Question about Operational Amplifiers

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Rho_HV

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What is the typical open loop gain for an op-amp? I am using an op-amp as a switch and need the output to be just about the supply voltage (rail voltage) when input is 100uV or higher. My supply voltage is 24 so that means I need open loop gain to be at least 240000. Is this the typical gain of an op amp? I read the datasheet but did not get a clue.
Link to op-amp datasheet: http://www.analog.com/static/imported-files/data_sheets/AD8226.pdf

I appreciate all comments.

Edit: it says max gain is 1000 but I am no sure if it is referring to open loop gain???
 
Last edited:
120612-1541 EDT

https://www.national.com/ds/LM/LM741.pdf
The large signal voltage gain is given as typical for the 741 at 25 Deg C, +/-15 V supply, +/-10 V output as 200,000.

http://www.analog.com/static/imported-files/data_sheets/OP07.pdf
About 500,000.

https://www.national.com/ds/LM/LM111.pdf

Any of the above can drive another amplifier for more gain. You may want hysteresis. You may need to consider bandwidth and oscillation. And there are many other choices. More clearly define your application. Note input offset voltage can be balanced,

You may need to specify a lot more than what you have.

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An Op-Amp used in a open loop circuit will not be happy! Open loop gain is not a will defined perimeter. The feed-back loop is what makes the op-amp stable. If you need a continuously variable output from about 0V to 24V consider a multi-stage circuit somewhat like a microphone pre-amplifier. If you want an either/or output (0v or 24V) consider a Comparator switching circuit and then an amplifier.
 
What you described sounds more like a comparator.
Do you need analog values or simply a change of states when 100uv is exceeded.
 
Thanks for the comments everyone; it seems like that I really need a comparator. I am going to try both of them (op-amp and comparator) anyways.
 
120613-1032 EDT

LMAO:

You can do what you want with either an op-amp or comparator. You can even use the instrument amplifier you originally referenced.

What is it that you really want to do? What response time? What stability? What hysteresis? Single supply or dual? Common mode voltage relative to supply? Oscillation problems? Etc.?

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gar said:
120613-1032 EDT

LMAO:

You can do what you want with either an op-amp or comparator. You can even use the instrument amplifier you originally referenced.

What is it that you really want to do? What response time? What stability? What hysteresis? Single supply or dual? Common mode voltage relative to supply? Oscillation problems? Etc.?

.
Click to expand...

gar, I am trying to make a current sensing circuit. I am thinking of placing a 0.02OHM current sensing resistor across the input channels of an op-amp/comparator and connect the output to an LED to detect reverse power situations.
 
120613-2204 EDT

LMAO:

Consider a power diode instead of the 0.02 ohm resistor. A PNP transistor with suitable input protection, and a current limiting resistor and LED in the collector.

A 2N4125 requires around 0.75 V base to emitter for collector saturation.

.
 
LMAO said:
gar, I am trying to make a current sensing circuit. I am thinking of placing a 0.02OHM current sensing resistor across the input channels of an op-amp/comparator and connect the output to an LED to detect reverse power situations.
Click to expand...

like something to warn when batteries are being discharged instead of being charged? or PV system supplying to the grid instead of drawing from the grid?
 
hurk27 said:
like something to warn when batteries are being discharged instead of being charged? or PV system supplying to the grid instead of drawing from the grid?
Click to expand...

We supply 24VDC powered consoles and cabinets for drilling applications and would like to know if somehow power supply is placed in the wrong place (upstream to the OCPD).
 
What about a red LED and current-limiter (resistor or Supertex CL2) wired to light if the polarity is backwards, upstream of the crowbar/reverse polarity protection circuit. If you have the headroom, you can always put a bridge rectifier in the front and work from any polarity, but you would get 1.4v drop and the corresponding heating...
/mike
 
LMAO said:
We supply 24VDC powered consoles and cabinets for drilling applications and would like to know if somehow power supply is placed in the wrong place (upstream to the OCPD).
Click to expand...

If you are trying to build a circuit into the power supply to detect this and it is being fed with AC current it's not going to care where in the system it is placed as long as it has the correct voltage and current, I know of no circuit that could be put into the power supply to achieve this, why not just provide a OCPD in the supply cord of the power supply encase this happens?

Who is installing these, and are they qualified? as this is a training/installation problem not a design problem.
 
hurk27 said:
If you are trying to build a circuit into the power supply to detect this and it is being fed with AC current it's not going to care where in the system it is placed as long as it has the correct voltage and current, I know of no circuit that could be put into the power supply to achieve this, why not just provide a OCPD in the supply cord of the power supply encase this happens?

Who is installing these, and are they qualified? as this is a training/installation problem not a design problem.
Click to expand...

Oh I AM going to find a way to detect power flow direction, I promise you!

These are installed in shipyards allover the world. A couple of years ago some idiot in Malaysia decided to connect 120VAC to 24VDC bus resulting in destruction of tens of thousands of dollars of equipment. Then I get kicked over there to figure out what caused this (duh!). Never again I will end up in the middle of nowhere eating noodles for breakfast/lunch/dinner.
This was a different problem (overvoltage) but we'd like our monitoring device to be able to detect both overvoltage and reverse power.
 
120615-1455 EDT

LMAO:

Until you can write a complete set of specifications for what you want your device to do, and the limitations that need to be imposed on the device you will not be able to get successful help.

You now seemed to have introduced some new information that may or may not apply to your present problem.

.
 
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