TL431 versus Zener

[[email protected]]

Aside from the fact that the TL431 is adjustable, does
it have any advantages over a Zener ?

 

[Watson A.Name - Watt Sun, Dark Remover]

Aside from the fact that the TL431 is adjustable, does
it have any advantages over a Zener ?

Well, I believe it has lower resistance, or in other words, the V-I
curve is straighter. So it should give less variation in voltage as
the current changes.

But obviously the 431 costs more and can't handle as much current. So
it depends on what you're using it for. If you're using it for a
reference, then the 431 is a better choice. If you're using it for a
Q & D regulated PS for something like an oscillator, then the zener is
the common choice, especially if you use a zener that's close to 6V,
where the tempco is at its minimum. But today a 78L05 or 78L08 is a
convenient way to get a stable voltage at less than 100 mA, for an
osc, for example.

And now they (Maxim, LTC, etc.) have micropower references that need
only a few tens or hundreds of microamps. If you try to run a zener
at that low a current, you'll be lucky to get a fraction of its rated
V.

I bought a hundred 1N4734A 5.6V zeners on Ebay for $3.50 and a buck
and a quarter shipping. That's a nickel apiece, a lot cheaper than
the 431.


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[Bruce Kizerian]

Zener? Never used it for that.

But it makes a great audio amplifier!

Bought a bunch for pennies apiece on ebay...

See http://www.techlib.com/electronics/audioamps.html

Bruce

www.elmerdude.com

 

[Watson A.Name - Watt Sun, Dark Remover]

Zener? Never used it for that.

But it makes a great audio amplifier!

Well, in this URL below it's acting like an emitter follower. But
that bad thing about it is that it's Class A, i.e. the current drain
from the battery is high, and the same no matter what the speaker
volume is. So it's wasteful of battery power. The first few circuits
in this URL that use complementary output transistors are Class AB or
B, so they take only a few mA idling current when the volume is very
low. A better chip to use is the LM386, which can also be found on
Ebay for cheap. It takes a minimum of parts, and can put our a half
watt I believe, at 9V. But then these are really a radical departure
from a true crystal radio. Even the single chip MK484 radio is
getting a lot more exotic compared to an xtal radio.

I ran a 50 foot (15M) wire out of my bedroom window and connected it
to a crystal radio, and the music was free. But futile, because we
lived a few miles from a 50 kW AM radio transmitter and another 5 kW a
few more miles away. So about the only thing heard during the daytime
was either of those, and usually a mixture of those, since the radio
wasn't that selective. But at night the 50 kW shut down, and the 5 kW
went to 500W, so it was a little bit better. But about the only thing
I heard at night was KFI 640 AM, which is clear channel and can be
heard all over the western U.S. at night.

Bought a bunch for pennies apiece on ebay...

See http://www.techlib.com/electronics/audioamps.html

Bruce

www.elmerdude.com

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[Exador]

Aside from the fact that the TL431 is adjustable, does
it have any advantages over a Zener ?

One advantage of the TL431 over a typical zener is that it is much more precise.
Another is that it requires less current to operate.

It may be slower than a zener, but I'm not sure about that.

Mike

 

[Bruce Kizerian]

Well, in this URL below it's acting like an emitter follower. But
that bad thing about it is that it's Class A, i.e. the current drain
from the battery is high, and the same no matter what the speaker
volume is. So it's wasteful of battery power. The first few circuits
in this URL that use complementary output transistors are Class AB or
B, so they take only a few mA idling current when the volume is very
low. A better chip to use is the LM386, which can also be found on
Ebay for cheap. It takes a minimum of parts, and can put our a half
watt I believe, at 9V. But then these are really a radical departure
from a true crystal radio. Even the single chip MK484 radio is
getting a lot more exotic compared to an xtal radio.

I tested the parts. The LM386 at 46dB gain specified uses about 5mA
quiescent current. The TL431, dependending on how you set it up, uses
more. That is correct. I set it up to use about 7mA quiescent and get
at least 40dB gain plus the advantage in my regen circuit of being a
more efficient rectifier than a germanium diode. The two are about
equal in performance as a small radio amplifier. The TL431 is not
nearly as bad as you make it sound.

The Tl431 is a bit noisier, will drive stereo headphones, but not an 8
ohm load. But as a three terminal device folks are finding it very
easy to solder into a circuit. The kit is designed for kids and they
are having great success. So it not the ultmate answer to every small
audio amplifier question, but it is a very good one.

And I payed less than a nickel each for my TL431 parts.

Bruce
www.elmerdude.com

 

[[email protected]]

Zener? Never used it for that.

But it makes a great audio amplifier!

Bought a bunch for pennies apiece on ebay...

See http://www.techlib.com/electronics/audioamps.html

Bruce

www.elmerdude.com

I may never build another general purpose audio amp
from scratch again. For less than 5 bucks, you can
get a kit of all parts for a LM386 amp or a TDA7052
amp at Marlin Jones - KT 6017 and KT 6027 respectively.
http://www.mpja.com/listitems.asp?dept=68&main=61

I built both, and they work real well.

That said, you've whet my appetite to play with the
TL431 as an audio amp.

 

[[email protected]]

Well, I believe it has lower resistance, or in other words, the V-I
curve is straighter. So it should give less variation in voltage as
the current changes.

But obviously the 431 costs more and can't handle as much current. So
it depends on what you're using it for. If you're using it for a
reference, then the 431 is a better choice. If you're using it for a
Q & D regulated PS for something like an oscillator, then the zener is
the common choice, especially if you use a zener that's close to 6V,
where the tempco is at its minimum. But today a 78L05 or 78L08 is a
convenient way to get a stable voltage at less than 100 mA, for an
osc, for example.

And now they (Maxim, LTC, etc.) have micropower references that need
only a few tens or hundreds of microamps. If you try to run a zener
at that low a current, you'll be lucky to get a fraction of its rated
V.

I bought a hundred 1N4734A 5.6V zeners on Ebay for $3.50 and a buck
and a quarter shipping. That's a nickel apiece, a lot cheaper than
the 431.

Thanks. I fiddled with a test circuit today, and the
TL431 outperformed 2 zeners in series - a 5.1 and a 6.2
by about 50 mV. But I reached the limit of my test
setup, so the ~50 mV isn't a solid figure. The circuit
was simple - the base of a 2N2904 is fed by the zeners
or the TL431 circuit thru 1K to drop a relay out when
voltage is reduced to 12.00. Trouble is, the supply I
used is not that precise. In any event, in numerous
tests I got worst case of 12.03 dropout with the TL431
and 12.08 with the zeners, and the dropout voltage
varied more often with the zeners, too.

 

[Watson A.Name - Watt Sun, Dark Remover]

I tested the parts. The LM386 at 46dB gain specified uses about 5mA
quiescent current. The TL431, dependending on how you set it up, uses
more. That is correct. I set it up to use about 7mA quiescent and get
at least 40dB gain plus the advantage in my regen circuit of being a
more efficient rectifier than a germanium diode. The two are about
equal in performance as a small radio amplifier. The TL431 is not
nearly as bad as you make it sound.

If you're talking about the followup I posted about what the circuits
are and aren't, then I didn't try to make is sound good or bad. I was
telling it like it is. If you want to use a TL431 for an amp, then
it's just like an emitter follower. And it is running Class A, so
there is the same current continuously thru it, no matter what volume.

The LM386 is simple enough, and probably cheaper than the TL431,
except if you buy it at Rat Shack. Of course at least you can get the
LM386 at Rat shack, but not the TL431. But you can also buy two
transistors, a PN2222 and PN2907 and build a Class B complementary
circuit that draws very little power from the battery when the sound
level is real low, yet can put out several tens of milliwatts to the
speaker when the volume is increased. Like you say below, the TL431
can't put out much power.

But here's a circuit that uses a single AA cell, and drives a pair of
stereo earphones. Check it out.
http://www.redcircuits.com/Page38.htm
The one I built draws a half dozen mA from the AA cell, and the
compression doesn't do much, but the volume is adequate. The output
stage is about the same complexity as the TL431. Notice how they
don't use the headphone ground, but drive the two earphone elements in
series. This helps increase the impedance.

The Tl431 is a bit noisier, will drive stereo headphones, but not an 8
ohm load. But as a three terminal device folks are finding it very
easy to solder into a circuit. The kit is designed for kids and they
are having great success. So it not the ultmate answer to every small
audio amplifier question, but it is a very good one.

And I payed less than a nickel each for my TL431 parts.

Bruce
www.elmerdude.com

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###Got a Question about ELECTRONICS? Check HERE First:###
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My email address is whitelisted. *All* email sent to it
goes directly to the trash unless you add NOSPAM in the
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Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
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[Watson A.Name - Watt Sun, Dark Remover]

I may never build another general purpose audio amp
from scratch again. For less than 5 bucks, you can
get a kit of all parts for a LM386 amp or a TDA7052
amp at Marlin Jones - KT 6017 and KT 6027 respectively.
http://www.mpja.com/listitems.asp?dept=68&main=61

I built both, and they work real well.

That said, you've whet my appetite to play with the
TL431 as an audio amp.

For Gp amps, that's true, but for some amps, building your own can
help cut down the quiescent current and match the amp output to higher
speaker impedances such as headphones, which may be 150, 300 or more
ohms.

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###Got a Question about ELECTRONICS? Check HERE First:###
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Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
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[Watson A.Name - Watt Sun, Dark Remover]

Thanks. I fiddled with a test circuit today, and the
TL431 outperformed 2 zeners in series - a 5.1 and a 6.2
by about 50 mV. But I reached the limit of my test
setup, so the ~50 mV isn't a solid figure. The circuit
was simple - the base of a 2N2904 is fed by the zeners
or the TL431 circuit thru 1K to drop a relay out when
voltage is reduced to 12.00. Trouble is, the supply I
used is not that precise. In any event, in numerous
tests I got worst case of 12.03 dropout with the TL431
and 12.08 with the zeners, and the dropout voltage
varied more often with the zeners, too.

Don't be afraid to use the TL431 itself as the comparator. In other
words, connect the relay between the 'cathode' of the Tl431 and the
supply, with a spike clamping diode across the relay coil. Then just
divide the supply voltage down to the 2.5V with two resistors or a
pot, with the wiper connected to the adjust terminal of the TL431.


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goes directly to the trash unless you add NOSPAM in the
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Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
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[[email protected]]

Don't be afraid to use the TL431 itself as the comparator. In other
words, connect the relay between the 'cathode' of the Tl431 and the
supply, with a spike clamping diode across the relay coil. Then just
divide the supply voltage down to the 2.5V with two resistors or a
pot, with the wiper connected to the adjust terminal of the TL431.

Good advice (as usual). I changed relays - the first one drew
about 160 mA, the new one draws about 70 - and fiddled. Your
idea works. But I'm leery of using just a pot - I put a fixed
resistor between the pot and + - I dunno what would happen to
the 431 if the adj terminal was connected directly to +. Anyway,
that worked. The reason I was using the transistor was to
supply the bigger relay, because max for the 431 is 100 mA.

The adjustment on the pot is damn touchy - with the values I
fiddled with, a 1 ohm swing on the pot makes a lot of difference.
(I'm calling 100 mv a lot of difference). I may have to table
further experiments until I can dig up a 20 turn pot at
500 ohms - I can series resistors with it to get it to the
sweet spot. I can also fiddle with the fixed R.

Sigh. I think I'm going to have to build a more precise
power supply, too, in order to get more solid data. I do
not have precise control below 100 mV, and it looks like
I need precision down to 1 mV for these tests. Is there a
simple "add-on" circuit I can use?

 

[[email protected]]

Sigh. I think I'm going to have to build a more precise
power supply, too, in order to get more solid data. I do
not have precise control below 100 mV, and it looks like
I need precision down to 1 mV for these tests. Is there a
simple "add-on" circuit I can use?

Well, I re-thought this - I only need adjustment precision,
not "power supply precision". I don't care about no load
to full load variation for these tests. I watch the meter
while I lower the supply voltage until the relay drops
out. All I have to do is record the last "good" voltage -
the voltage at which the relay is energized. Doesn't matter
what the voltage increases to when the load is removed. To
that end, I performed surgery on the supply today, and added
a 100K resistor and 50 K pot to the adjustment circuit. That
allows me to adjust the supply with a lot more precision.
The modified circuit:
20K
---+---/\/\/\/-+----------------+--- To regulator
| ^ | |
o | | |
Sw / +----+ +-------+
o | |
| v |
+---/\/\/\/\------/\/\/\/\---+
100K 50K

To use it, I crank the 50 K pot for max voltage, then
adjust the 20K pot (coarse adjustment) to about 12.15.
Then, with the 50K pot (fine adjustment) I can crank
the voltage down to ~ 11.90

The effect is like having a fixed resistance in series
with a ~200 ohm pot totaling about 9.2K when set for
12.15 on the coarse pot.

Hopefully, I'll be able to get back to the TL431 vs Zener
experiment tomorrow, now that I can adjust the supply
with better precision.

 

[Watson A.Name - Watt Sun, Dark Remover]

Good advice (as usual). I changed relays - the first one drew
about 160 mA, the new one draws about 70 - and fiddled. Your
idea works. But I'm leery of using just a pot - I put a fixed
resistor between the pot and + - I dunno what would happen to
the 431 if the adj terminal was connected directly to +. Anyway,
that worked. The reason I was using the transistor was to
supply the bigger relay, because max for the 431 is 100 mA.

The adjustment on the pot is damn touchy - with the values I
fiddled with, a 1 ohm swing on the pot makes a lot of difference.
(I'm calling 100 mv a lot of difference). I may have to table
further experiments until I can dig up a 20 turn pot at
500 ohms - I can series resistors with it to get it to the
sweet spot. I can also fiddle with the fixed R.

Sigh. I think I'm going to have to build a more precise
power supply, too, in order to get more solid data. I do
not have precise control below 100 mV, and it looks like
I need precision down to 1 mV for these tests. Is there a
simple "add-on" circuit I can use?

I've bought a couple HP 6214 and 6216 PSes on ebay, and they have a
dual concentric pot with coarse and fine V adj. The coarse is a 5k
pot, and the fine is a 50 ohm pot in series with it. The fine does
almost nothing, unless the situation is like what you're doing. ;-)

Don't forget to include some bypass caps to minimize the noise and
maybe add some delay or hysteresis.

I just realized that you might have the pot on the lower end of the
divider. If so, then you're adjusting only 2.5V, which may cause it
to be real tetchy. Might be better to put the pot in the high side,
with a series resistor.


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Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
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[[email protected]]

I've bought a couple HP 6214 and 6216 PSes on ebay, and they have a
dual concentric pot with coarse and fine V adj. The coarse is a 5k
pot, and the fine is a 50 ohm pot in series with it. The fine does
almost nothing, unless the situation is like what you're doing. ;-)

I guess that's essentially what I've done to my supply, in
terms of function. Your 2 pots are 100:1 - my "effective
pot" gives me about 200 ohms or ~ 50:1 at 12 V (the 20 K
pot is about 10K at 12 volts). Mathematically it's effectively
over 900 ohms in series with 20 K when the 20 K pot is at max.

I was thinking of you when messing with my power supply, and
wondering if those supplies you bought had a fine and coarse
adjustment.

Don't forget to include some bypass caps to minimize the noise and
maybe add some delay or hysteresis.

I just realized that you might have the pot on the lower end of the
divider. If so, then you're adjusting only 2.5V, which may cause it
to be real tetchy. Might be better to put the pot in the high side,
with a series resistor.

Yup - the TL431 circuit uses a pot on the low side - 500 ohm
pot in series with 1K, and the resistor to the high side is
1K. When I adjust to get 12.00 V, the pot reads 208 ohms.
Its a big wirewound pot. I tried the high side originally,
with a different pot and a series resistor - that was before
I modified the supply. I'll have to try that again. But
I get very good results out of the TL431 circuit. I can
set it right on the gnat's ass. I haven't gotten near as
good results with the zener - but I have more tests to do.

I have to use the xsistor with the zener, but the 431 can
drive a relay directly. However, I'm going to test it
again with the transistor circuit and original relay so that
I can switch between the zener and the 431 to get a more valid
comparison. That's the way I had it originally, and there
was not as much difference between the two devices
that way as there was in today's test. Actually, I couldn't
get the zener to perform as well as it had in the past
today using an npn. Gonna go back to the original
xsistor & circuit.

The original circuit that got me wondering about this
is long since designed - I'm having too much fun with
the experiments to finish that project now. Besides,
that project is now a matter of designing the physical
layout, and I don't enjoy doing that.