Current source design (tricky?)

In article <4236C693.4020904@nospam.com>,
Fred Bloggs <nospam@nospam.com> wrote:
[.. saving band width ..]
Go back one level to see where I added the capacitor.
That addition definitely makes for an improved transient response- by a
factor of 50x from what I can see.
Click to expand...
I think the 50x factor assumes a tighter tolerance than we can expect in
real life. The slightly mismatched capacitors we are likely to have in
real life will still match near exactly on the initial step but the
ext(-t/T) curves will not match.

It would be nice if we could make the circuit not rely on the match. If
we had a rail-to-rail op-amp, or something like the LM301, we could shift
both inputs of the op-amp up near the positivve rail. As it is, the LM741
will work with its inputs up within about 4V of the positive supply. If
we changed the resistor dividers to make them a 1/3,2/3 ratio, we would
get an improved transient performance.


The big 1000u across the regulator
limits the current perturbations to under 5% when subjected to things
like that +30/-300V 20u/30u transient, and the 22 ohm resistor makes for
a nice low pass attenuating Volts-transient to mV-transient applied to
the circuit and on a time scale it can follow.
Click to expand...
Since the LM741, IIRC, only draws about 3mA, the 22 ohm resistor in the
minus supply to that section could be increased to lets say 100.


Since we can't put a
component in series with the main current path, the MOSFET is on its
own, with the body diode saving it during negative transients, but no
protection from positive transients. The part numbers suggested thus far
are shy on Vbr,dss.
Click to expand...
We can put a small inductance in series. It won't help much except to
round off very short timed pulses. I don't think such things are common
in automotive power systems though.

The problem with looking for a higher Vbr(dss) is that Rds(on) runs about
as the square of Vbr(ds) for the same sized part.



--
--
kensmith@rahul.net forging knowledge
 
John Woodgate wrote...
Winfield Hill wrote ...
Tony Williams wrote...

... we add a modest 33C penalty to the
heat-sink's job when dissipating 30W.

Since this appears to be a vehicle application there
may be limited facility for a large heat sink.

Ahh, but the unit may well be bolted to a slab of metal.


...at 150 C when the engine is running, on a hot day.
Click to expand...
Hey, I said "slab" of metal (firewall) not "block" of metal!


--
Thanks,
- Win
 
On 15 Mar 2005 13:21:00 -0800, Winfield Hill
<hill_a@t_rowland-dotties-harvard-dot.s-edu> wrote:

John Woodgate wrote...

Winfield Hill wrote ...
Tony Williams wrote...

... we add a modest 33C penalty to the
heat-sink's job when dissipating 30W.

Since this appears to be a vehicle application there
may be limited facility for a large heat sink.

Ahh, but the unit may well be bolted to a slab of metal.


...at 150 C when the engine is running, on a hot day.

Hey, I said "slab" of metal (firewall) not "block" of metal!
Click to expand...
Why do you think most automotive Electronic Control Units are located
under the driver's seat or in the kick panel to the left of the
"clutch" foot?

Under-hood is VERY inhospitable... when I was in the automotive
electronics biz I designed for a normal high of +140°C.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
 
I read in sci.electronics.design that Winfield Hill <hill_a@t_rowland-
dotties-harvard-dot.s-edu> wrote (in <d17jjs023jq@drn.newsguy.com>)
about 'Current source design (tricky?)', on Tue, 15 Mar 2005:
John Woodgate wrote...

Winfield Hill wrote ...
Tony Williams wrote...

... we add a modest 33C penalty to the
heat-sink's job when dissipating 30W.

Since this appears to be a vehicle application there
may be limited facility for a large heat sink.

Ahh, but the unit may well be bolted to a slab of metal.


...at 150 C when the engine is running, on a hot day.

Hey, I said "slab" of metal (firewall) not "block" of metal!
Click to expand...
Practically anything in the engine compartment gets very hot. But it was
just a hint to consider Murphy's Law.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
John Woodgate wrote...
Winfield Hill wrote ...
John Woodgate wrote...
Winfield Hill wrote ...
Tony Williams wrote...

... we add a modest 33C penalty to the heat-sink's
job when dissipating 30W.
Since this appears to be a vehicle application there
may be limited facility for a large heat sink.
Ahh, but the unit may well be bolted to a slab of metal.
...at 150 C when the engine is running, on a hot day.
Hey, I said "slab" of metal (firewall) not "block" of metal!
Practically anything in the engine compartment gets very hot.
But it was just a hint to consider Murphy's Law.
Click to expand...
Indeed. The OP didn't say anything about operating inside an
actual automobile, let alone in the engine compartment, but if
he has this in mind, he should speak up and let us deal with it!


--
Thanks,
- Win
 
I read in sci.electronics.design that Ken Smith
<kensmith@green.rahul.net> wrote (in <d11rpa$m5o$2@blue.rahul.net>)
about 'Current source design (tricky?)', on Sun, 13 Mar 2005:
In article <5h_Yd.50990$BX6.1154@fe06.lga>,
Jamie <jamie_5_not_valid_after_5_Please@charter.net> wrote:
[...]
i remember the days of using an incandescent lamp! :)

There used to be a "constant current" tube that was really a specially
tricked out light bulb.


Well, yes, for sufficiently basic differences. A 'barretter' has an iron
Click to expand...
wire filament in a hydrogen-filled envelope.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
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