A TL431-Based Turn On Delay With Mute and Power Fail Detection

[mediatechnology]

A TL431-Based Turn On Delay With Mute and Power Fail Detection

This simple little circuit popped into my head the other day and I was anxious to try it.


A TL431-Based Turn On Delay With Mute and Power Fail Detection

The TL431, configured as a comparator, prevents turn-on thumps and pops.

Ideally the supply must be at the proper voltage and stable before the output is unmuted.
Relays typically have low drop-out voltages and can't reliably be used for threshold detection.
Connecting the coil directly to the supply and without delay may allow turn-on/turn-off pops and clicks to pass through.

A relay turned on at a coil voltage significantly lower than its rated voltage may also distort due to lack of snap action.
The TL431 provides crisp turn on and off.
The threshold voltage, set by R1 and R2, is 27V or +/-13.5V.
If R1 is 90KΩ the threshold voltage for power good/fail is about 25V or +/-12.5V

Even though the supplies may have stabilized near the operating voltage a delay is needed to make sure upstream coupling capacitors have had time to discharge their thumps.
C1 introduces a turn-on delay.

The Vce saturation of the TL431 is about 2V.
D1 adds an additional 3.9V voltage drop to provide 24V to the relay coil.
The TL431 has a maximum cathode current of 100 mA and can drive multiple relay coils in parallel.

An NPN transistor CE connected across R2 (along with a PNP level shift) could be used to provide a logic level external Mute function.

Here's the results.
The blue trace is a THAT1646 output driving a single-ended load at turn on.
The peak-to-peak level is about 4.5V.
The 4.5V p-p is not necessarily indicative of the 1646s turn-on behavior: Power supply start-up also comes into play.
The red trace is the output.


A TL431-Based Turn On Delay With Mute and Power Fail Detection

A good application for this circuit would be for a Monitor Controller output.

Another application for the TL431 can be found here:
A Super-Simple LED Peak Flasher Audio Overload Clipping Indicator Using the TL431 viewtopic.php?f=6&t=960

Rod Elliot has an excellent article about muting circuits:
"Muting Circuits for Audio" https://sound-au.com/articles/muting.html

[mediatechnology]

Before I set this circuit aside I wanted to post some oscillograms of the turn-on and turn-off behavior.

The blue trace is the +15V rail.
The red trace is the relay coil voltage which is active low.
The trigger level was set to +13.5V.
The power supply was switched using its AC On/Off switch.

This is the turn-on behavior. There's about a 240 ms delay until the relay turns on.


TL431 Mute Turn On Delay Blue Supply Red Relay Coil

Turn-off behavior. The relay begins to turn off about 10 ms after the supply rail drops through +13.5V.


TL431 Turn-Off Delay Blue Supply Red Relay Coil

[mediatechnology]

I updated the TL431-based turn-on delay to decrease the turn-off time.



Oscillograms of the galvanically-isolated version.

Blue Trace is +15V rail.
Red trace is relay coil, active low.
The trigger is set for 13V.
The On delay is about 260 ms.





Blue Trace is +15V rail.
Red trace is relay coil, active low.
The trigger is set for 13V.
The Off delay is about 8 ms.