how can I deal with a current spike?

[panfilero]

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]

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.

 

[panfilero]

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.

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!

 

[Jamie]

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"

 

[panfilero]

   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

 

[[email protected]]

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.

[...]

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- 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

 

[panfilero]

panfilero wrote:
On Apr 7, 5:08 pm, Jamie
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 intothem,
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 ;-)

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?