LM3914 --help :/

[[email protected]]

Hi all!

Could someone explain me how does design from page 12 of
http://boss.iele.polsl.gliwice.pl/pdf/National_Semiconductor/lm3914.pdf
works.

Why all the LEDs are blinkink while the last one begins to lighth ??

Best regards

DM

 

[petrus bitbyter]

Hi all!

Could someone explain me how does design from page 12 of
http://boss.iele.polsl.gliwice.pl/pdf/National_Semiconductor/lm3914.pdf
works.

Why all the LEDs are blinkink while the last one begins to lighth ??

Best regards

DM

Most likely your circuit starts to oscillate when that last LED comes on.
Carefully decouple the power supply (pins 2 and 3.) Let's say 10uF elco //
100nF ceramic. Make sure the power traces are wide (thick) enough.
Especially the GND trace should have a very low resistance.

petrus bitbyter

 

[[email protected]]

petrus bitbyter napisal(a):

Most likely your circuit starts to oscillate when that last LED comes on.
Carefully decouple the power supply (pins 2 and 3.) Let's say 10uF elco //
100nF ceramic. Make sure the power traces are wide (thick) enough.
Especially the GND trace should have a very low resistance.

petrus bitbyter

You did not understand me. The circuit does oscilate as it should, but
I do not understand the idea of oscilating, I know that whe the last
LED begins to ligth then something is going on with capacitor and ...?
why does it blink ?

 

[John B]

petrus bitbyter napisal(a):




You did not understand me. The circuit does oscilate as it should, but
I do not understand the idea of oscilating, I know that whe the last
LED begins to ligth then something is going on with capacitor and ...?
why does it blink ?

First, you need to look at the block diagram of the chip on page 6 of
the data sheet. You will see that the comparator chain comes out on the
'Rhi' pin. In the circuit on page 12 the 'Rhi' pin is connected to one
end of the capacitor via a 470 ohm resistor and the other end of the
capacitor to the last LED driver pin. The capacitor acts as feedback
and as LED 10 lights it discharges the capacitor and causes the
comparator chain input to be modulated thus turning all the LED's off
again. Then the cycle repeats.

 

[[email protected]]

but exactly how does it modulate the chain inputof comarators, to
disable all leds the + input of comparators should become higher than
-... am I rigth ?

 

[John B]

but exactly how does it modulate the chain inputof comarators, to
disable all leds the + input of comparators should become higher than
-... am I rigth ?

You need to understand how an Eccles-Jordan Multivibrator works, then
all will become clear.

Study this page:

http://www.du.edu/~etuttle/electron/elect36.htm

 

[John Devereux]

but exactly how does it modulate the chain inputof comarators, to
disable all leds the + input of comparators should become higher than
-... am I rigth ?

I looked at the datasheet and came to the same conclusion. I don't
understand how it works, either.

 

[John Devereux]

You need to understand how an Eccles-Jordan Multivibrator works, then
all will become clear.

But it seems to be going the wrong way! When the final led turns on,
the voltage drops. This is coupled through the capacitor to Rhi. This
*reduces* the voltage accross the resistor chain so the indication
will go overrange. I would expect that to turn off all the lights the
indication would need to go to zero instead.

However, looking at the block diagram more closely, it appears that
the "load" on the reference also determines the LED brightness. So
presumably injecting a current into it switches the leds off!

 

[Jim Thompson]

I looked at the datasheet and came to the same conclusion. I don't
understand how it works, either.

The LM3914 schematic is "simplified" ;-)

...Jim Thompson

 

[John B]

On 02/12/2005 the venerable Jim Thompson etched in runes:

..
..

The LM3914 schematic is "simplified" ;-)

...Jim Thompson

Now, how does he know that? My guess is that he had a hand in the
design somewhere.

 

[Jim Thompson]

On 02/12/2005 the venerable Jim Thompson etched in runes:

.
.

Now, how does he know that? My guess is that he had a hand in the
design somewhere.

Nope, wasn't me ;-)

I think it was Leroy Long or <first name lost in my head :> Davis.

...Jim Thompson

 

[[email protected]]

So... which version is the one ? that with Eccles-Jordan Multivibrator
or with the "load" on the reference which determines the LED brightness
?

Best regards
DM

 

[Spehro Pefhany]

But it seems to be going the wrong way! When the final led turns on,
the voltage drops. This is coupled through the capacitor to Rhi. This
*reduces* the voltage accross the resistor chain so the indication
will go overrange. I would expect that to turn off all the lights the
indication would need to go to zero instead.

However, looking at the block diagram more closely, it appears that
the "load" on the reference also determines the LED brightness. So
presumably injecting a current into it switches the leds off!

Yes, there's a 10:1 current mirror thing going on with the comparator
outputs vis-a-vis the reference output current.

But why the 100 ohm resistor?


Best regards,
Spehro Pefhany

 

[John Devereux]

On 02 Dec 2005 16:44:24 +0000, the renowned John Devereux


Yes, there's a 10:1 current mirror thing going on with the comparator
outputs vis-a-vis the reference output current.

But why the 100 ohm resistor?

I guess it drives the capacitor with the full 5V instead of 3V or so?

 

[Spehro Pefhany]

I guess it drives the capacitor with the full 5V instead of 3V or so?

Okay, yes- increases the swing. Could be as low as ~1.6V with an old
GaAs red LED.


Best regards,
Spehro Pefhany

 

[[email protected]]

Spehro Pefhany napisal(a):


so all in all what does make all the leds blinking while the last one
is ligthing ?

David

 

[Bob Monsen]

so all in all what does make all the leds blinking while the last one
is ligthing ?

The output current for the LEDs is a 10x current mirror of the output
of the REF OUT pin. When the signal input goes above the reference
voltage of 1.25, The last LED turns on. Because of C1, this causes MORE
current to be output by the ref out pin, causing more current in the
output of all the LEDs, and making the voltage across C1 drop even more
due to the 100 ohm resistor. Over time, the voltage at the REF OUT pin
climbs back up to 1.25, and the current out of REF OUT diminishes. If you
recall, this current is amplified 10x by the mirror, so this decrease
causes the voltage at pin 10 to start to increase, because the voltage
across the 100 ohm resistor depends on the current. This voltage drop
decreases the current out of REF OUT even more; thus, this positive
feedback turns off the output at pin 10, causing the voltage to spike up
on the REF OUT pin. This turns off the output completely, because now the
ref high pin is > the signal input. While the voltage at the REF OUT node
is higher than the reference, the current out of the ref out pin is
basically 0. This is what shuts off all the LEDs.

The voltage decays for a while through the 1.2k resistor. When it gets
down to the signal level, the LED at pin 10 is turned on again, causing
the CAP to pull the voltage down, restarting the cycle...

The off time depends on the cap and the resistance to ground it drains
through when it spikes, I think. The on time depends on how quickly the
REF OUT pin can charge the 100uF cap.

---
Regards,
Bob Monsen

The question of the ultimate foundations and the ultimate meaning of
mathematics remains open; we do not know in what direction it will find its
final solution or even whether a final objective answer can be expected at
all. "Mathematizing" may well be a creative activity of man, like language
or music, of primary originality, whose historical decisions defy complete
objective rationalization.
- Hermann Weyl in 1944