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How to find if a simple regulator is oscillating

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tip35

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I am working at a project which involves the TL431 shunt regulator. The schematic of the regulator is in the attachment.
I want to see on the oscilloscope if the power supply is oscillating.
Are there any standard procedures when viewing the output of the regulator on the oscilloscope ? I watched some videos on the internet before posting, so I know how to set the oscilloscope ...

More exactly, I am interested in how an oscillation looks on the oscilloscope screen (amplitude, frequency) ?
 

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  • power supply tl431.PNG
    power supply tl431.PNG
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Hi,

I don't expect this circuit to oscillate.

But to stabilize the output voltage I recommend to use a ceramics capacitor at the output.
Depending on load current (waveform): 100nF ... 10uF
.... from a personal taste
.... but the datasheet says especially this is the most critical capacitance range.
--> thus you need to design it carefully.

"Oscillation" occurs when the circuit is "not stable".
Thus do a datasheet search for "oscillation", "stable", "stability" ... and so on.
The dataseet is full of important informations ... and links (to application notes..)

The device is considered to be stable without capacitor. But this does not mean that the output voltage is "clean".

To detect oscillation with a scope:
* use "AC" mode
* adjust the voltage to see some noise
* adjust the timing to microseconds

Oscillation waveform may be somewhere inbetween square wave and sine.

Ringing waveform is sine, but with decreasing amplitude.
It is considered as "stable" but at the edge to "unstable".
The faster the decay the more stable.

To check for stability you should cause some jump in the load current ... this causes some ringing excitation.
Best is a square wave current. Focus on the time when the current is released.

Klaus
 
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    tip35

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Same problem as with many linear voltage regulators. TL431 can oscillate if bypassed with too low ESR capacitor. A lossy electrolytic capacitor will usually suffice to stabilize the circuit. Or in the present case a low ESR capacitor with larger capacitance, e.g. 10 µF ceramics.

Check the appended Ltspice simulation.

Without additional load capacitance, your circuit circuit won't oscillate.
 

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  • tl431 instable.zip
    762 bytes · Views: 76

Besides instability I wonder about its general configuration.

The circuit is set for 5Vout but with 150ohms in R9 it can only tolerate 8Vin (at C1) before risk of hitting the 20mA limit TL431 limit (at no load anyway)

So C1 needs to remain between 5 and 8 volts for the circuit to work properly.... a narrow range unless that AC input is very well known (and/or the load).
 

TL431 rated current is 100 mA according to TI datasheet. It's achievable at least with the "bigger" packages (DIP, SO8, SOT89) at 5V.
 

Ok fair enough. I'm probably recalling the small lower voltage variants I typically use.
 

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