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'[EE] TL431 as unintended oscillator'
2017\04\28@193451 by Bob Blick

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I am attaching a portion of a circuit that forms a battery indicator. It uses a TL431 as a YES/NO battery indicator activating an LED. It does not oscillate, but it does have a bit of gain to any low-to-medium frequency ripple on the power line. Should I go to the extra expense of adding a small capacitor from reference terminal to cathode? Adding one definitely shuts it up. Perhaps I should try to squeeze room for one and leave it unpopulated in the first board run.

Thanks in advance for any feedback,

Bob

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2017\04\28@220833 by RussellMc

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On 29 April 2017 at 11:34, Bob Blick <spam_OUTbobblickTakeThisOuTspamoutlook.com> wrote:

> I am attaching a portion of a circuit that forms a battery indicator. It
> uses a TL431 as a YES/NO battery indicator activating an LED. It does not
> oscillate, but it does have a bit of gain to any low-to-medium frequency
> ripple on the power line. Should I go to the extra expense of adding a
> small capacitor from reference terminal to cathode? Adding one definitely
> shuts it up. Perhaps I should try to squeeze room for one and leave it
> unpopulated in the first board run.
>

​They can definitely be made to oscillate (as you know) but I've not had
them do ​so in practice.

In your circuit, if the input was "stiff" you would not get oscillation as
there is then no feedback path.
To oscillate the LED drain has to drop the supply enough to take Vref
across the thrteshold again.
Adding a small amount of hystersis would make sure of the result.

Adding a cap to Vref can help BUT can also enhance ods of oscillation as if
I_LED_on drops supply significantly the cap will hold Vrf hi for about R3.C
and then lower Vref enough to turn LED off. ie it lowers Fosc. This can be
a "feature"  with flashing indicator at critical point :-).
This could be induced if R5 not used (see below) by adding a small R on Vin
side of 1k2 and taking ref divider to it, so I_LED_on DOES drop ref enough
to cause oscillation :-).

Vin high - LED on
Vin marginal - LED flashes
Vin low - LED off


Other:

I'm not sure why R5 is present - perhaps to ensure leakage current does not
make the LED glimmer - but TL431 off current is extremely low.
If not needed it would make room for a capacitor elsewhere.


R
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2017\04\29@072346 by Clint Jay

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A tidbit of trivia, I have in my time repaired thousands of SMPSUs and they
almost all had a TL431. My diagnostic tool of choice was/is the component
tester on a Hameg 'scope and I can instantly recognise a fault TL because a
working one produces a rather pretty junction with an umbrella above (or
below depending on probe polarity), I believe this is because they
oscillate when a fairly slow AC signal is applied

On 29 Apr 2017 12:35 am, "Bob Blick" <.....bobblickKILLspamspam@spam@outlook.com> wrote:

I am attaching a portion of a circuit that forms a battery indicator. It
uses a TL431 as a YES/NO battery indicator activating an LED. It does not
oscillate, but it does have a bit of gain to any low-to-medium frequency
ripple on the power line. Should I go to the extra expense of adding a
small capacitor from reference terminal to cathode? Adding one definitely
shuts it up. Perhaps I should try to squeeze room for one and leave it
unpopulated in the first board run.

Thanks in advance for any feedback,

Bob
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2017\04\29@102708 by RussellMc

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On 29 April 2017 at 23:23, Clint Jay <cjaysharpspamKILLspamgmail.com> wrote:

> A tidbit of trivia, I have in my time repaired thousands of SMPSUs and they
> almost all had a TL431. My diagnostic tool of choice was/is the component
> tester on a Hameg 'scope and I can instantly recognise a fault TL because a
> working one produces a rather pretty junction with an umbrella above (or
> below depending on probe polarity), I believe this is because they
> oscillate when a fairly slow AC signal is applied
>
> ​It seems likely that the oscillation you see is an intended property of
the overall SMPS circuit rather than the TL431 (although there is no doubt
that it could add its own magic as well.)

A typical volatage controlled SMPS which is "in control" hunts perpetually
around the target voltage unless extra effort has been made in the design
to not do so.It would be possible to eg try and hold the power switch at a
duty cycle that produced a Vout eg just above the TL431 switch point for
curremt load and Vin, but this then loses one direction of control - the
added control loop has to ensure Vout does not wander high by using only
input side information. While this would be possible I can see no point in
doing so in most cases - so that the TL431 will then be forced to oscillate
at a rate set by hysteresis, load and output reservoir capacity (at least
:-) ).


    Russell
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2017\04\29@131328 by Clint Jay

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Nope, I understand what you're saying but they exhibit the same behaviour
out of circuit.

On 29 Apr 2017 3:27 pm, "RussellMc" <.....apptechnzKILLspamspam.....gmail.com> wrote:

{Quote hidden}

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'[EE] TL431 as unintended oscillator'
2017\05\02@113754 by Bob Blick
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R5 is indeed there to prevent LED glimmer, although it is also needed to bring the TL431 into voltage spec.

The glimmer is an issue depending on the brand of TL431 used. All the "name brands" consume about 300uA, which is enough to light an LED indoors. The TL431 that I buy for a penny apiece (unisonic and worse) only draw about 60uA, but they won't be used in this application.

I thrashed this circuit a bit and never got it to actually oscillate so I am leaving it as in.

Thanks for your help.

Best regards,
Bob

________________________________________
From: piclist-bouncesspamspam_OUTmit.edu <@spam@piclist-bouncesKILLspamspammit.edu> on behalf of RussellMc
Sent: Friday, April 28, 2017 7:07 PM
To: Microcontroller discussion list - Public.
Subject: Re: [EE] TL431 as unintended oscillator

On 29 April 2017 at 11:34, Bob Blick <KILLspambobblickKILLspamspamoutlook.com> wrote:

> I am attaching a portion of a circuit that forms a battery indicator. It
> uses a TL431 as a YES/NO battery indicator activating an LED. It does not
> oscillate, but it does have a bit of gain to any low-to-medium frequency
> ripple on the power line. Should I go to the extra expense of adding a
> small capacitor from reference terminal to cathode? Adding one definitely
> shuts it up. Perhaps I should try to squeeze room for one and leave it
> unpopulated in the first board run.
>

​They can definitely be made to oscillate (as you know) but I've not had
them do ​so in practice.

In your circuit, if the input was "stiff" you would not get oscillation as
there is then no feedback path.
To oscillate the LED drain has to drop the supply enough to take Vref
across the thrteshold again.
Adding a small amount of hystersis would make sure of the result.

Adding a cap to Vref can help BUT can also enhance ods of oscillation as if
I_LED_on drops supply significantly the cap will hold Vrf hi for about R3.C
and then lower Vref enough to turn LED off. ie it lowers Fosc. This can be
a "feature"  with flashing indicator at critical point :-).
This could be induced if R5 not used (see below) by adding a small R on Vin
side of 1k2 and taking ref divider to it, so I_LED_on DOES drop ref enough
to cause oscillation :-).

Vin high - LED on
Vin marginal - LED flashes
Vin low - LED off


Other:

I'm not sure why R5 is present - perhaps to ensure leakage current does not
make the LED glimmer - but TL431 off current is extremely low.
If not needed it would make room for a capacitor elsewhere.

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2017\05\03@095209 by RussellMc

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On 3 May 2017 at 03:37, Bob Blick <RemoveMEbobblickTakeThisOuTspamoutlook.com> wrote:

> R5 is indeed there to prevent LED glimmer, although it is also needed to
> bring the TL431 into voltage spec.
>

​When the TL431 is off it is subject to full Vin, so I can't see how iR5
affects voltage spec.
(Which is not to say it doesn't, I just don't know why - maybe related to
unseen parts of the circuit.

A TL431 or TLV431 nominally have V_Cathode min = Vref but in practice the
will pull down to about 1 diode drop below that - ie until the internal
ref-Cathodediode becomes forward biased.
TL431 are usually spcd at Vmax of 36V more or less and T:V431 to about half
that.
Some Chinese TLV431 equivalent have a data sheet rating of 30V+ but that
may be due to lax datasheet updating on reuse.

300 uA and 60 uA sound like the ​minimum Cathode current for regulation.
When off Icathode is usually well under 1 uA.
I'd have thoughtthat when properly off glimmer would be negligible.
(Say 1 uA x 2.5V = 2.5 uW or about say 0.5 milli-lumen.
Illuminating say 10mm square = 10^-4 m^2 so about 5 lux +/- N00% :-)
So maybe perhaps visible as a change of tone in dim conditions.
Such calculations are worse than magic or dice throwing in practice :-).


Typical TL431 are about 300 uA as you say. Zetex devices (now Diodes Inc)
are much lower.


​ Russell​
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