LM3914 --help :/

Hi all!

Could someone explain me how does design from page 12 of

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works.

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

Best regards

DM

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blender
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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 ?

Reply to
blender

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 ?

Reply to
blender

The LM3914 schematic is "simplified" ;-)

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Jim Thompson

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

Reply to
blender

Nope, wasn't me ;-)

I think it was Leroy Long or Davis.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Jim Thompson

Spehro Pefhany napisal(a):

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

David

Reply to
blender

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

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"it\'s the network..."                          "The Journey is the reward"
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Reply to
Spehro Pefhany

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

Best regards, Spehro Pefhany

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"it\'s the network..."                          "The Journey is the reward"
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Reply to
Spehro Pefhany

On 02/12/2005 the venerable snipped-for-privacy@jaskom.pl etched in runes:

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.

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John B
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John B

schreef in bericht news: snipped-for-privacy@g14g2000cwa.googlegroups.com...

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

Reply to
petrus bitbyter

On 02/12/2005 the venerable snipped-for-privacy@jaskom.pl etched in runes:

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

Study this page:

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John B
Reply to
John B

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

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John Devereux
Reply to
John Devereux

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!

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John Devereux
Reply to
John Devereux

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.

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John B
Reply to
John B

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

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John Devereux
Reply to
John Devereux

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.

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  Bob Monsen
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Bob Monsen

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