how can I deal with a current spike?

panfilero · Apr 7, 2009

Hello,

I'm going to be driving a capcitive load out from a couple opamps,
both the opamps are acting as buffers, and normally my capacitve load
will have a very small current through it... but initially it will
have a current surge going through it as the voltage builds across
it...

My question, my opamps can't handle this (it's about 400mA my opamps
can handle about 60mA) can someone recommend a way of me handling the
current surge without it damaging my opamps? Would an inductor in
series with my load help soften the current surge?

The voltage across my load swings from -5V to +5V so, I need to be
able to handle that current surge regardless of what direction its
going in....

much thanks!

Sjouke Burry · Apr 7, 2009

panfilero wrote:

Hello,

I'm going to be driving a capcitive load out from a couple opamps,
both the opamps are acting as buffers, and normally my capacitve load
will have a very small current through it... but initially it will
have a current surge going through it as the voltage builds across
it...

My question, my opamps can't handle this (it's about 400mA my opamps
can handle about 60mA) can someone recommend a way of me handling the
current surge without it damaging my opamps? Would an inductor in
series with my load help soften the current surge?

The voltage across my load swings from -5V to +5V so, I need to be
able to handle that current surge regardless of what direction its
going in....

much thanks!
As long as the opamp has current limiting(a lot of them), there

is no problem, the current wont rise above the stated current
capability.
If there is no inherent current limiting, use a resistor to ensure
that the current does not damage the opamp.
You might use a reed relay to short the resistor, after the worst
initial surge has abated.

Joerg · Apr 7, 2009

Sjouke Burry wrote:

panfilero wrote:
Hello,

I'm going to be driving a capcitive load out from a couple opamps,
both the opamps are acting as buffers, and normally my capacitve load
will have a very small current through it... but initially it will
have a current surge going through it as the voltage builds across
it...

My question, my opamps can't handle this (it's about 400mA my opamps
can handle about 60mA) can someone recommend a way of me handling the
current surge without it damaging my opamps? Would an inductor in
series with my load help soften the current surge?

The voltage across my load swings from -5V to +5V so, I need to be
able to handle that current surge regardless of what direction its
going in....

much thanks!
As long as the opamp has current limiting(a lot of them), there
is no problem, the current wont rise above the stated current
capability.
If there is no inherent current limiting, use a resistor to ensure
that the current does not damage the opamp.
You might use a reed relay to short the resistor, after the worst
initial surge has abated.

Anotehr limit might be to current-limit the supply rails if it's a
single opamp package. But remember, Opamp do not like to drive
capacitive loads as that can drive them unstable, the usual tricks are
needed.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

panfilero · Apr 7, 2009

On Apr 6, 7:37 pm, Joerg <notthisjoerg...@removethispacbell.net>
wrote:

Sjouke Burry wrote:
panfilero wrote:
Hello,

I'm going to be driving a capcitive load out from a couple opamps,
both the opamps are acting as buffers, and normally my capacitve load
will have a very small current through it... but initially it will
have a current surge going through it as the voltage builds across
it...

My question, my opamps can't handle this (it's about 400mA my opamps
can handle about 60mA) can someone recommend a way of me handling the
current surge without it damaging my opamps? Would an inductor in
series with my load help soften the current surge?

The voltage across my load swings from -5V to +5V so, I need to be
able to handle that current surge regardless of what direction its
going in....

much thanks!
As long as the opamp has current limiting(a lot of them), there
is no problem, the current wont rise above the stated current
capability.
If there is no inherent current limiting, use a resistor to ensure
that the current does not damage the opamp.
You might use a reed relay to short the resistor, after the worst
initial surge has abated.

Anotehr limit might be to current-limit the supply rails if it's a
single opamp package. But remember, Opamp do not like to drive
capacitive loads as that can drive them unstable, the usual tricks are
needed.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

Oh? I didn't know about the op-amps not liking the capacitive loads...
is this something that is an issue even for a buffer passing a dc
voltage... or is it more for passing ac signals... or frequencies....
I'm googling this and most of the hits are talking about op-amps with
some gain and for some ac signal at the input... I wonder though if
the issue still applies to my dc buffer?

much thanks!

Bob Masta · Apr 7, 2009

On Mon, 6 Apr 2009 19:09:05 -0700 (PDT), panfilero
<panfilero@gmail.com> wrote:

On Apr 6, 7:37=A0pm, Joerg <notthisjoerg...@removethispacbell.net
wrote:
Sjouke Burry wrote:
panfilero wrote:
Hello,

I'm going to be driving a capcitive load out from a couple opamps,
both the opamps are acting as buffers, and normally my capacitve load
will have a very small current through it... but initially it will
have a current surge going through it as the voltage builds across
it...

My question, my opamps can't handle this (it's about 400mA my opamps
can handle about 60mA) can someone recommend a way of me handling the
current surge without it damaging my opamps? =A0Would an inductor in
series with my load help soften the current surge?

The voltage across my load swings from -5V to +5V so, I need to be
able to handle that current surge regardless of what direction its
going in....

much thanks!
As long as the opamp has current limiting(a lot of them), there
is no problem, the current wont rise above the stated current
capability.
If there is no inherent current limiting, use a resistor to ensure
that the current does not damage the opamp.
You might use a reed relay to short the resistor, after the worst
initial surge has abated.

Anotehr limit might be to current-limit the supply rails if it's a
single opamp package. But remember, Opamp do not like to drive
capacitive loads as that can drive them unstable, the usual tricks are
needed.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

Oh? I didn't know about the op-amps not liking the capacitive loads...
is this something that is an issue even for a buffer passing a dc
voltage... or is it more for passing ac signals... or frequencies....
I'm googling this and most of the hits are talking about op-amps with
some gain and for some ac signal at the input... I wonder though if
the issue still applies to my dc buffer?

much thanks!

Yes, it still applies to buffers. And the fact
that you may *intend* to only pass DC is not
recognized by the op-amp. If the capacitive load
causes the amp to be unstable, you'll have an
oscillator instead of a DC signal.

The fix is to add a small resistor between the
op-amp output and the load. Then, instead of
taking the feedback from the op-amp output, you
take it from the junction of the resistor and
load. You also need a small cap from the op-amp
output to the inverting input. I've forgotten the
rule for calculating the R and C (anybody?) but
for normal op-amps driving long cables and such
you can use R = 47-100 ohms and C = 22-47 pF.

In addition, you can also use (for example) a 10k
resistor instead of the normal solid-wire feedback
that is typically used for buffers. (Note that
per the above it will now go between the inverting
input and the load, not the op-amp output.) That
won't change the DC gain, but may slow down the
transients in conjuction with the above C.

Best regards,

Bob Masta

DAQARTA v4.51
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Sound Level Meter
FREE Signal Generator
Science with your sound card!

Joerg · Apr 7, 2009

Bob Masta wrote:

On Mon, 6 Apr 2009 19:09:05 -0700 (PDT), panfilero
panfilero@gmail.com> wrote:

On Apr 6, 7:37=A0pm, Joerg <notthisjoerg...@removethispacbell.net
wrote:
Sjouke Burry wrote:
panfilero wrote:
Hello,
I'm going to be driving a capcitive load out from a couple opamps,
both the opamps are acting as buffers, and normally my capacitve load
will have a very small current through it... but initially it will
have a current surge going through it as the voltage builds across
it...
My question, my opamps can't handle this (it's about 400mA my opamps
can handle about 60mA) can someone recommend a way of me handling the
current surge without it damaging my opamps? =A0Would an inductor in
series with my load help soften the current surge?
The voltage across my load swings from -5V to +5V so, I need to be
able to handle that current surge regardless of what direction its
going in....
much thanks!
As long as the opamp has current limiting(a lot of them), there
is no problem, the current wont rise above the stated current
capability.
If there is no inherent current limiting, use a resistor to ensure
that the current does not damage the opamp.
You might use a reed relay to short the resistor, after the worst
initial surge has abated.
Anotehr limit might be to current-limit the supply rails if it's a
single opamp package. But remember, Opamp do not like to drive
capacitive loads as that can drive them unstable, the usual tricks are
needed.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
Oh? I didn't know about the op-amps not liking the capacitive loads...
is this something that is an issue even for a buffer passing a dc
voltage... or is it more for passing ac signals... or frequencies....
I'm googling this and most of the hits are talking about op-amps with
some gain and for some ac signal at the input... I wonder though if
the issue still applies to my dc buffer?

much thanks!

Yes, it still applies to buffers. And the fact
that you may *intend* to only pass DC is not
recognized by the op-amp. If the capacitive load
causes the amp to be unstable, you'll have an
oscillator instead of a DC signal.

The fix is to add a small resistor between the
op-amp output and the load. Then, instead of
taking the feedback from the op-amp output, you
take it from the junction of the resistor and
load. You also need a small cap from the op-amp
output to the inverting input. I've forgotten the
rule for calculating the R and C (anybody?) but
for normal op-amps driving long cables and such
you can use R = 47-100 ohms and C = 22-47 pF.

In addition, you can also use (for example) a 10k
resistor instead of the normal solid-wire feedback
that is typically used for buffers. (Note that
per the above it will now go between the inverting
input and the load, not the op-amp output.) That
won't change the DC gain, but may slow down the
transients in conjuction with the above C.

Yep, well said. There is an app note from National about it but I
didn't find it back. Probably tossed it but maybe it can be found on
their server und "opamps" plus "capacitive load".

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

Jamie · Apr 7, 2009

panfilero wrote:

Hello,

I'm going to be driving a capcitive load out from a couple opamps,
both the opamps are acting as buffers, and normally my capacitve load
will have a very small current through it... but initially it will
have a current surge going through it as the voltage builds across
it...

My question, my opamps can't handle this (it's about 400mA my opamps
can handle about 60mA) can someone recommend a way of me handling the
current surge without it damaging my opamps? Would an inductor in
series with my load help soften the current surge?

The voltage across my load swings from -5V to +5V so, I need to be
able to handle that current surge regardless of what direction its
going in....

much thanks!
I don't know how you're applying your op-amps to the circuit how ever,

if you're using some kind of feed back to the inputs like (-) for
example, you can use a low value resistor on the output that will be
calculated for the max load, from there, that network point is your
output and also the point where you feed the remaining r value back to
the input.
Many op-amps already have a short term limiting designed into them,
but you don't know this until you check the specs.

If you are not employing a feed back circuit where this idea will be a
solution for you, you could then use an inductor on the output to help
reduce the inrush currents which is a common practice or fix a permanent
series resistor there if the small drop isn't going to effect you much.

http://webpages.charter.net/jamie_5"

Joerg · Apr 7, 2009

panfilero wrote:

On Apr 7, 5:08 pm, Jamie
jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:
panfilero wrote:
Hello,
I'm going to be driving a capcitive load out from a couple opamps,
both the opamps are acting as buffers, and normally my capacitve load
will have a very small current through it... but initially it will
have a current surge going through it as the voltage builds across
it...
My question, my opamps can't handle this (it's about 400mA my opamps
can handle about 60mA) can someone recommend a way of me handling the
current surge without it damaging my opamps? Would an inductor in
series with my load help soften the current surge?
The voltage across my load swings from -5V to +5V so, I need to be
able to handle that current surge regardless of what direction its
going in....
much thanks!
I don't know how you're applying your op-amps to the circuit how ever,
if you're using some kind of feed back to the inputs like (-) for
example, you can use a low value resistor on the output that will be
calculated for the max load, from there, that network point is your
output and also the point where you feed the remaining r value back to
the input.
Many op-amps already have a short term limiting designed into them,
but you don't know this until you check the specs.

If you are not employing a feed back circuit where this idea will be a
solution for you, you could then use an inductor on the output to help
reduce the inrush currents which is a common practice or fix a permanent
series resistor there if the small drop isn't going to effect you much.

http://webpages.charter.net/jamie_5"

Both my opamps are set up with their outputs being fedback into their
inverting inputs.... and on one I am controlling the input voltage at
the non-inverting input (and varying this between 0-10V) and on the
other I have a regulated 5V going into the non inverting input... my
load then goes across the two op-amp buffer's outputs.... and when I
vary my voltage between 0-10V I see -5 to 5V across my load.... I'm
still trying to figure out why I could have a potential issue here
with a capacitive load... it seems most the stuff I've found online,
talk about the capacitive load adding an extra pole to the transfer
funciton... but it looks like this is at a frequency above 1Hz... so I
don't know if this applies to me at a steady DC output...

As Bob wrote, the opamp doesn't have a crystal ball, it doesn't know
that you only want DC. Worst case it barely works, you turn around and
walk away, then it begins to oscillate and all hell breaks loose.

I'm thinking about using the inductor at my opamp outputs to ease up
the current...

Here is how it's done when you have a capacitive load, and they also
explain the theory quite well:

http://www.analog.com/Analog_Root/static/techSupport/designTools/interactiveTools/stability/stability.html

You could, for example, dimension the resistor so that the desired
maximum current results when the opamp does a full swing. Its output
will peg close to the rails. How close would be mentioned in the
sink/source capabilities in the datasheet.

[...]

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

panfilero · Apr 8, 2009

On Apr 7, 5:08 pm, Jamie
<jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:

panfilero wrote:
Hello,

I'm going to be driving a capcitive load out from a couple opamps,
both the opamps are acting as buffers, and normally my capacitve load
will have a very small current through it... but initially it will
have a current surge going through it as the voltage builds across
it...

My question, my opamps can't handle this (it's about 400mA my opamps
can handle about 60mA) can someone recommend a way of me handling the
current surge without it damaging my opamps? Would an inductor in
series with my load help soften the current surge?

The voltage across my load swings from -5V to +5V so, I need to be
able to handle that current surge regardless of what direction its
going in....

much thanks!

I don't know how you're applying your op-amps to the circuit how ever,
if you're using some kind of feed back to the inputs like (-) for
example, you can use a low value resistor on the output that will be
calculated for the max load, from there, that network point is your
output and also the point where you feed the remaining r value back to
the input.
Many op-amps already have a short term limiting designed into them,
but you don't know this until you check the specs.

If you are not employing a feed back circuit where this idea will be a
solution for you, you could then use an inductor on the output to help
reduce the inrush currents which is a common practice or fix a permanent
series resistor there if the small drop isn't going to effect you much.

http://webpages.charter.net/jamie_5"

Both my opamps are set up with their outputs being fedback into their
inverting inputs.... and on one I am controlling the input voltage at
the non-inverting input (and varying this between 0-10V) and on the
other I have a regulated 5V going into the non inverting input... my
load then goes across the two op-amp buffer's outputs.... and when I
vary my voltage between 0-10V I see -5 to 5V across my load.... I'm
still trying to figure out why I could have a potential issue here
with a capacitive load... it seems most the stuff I've found online,
talk about the capacitive load adding an extra pole to the transfer
funciton... but it looks like this is at a frequency above 1Hz... so I
don't know if this applies to me at a steady DC output...

I'm thinking about using the inductor at my opamp outputs to ease up
the current...

thanks for all the comments

Apr 8, 2009

On Apr 7, 7:51 pm, Joerg <notthisjoerg...@removethispacbell.net>
wrote:

panfilero wrote:
On Apr 7, 5:08 pm, Jamie
jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:
panfilero wrote:
Hello,
I'm going to be driving a capcitive load out from a couple opamps,
both the opamps are acting as buffers, and normally my capacitve load
will have a very small current through it... but initially it will
have a current surge going through it as the voltage builds across
it...
My question, my opamps can't handle this (it's about 400mA my opamps
can handle about 60mA) can someone recommend a way of me handling the
current surge without it damaging my opamps? Would an inductor in
series with my load help soften the current surge?
The voltage across my load swings from -5V to +5V so, I need to be
able to handle that current surge regardless of what direction its
going in....
much thanks!
I don't know how you're applying your op-amps to the circuit how ever,
if you're using some kind of feed back to the inputs like (-) for
example, you can use a low value resistor on the output that will be
calculated for the max load, from there, that network point is your
output and also the point where you feed the remaining r value back to
the input.
Many op-amps already have a short term limiting designed into them,
but you don't know this until you check the specs.

If you are not employing a feed back circuit where this idea will be a
solution for you, you could then use an inductor on the output to help
reduce the inrush currents which is a common practice or fix a permanent
series resistor there if the small drop isn't going to effect you much..

http://webpages.charter.net/jamie_5"

Both my opamps are set up with their outputs being fedback into their
inverting inputs.... and on one I am controlling the input voltage at
the non-inverting input (and varying this between 0-10V) and on the
other I have a regulated 5V going into the non inverting input... my
load then goes across the two op-amp buffer's outputs.... and when I
vary my voltage between 0-10V I see -5 to 5V across my load.... I'm
still trying to figure out why I could have a potential issue here
with a capacitive load... it seems most the stuff I've found online,
talk about the capacitive load adding an extra pole to the transfer
funciton... but it looks like this is at a frequency above 1Hz... so I
don't know if this applies to me at a steady DC output...

As Bob wrote, the opamp doesn't have a crystal ball, it doesn't know
that you only want DC. Worst case it barely works, you turn around and
walk away, then it begins to oscillate and all hell breaks loose.

I'm thinking about using the inductor at my opamp outputs to ease up
the current...

Here is how it's done when you have a capacitive load, and they also
explain the theory quite well:

http://www.analog.com/Analog_Root/static/techSupport/designTools/inte...

You could, for example, dimension the resistor so that the desired
maximum current results when the opamp does a full swing. Its output
will peg close to the rails. How close would be mentioned in the
sink/source capabilities in the datasheet.

[...]

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.- Hide quoted text -

- Show quoted text -

Thanks for the link Joerg. I never thought of diddling with the noise
gain of the opamp to help fix the oscillation problem.

George Herold

Joerg · Apr 8, 2009

ggherold@gmail.com wrote:

On Apr 7, 7:51 pm, Joerg <notthisjoerg...@removethispacbell.net
wrote:
panfilero wrote:
On Apr 7, 5:08 pm, Jamie
jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:
panfilero wrote:
Hello,
I'm going to be driving a capcitive load out from a couple opamps,
both the opamps are acting as buffers, and normally my capacitve load
will have a very small current through it... but initially it will
have a current surge going through it as the voltage builds across
it...
My question, my opamps can't handle this (it's about 400mA my opamps
can handle about 60mA) can someone recommend a way of me handling the
current surge without it damaging my opamps? Would an inductor in
series with my load help soften the current surge?
The voltage across my load swings from -5V to +5V so, I need to be
able to handle that current surge regardless of what direction its
going in....
much thanks!
I don't know how you're applying your op-amps to the circuit how ever,
if you're using some kind of feed back to the inputs like (-) for
example, you can use a low value resistor on the output that will be
calculated for the max load, from there, that network point is your
output and also the point where you feed the remaining r value back to
the input.
Many op-amps already have a short term limiting designed into them,
but you don't know this until you check the specs.
If you are not employing a feed back circuit where this idea will be a
solution for you, you could then use an inductor on the output to help
reduce the inrush currents which is a common practice or fix a permanent
series resistor there if the small drop isn't going to effect you much.
http://webpages.charter.net/jamie_5"
Both my opamps are set up with their outputs being fedback into their
inverting inputs.... and on one I am controlling the input voltage at
the non-inverting input (and varying this between 0-10V) and on the
other I have a regulated 5V going into the non inverting input... my
load then goes across the two op-amp buffer's outputs.... and when I
vary my voltage between 0-10V I see -5 to 5V across my load.... I'm
still trying to figure out why I could have a potential issue here
with a capacitive load... it seems most the stuff I've found online,
talk about the capacitive load adding an extra pole to the transfer
funciton... but it looks like this is at a frequency above 1Hz... so I
don't know if this applies to me at a steady DC output...
As Bob wrote, the opamp doesn't have a crystal ball, it doesn't know
that you only want DC. Worst case it barely works, you turn around and
walk away, then it begins to oscillate and all hell breaks loose.

I'm thinking about using the inductor at my opamp outputs to ease up
the current...
Here is how it's done when you have a capacitive load, and they also
explain the theory quite well:

http://www.analog.com/Analog_Root/static/techSupport/designTools/inte...

You could, for example, dimension the resistor so that the desired
maximum current results when the opamp does a full swing. Its output
will peg close to the rails. How close would be mentioned in the
sink/source capabilities in the datasheet.

[...]

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.- Hide quoted text -

- Show quoted text -

Thanks for the link Joerg. I never thought of diddling with the noise
gain of the opamp to help fix the oscillation problem.

But brace yourself for design reviews. Some of those tricks are met with
disgust there ;-)

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

panfilero · Apr 9, 2009

On Apr 8, 4:18 pm, Joerg <notthisjoerg...@removethispacbell.net>
wrote:

ggher...@gmail.com wrote:
On Apr 7, 7:51 pm, Joerg <notthisjoerg...@removethispacbell.net
wrote:
panfilero wrote:
On Apr 7, 5:08 pm, Jamie
jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:
panfilero wrote:
Hello,
I'm going to be driving a capcitive load out from a couple opamps,
both the opamps are acting as buffers, and normally my capacitve load
will have a very small current through it... but initially it will
have a current surge going through it as the voltage builds across
it...
My question, my opamps can't handle this (it's about 400mA my opamps
can handle about 60mA) can someone recommend a way of me handling the
current surge without it damaging my opamps? Would an inductor in
series with my load help soften the current surge?
The voltage across my load swings from -5V to +5V so, I need to be
able to handle that current surge regardless of what direction its
going in....
much thanks!
I don't know how you're applying your op-amps to the circuit how ever,
if you're using some kind of feed back to the inputs like (-) for
example, you can use a low value resistor on the output that will be
calculated for the max load, from there, that network point is your
output and also the point where you feed the remaining r value back to
the input.
Many op-amps already have a short term limiting designed into them,
but you don't know this until you check the specs.
If you are not employing a feed back circuit where this idea will be a
solution for you, you could then use an inductor on the output to help
reduce the inrush currents which is a common practice or fix a permanent
series resistor there if the small drop isn't going to effect you much.
http://webpages.charter.net/jamie_5"
Both my opamps are set up with their outputs being fedback into their
inverting inputs.... and on one I am controlling the input voltage at
the non-inverting input (and varying this between 0-10V) and on the
other I have a regulated 5V going into the non inverting input... my
load then goes across the two op-amp buffer's outputs.... and when I
vary my voltage between 0-10V I see -5 to 5V across my load.... I'm
still trying to figure out why I could have a potential issue here
with a capacitive load... it seems most the stuff I've found online,
talk about the capacitive load adding an extra pole to the transfer
funciton... but it looks like this is at a frequency above 1Hz... so I
don't know if this applies to me at a steady DC output...
As Bob wrote, the opamp doesn't have a crystal ball, it doesn't know
that you only want DC. Worst case it barely works, you turn around and
walk away, then it begins to oscillate and all hell breaks loose.

I'm thinking about using the inductor at my opamp outputs to ease up
the current...
Here is how it's done when you have a capacitive load, and they also
explain the theory quite well:

http://www.analog.com/Analog_Root/static/techSupport/designTools/inte....

You could, for example, dimension the resistor so that the desired
maximum current results when the opamp does a full swing. Its output
will peg close to the rails. How close would be mentioned in the
sink/source capabilities in the datasheet.

[...]

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.- Hide quoted text -

- Show quoted text -

Thanks for the link Joerg. I never thought of diddling with the noise
gain of the opamp to help fix the oscillation problem.

But brace yourself for design reviews. Some of those tricks are met with
disgust there ;-)

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

ok.... the potential to oscillate really took me off on a tangent, but
while my original question and problem was about what to do about the
current surge from the capacitive load... it seems that adding those
resistors at the outputs of my opamps may get rid of my current surge?
by limiting the current to the voltage out of the opamp divded by my
resistor... thus allowing me to control the amount of current by
selecting the right resistor?

Joerg · Apr 9, 2009

panfilero wrote:

On Apr 8, 4:18 pm, Joerg <notthisjoerg...@removethispacbell.net
wrote:
ggher...@gmail.com wrote:
On Apr 7, 7:51 pm, Joerg <notthisjoerg...@removethispacbell.net
wrote:
panfilero wrote:
On Apr 7, 5:08 pm, Jamie
jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:
panfilero wrote:
Hello,
I'm going to be driving a capcitive load out from a couple opamps,
both the opamps are acting as buffers, and normally my capacitve load
will have a very small current through it... but initially it will
have a current surge going through it as the voltage builds across
it...
My question, my opamps can't handle this (it's about 400mA my opamps
can handle about 60mA) can someone recommend a way of me handling the
current surge without it damaging my opamps? Would an inductor in
series with my load help soften the current surge?
The voltage across my load swings from -5V to +5V so, I need to be
able to handle that current surge regardless of what direction its
going in....
much thanks!
I don't know how you're applying your op-amps to the circuit how ever,
if you're using some kind of feed back to the inputs like (-) for
example, you can use a low value resistor on the output that will be
calculated for the max load, from there, that network point is your
output and also the point where you feed the remaining r value back to
the input.
Many op-amps already have a short term limiting designed into them,
but you don't know this until you check the specs.
If you are not employing a feed back circuit where this idea will be a
solution for you, you could then use an inductor on the output to help
reduce the inrush currents which is a common practice or fix a permanent
series resistor there if the small drop isn't going to effect you much.
http://webpages.charter.net/jamie_5"
Both my opamps are set up with their outputs being fedback into their
inverting inputs.... and on one I am controlling the input voltage at
the non-inverting input (and varying this between 0-10V) and on the
other I have a regulated 5V going into the non inverting input... my
load then goes across the two op-amp buffer's outputs.... and when I
vary my voltage between 0-10V I see -5 to 5V across my load.... I'm
still trying to figure out why I could have a potential issue here
with a capacitive load... it seems most the stuff I've found online,
talk about the capacitive load adding an extra pole to the transfer
funciton... but it looks like this is at a frequency above 1Hz... so I
don't know if this applies to me at a steady DC output...
As Bob wrote, the opamp doesn't have a crystal ball, it doesn't know
that you only want DC. Worst case it barely works, you turn around and
walk away, then it begins to oscillate and all hell breaks loose.
I'm thinking about using the inductor at my opamp outputs to ease up
the current...
Here is how it's done when you have a capacitive load, and they also
explain the theory quite well:
http://www.analog.com/Analog_Root/static/techSupport/designTools/inte...
You could, for example, dimension the resistor so that the desired
maximum current results when the opamp does a full swing. Its output
will peg close to the rails. How close would be mentioned in the
sink/source capabilities in the datasheet.
[...]
--
Regards, Joerg
http://www.analogconsultants.com/
"gmail" domain blocked because of excessive spam.
Use another domain or send PM.- Hide quoted text -
- Show quoted text -
Thanks for the link Joerg. I never thought of diddling with the noise
gain of the opamp to help fix the oscillation problem.
But brace yourself for design reviews. Some of those tricks are met with
disgust there ;-)

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

ok.... the potential to oscillate really took me off on a tangent, but
while my original question and problem was about what to do about the
current surge from the capacitive load... it seems that adding those
resistors at the outputs of my opamps may get rid of my current surge?
by limiting the current to the voltage out of the opamp divded by my
resistor... thus allowing me to control the amount of current by
selecting the right resistor?

Yes. The opamp tries to maintain regulation but it would swing farther
than your desired output voltage. The worst case current would occur
when the capacitor is at its most negative voltage and the opamp swings
full positive (close to the rail) and vice versa. The maximum voltage
differential divided by the resistance would be the maximum current.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

how can I deal with a current spike?

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