# In rush chokes on op amp inputs

Discussion in 'General Electronics Discussion' started by Davewalker5, Sep 20, 2014.

1. ### Davewalker5

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Sep 20, 2014
How does this chokes work when there is an in rush current?

Does the chokes/coils delay the in rush current? the chokes/coils soak up and drain the in rush current? to protect the inputs of the op amps from getting damaged?

What other purpose does these chokes/coils and capacitor on the input of the op amp?

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2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
You have a series LCR circuit there. What it does is frequency dependent. It;s hard to talk about inrush current when there's a 22pF capacitor in series.

An inductor presents a high impedance to any signal which would seek to change the current through the circuit, and a low impedance to a constant current.

3. ### KrisBlueNZSadly passed away in 2015

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Nov 28, 2011
15 µH is a pretty low inductance. What's the frequency of the signal the circuit is monitoring? The inductors could just be for RF rejection or to stop the circuit from responding to very short transients or noise bursts.

4. ### Davewalker5

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Sep 20, 2014
The Function generator to the inputs of the chokes are at different test with these frequencys
1.) 0.1hz
2.) 4 hz
3.) 1 hz
4.) .71 hz
5.) 0.5hz
6.) .75 hz
7.) .6hz
8.) 10hz
9.) .1hz
10.) .35 hz
11.) 1.7hz
12.) 1.4hz
13.) 1.0hz
14.) 7.4hz
15.) .16hz

Why are these tests done at weird low hz?

.35hz , .6hz, .75hz, .16hz. these are weird and millihz?

The function generator is acting like servo motor or syncho motor

5. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
At frequencies like those, the inductors will act as shorts and the capacitor as an open circuit. I'E. they will have so little effect you can ignore them.

6. ### Davewalker5

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Sep 20, 2014
What does the inductors and capacitors do for a servo or synchro motors as an input?

When a servo motor or synchro motor is shorted or creates an in rush current what does the inductors and capacitor do?

7. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
The circuit has a high impedance. If you're driving it from a signal generator the inductors and capacitor will do very little.

If you're driving it from a noisy source with high frequency noise, the arrangement may filter it a little. But the exact action depends on the frequency. You could graph the voltage and phase shift across the capacitor with input frequency if you were really keen.

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8. ### Davewalker5

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Sep 20, 2014
servos and synchros have an AC output with a phase angle

Does the AC output has noise and ripple riding on it?

9. ### KrisBlueNZSadly passed away in 2015

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Nov 28, 2011
It could do. That would be the reason for the inductors and the tiny capacitor. They won't have any effect at all on the signals you're trying to measure. I think they're just there to reject RF and other high-frequency noise.