the drop in the sense resistor by biasing
(adding R in emitter of one of the transistors).

Temperature IS going to be an issue. Perhaps a dual transistor
package would help.

...Jim Thompson

I was going to glue the flat sides of a couple of TO-92 transistors
together using Krazy Glue. It's amazing how well that stuff works on
plastic transistors.

I'm going to study your circuit. Just two transistors. What's not to
like?

In the meantime, here's something that looks better than the first
circuit I posted. You guys can pick it apart if you have a mind to.



Icharge-->


------+-----Rs-------+--batt, 12v
| |
Re |
| |
| |
\ PN2907 /

| x2 |<

|---+----|
/| | |\
/ | \
| | |
| '-------+
| |
| |
| ,------------,
| | | 317-based |
| | | floating |
out<--+ | | current |
| | | source |
| | |I=Ilim*Rs/Re|
| v '------------'
| |
| ,-------+
| | |
\ | /
\| | |/
|---+----|
/| |\
< PN2222x2 >
| |
'------+-------'
|
gnd

Ilim = design limit current

Thanks everybody for the replies.

 

[Robert Baer]

[Snip ugly circuit]

Perhaps something along the lines of...

http://analog-innovations.com/SED/CurrentSense.pdf

Wow, talk about simple.

You need to make-up for the drop in the sense resistor by biasing
(adding R in emitter of one of the transistors).

Temperature IS going to be an issue. Perhaps a dual transistor
package would help.

...Jim Thompson

I was going to glue the flat sides of a couple of TO-92 transistors
together using Krazy Glue. It's amazing how well that stuff works on
plastic transistors.

I'm going to study your circuit. Just two transistors. What's not to
like?

In the meantime, here's something that looks better than the first
circuit I posted. You guys can pick it apart if you have a mind to.



Icharge-->


------+-----Rs-------+--batt, 12v
| |
Re |
| |
| |
\ PN2907 /

| x2 |<

|---+----|
/| | |\
/ | \
| | |
| '-------+
| |
| |
| ,------------,
| | | 317-based |
| | | floating |
out<--+ | | current |
| | | source |
| | |I=Ilim*Rs/Re|
| v '------------'
| |
| ,-------+
| | |
\ | /
\| | |/
|---+----|
/| |\
< PN2222x2 >
| |
'------+-------'
|
gnd

Ilim = design limit current

Thanks everybody for the replies.

Well...Krazy glue is a good thermal insulator, does not hold up to
temperature, and would make a "thick" interface between the transistors.
Better yet to use a surface mount dual and layout the PCB for a
thermal pad for the body to sit upon.

 

[Tolstoy]

[Snip ugly circuit]

This circuit will get bolted
somewhere on an internal combustion engine
operating outdoors, so temperature will vary
to extremes. The transistors' 2mV
tempco *should* cancel; but then the circuit
operation also relies on the exponential
voltage/current relationship, and
I haven't found any information about whether
the terms of this expression exhibit a
temperature dependence.
Does anybody know please?
Perhaps something along the lines of...
http://analog-innovations.com/SED/CurrentSense.pdf

Wow, talk about simple.

I was going to glue the flat sides of a couple of TO-92 transistors
together using Krazy Glue. It's amazing how well that stuff works on
plastic transistors.

I'm going to study your circuit. Just two transistors. What's not to
like?

In the meantime, here's something that looks better than the first
circuit I posted. You guys can pick it apart if you have a mind to.

------+-----Rs-------+--batt, 12v
| |
Re |
| |
| |
\ PN2907 /
|---+----|
/| | |\
/ | \
| | |
| '-------+
| |
| |
| ,------------,
| | | 317-based |
| | | floating |
out<--+ | | current |
| | | source |
| | |I=Ilim*Rs/Re|
| v '------------'
| |
| ,-------+
| | |
\ | /
\| | |/
|---+----|
/| |\
< PN2222x2 >
| |
'------+-------'
|
gnd

Ilim = design limit current

Thanks everybody for the replies.

Well...Krazy glue is a good thermal insulator, does not hold up to
temperature, and would make a "thick" interface between the transistors.
Better yet to use a surface mount dual and layout the PCB for a
thermal pad for the body to sit upon.- Hide quoted text -

- Show quoted text -

I need something in the largest surface mount package I can find so
that I can hand solder it, that I can buy in onesies and twosies, and
totally generic so that it won't get hard to find.
I found a couple of npn and pnp current mirrors that would have been
perfect, the BCV61 and BCV72; unfortunately, digikey has a large
minimum purchase for the npn version. Also, Mouser doesn't list
either of these parts, which isn't a good sign. So I'm looking for a
dual npn chip and a dual pnp chip, or maybe a quad with two of each.
I've never used surface mount, so I have to sort through all the
different surfmount packages according to size and find out the names
of the ones that are big enough. Then I can use the digikey
parametric search to see what's available.

 

[Fred Bloggs]

Yep. Looks like useful "popcorn" circuits are returning, as people
discover that uP's have limited capabilities ;-)

...Jim Thompson

That ZDS1009 has Camenzind written all over it. It wasn't too long ago
when you were bitching about it having start-up problems.

 

[Fred Bloggs]

In the meantime, here's something that looks better than the first
circuit I posted. You guys can pick it apart if you have a mind to.



Icharge-->


------+-----Rs-------+--batt, 12v
| |
Re |
| |
| |
\ PN2907 /
|---+----|
/| | |\
/ | \
| | |
| '-------+
| |
| |
| ,------------,
| | | 317-based |
| | | floating |
out<--+ | | current |
| | | source |
| | |I=Ilim*Rs/Re|
| v '------------'
| |
| ,-------+
| | |
\ | /
\| | |/
|---+----|
/| |\
< PN2222x2 >
| |
'------+-------'
|
gnd

Ilim = design limit current

Here is a much better circuit with linear output, better control over
sense transistor operating conditions, low current and low voltage
operation too, patented by Analog Devices:
View in a fixed-width font such as
Courier.

 

[Tolstoy]

In the meantime, here's something that looks better than the first
circuit I posted. You guys can pick it apart if you have a mind to.

------+-----Rs-------+--batt, 12v
| |
Re |
| |
| |
\ PN2907 /
|---+----|
/| | |\
/ | \
| | |
| '-------+
| |
| |
| ,------------,
| | | 317-based |
| | | floating |
out<--+ | | current |
| | | source |
| | |I=Ilim*Rs/Re|
| v '------------'
| |
| ,-------+
| | |
\ | /
\| | |/
|---+----|
/| |\
< PN2222x2 >
| |
'------+-------'
|
gnd

Ilim = design limit current

Here is a much better circuit with linear output, better control over
sense transistor operating conditions, low current and low voltage
operation too, patented by Analog Devices:
View in a fixed-width font such as
Courier.

.
. -Is->
.
. ----+------[Rs]------+-----
. | |
. [R] [R]
. | |
. .----+ |
. | | |
. | >| |<
. | Q1 |-----+------| Q2
. | /| | |\
. | | |< |
. | +-----| Q3 |
. | | |\ |
. >| | | |
. Q4 |-------------------+
. /| | | |
. | / \ | / \
. | | I | | | I |
. Io \ / | \ /
. | | |
. | | |
. '-------+--------'
. |
. ---
.
.- Hide quoted text -

- Show quoted text -

Nice. I'll file that one. But..
I want comparator action, not linear output (sorry, I didn't mention
that before). If my circuit were to have linear output I would just
end up having to add a load resistor, a voltage reference and a
comparator to get what I want, an output active high when charging
current exceeds Ilim, otherwise open or low output.
Just for grins, I jiggered the circuit around again:


Icharge-->

------+-----Rs-------+--batt, 12v
| |
Re |
| |
| |
\ PN2907 /

| x2 |<

|---+----|
/| | |\
/ | \
| | |
| | |
out-+ Rb |
| | |
| | |
\ | /
\| | |/
|---+----|
/| |\
< PN2222x2 >
| |
'------+-------'
|
current sink
I = 2 Ilim Re/Rs
|
gnd

 

[Tolstoy]

Tolstoy wrote:

Here is a much better circuit with linear output, better control over
sense transistor operating conditions, low current and low voltage
operation too, patented by Analog Devices:
View in a fixed-width font such as
Courier.

.
. -Is->
.
. ----+------[Rs]------+-----
. | |
. [R] [R]
. | |
. .----+ |
. | | |
. | >| |<
. | Q1 |-----+------| Q2
. | /| | |\
. | | |< |
. | +-----| Q3 |
. | | |\ |
. >| | | |
. Q4 |-------------------+
. /| | | |
. | / \ | / \
. | | I | | | I |
. Io \ / | \ /
. | | |
. | | |
. '-------+--------'
. |
. ---
.
.- Hide quoted text -

- Show quoted text -

Nice. I'll file that one. But..
I want comparator action, not linear output (sorry, I didn't mention
that before). If my circuit were to have linear output I would just
end up having to add a load resistor, a voltage reference and a
comparator to get what I want, an output active high when charging
current exceeds Ilim, otherwise open or low output.
Just for grins, I jiggered the circuit around again:

Icharge-->

------+-----Rs-------+--batt, 12v
| |
Re |
| |
| |
\ PN2907 /

| x2 |<

|---+----|
/| | |\
/ | \
| | |
| | |
out-+ Rb |
| | |
| | |
\ | /
\| | |/
|---+----|
/| |\
< PN2222x2 >
| |
'------+-------'
|
current sink
I = 2 Ilim Re/Rs
|
gnd- Hide quoted text -

- Show quoted text -

Sorry, that's I = 2 Ilim Rs/Re

 

[Jim Thompson]

That ZDS1009 has Camenzind written all over it. It wasn't too long ago
when you were bitching about it having start-up problems.

Good point. If it doesn't leak, how does it start?

...Jim Thompson