LM3914 advice / help

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dacflyer
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LM3914 advice / help

Post by dacflyer » Sat Apr 02, 2005 10:42 pm

i am tinkering with a project here i had to put on the back burner for a while because of parts and $$ lacking..anyway,i am back to the front burner now :D
i am making this led temperature thats on the National Parts website about the LM34
i think its on page 9.
this project uses 2 LM3914's and a LM34. It is a 20 led bar graph temp display
my problem is.. the 1st stage LM3916 goes up to the 10 led fine,,but as it goes to the 11th led. the 10th one remains lit and so on even up to the 20th step. i thought something was wrong, so i swapped the chips, it still does the same, is there an error in the diagram? is it supposed to be this way? i thought if they are daisy chained the whole thing should be linear right..or?
smooth count from 1 to 20, and the 10led not remain let,,can someone advise.. hope i am understandable.. if not i will try to clarafy.
thanx

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sofaspud
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Re: LM3914 advice / help

Post by sofaspud » Sat Apr 02, 2005 10:54 pm

Is one chip configured for dot display and one configured for bar display?
Can you post a url for the schematic? Is it in NatSemi's LM34 datasheet?<p>[ April 02, 2005: Message edited by: sofaspud ]</p>

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jollyrgr
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Re: LM3914 advice / help

Post by jollyrgr » Sat Apr 02, 2005 11:10 pm

I think I know what is causing your error. Do you have a 20k resistor in parallel with the LED connected to pin 11 of the first LM391x chip? See the diagram on page 9 of the document located here:
http://www.national.com/ds/LM/LM3914.pdf<p>The feedback resistor is used to mute the "tenth" LED of the first device a two cascade setup. It connects between pin 11 of the first chip and the Vcc line of the LED power supply. In simplistic terms it is a 20K resistor in parallel with the LED on pin 11 of the first chip. The text explaining all of this is on page 9 of the above referenced data sheet.<p>Hope this helps.
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Re: LM3914 advice / help

Post by viveguy » Sun Apr 03, 2005 7:23 am

You don't mention the rectifier configuration that you are using. You are going to have a voltage multiplication of 1.414 for a full wave bridge. You can get around the voltage differential problem by putting a zener on the reference (ground) pin. But, that will raise your minimum output voltage.<p>I suggest that you go to www.national.com for application information.

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Re: LM3914 advice / help

Post by viveguy » Sun Apr 03, 2005 7:26 am

Oops, sorry. I posted to the wrong topic. Another victim of 'sometimers disease'!

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Re: LM3914 advice / help

Post by dacflyer » Sun Apr 03, 2005 10:09 am

sofaspud >> yes it is on page 9 on LM34 data sheet.<p>Jollyrancher >> not too familiar with the term mute except in audio..
but if i understand you right, if i remove this 20k resistor it should remove the problem?
i will try it.. i didn't understand why it was being used in the 1st place..thanx.
correct me if i am wrong..

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Re: LM3914 advice / help

Post by dacflyer » Sun Apr 03, 2005 10:47 am

jolly rodger >> sorry mis-spelled your name..<p>but i just wanted to reply again, i found my problem,, i had the 20k in place , but retracing my layout, i forgot to connect pin 9 of 1st IC to the pin one of 2nd ic , this fixed the problem
thanks again guys..<p>this circuit sure uses a lot of critical values,little hard to get it calibrated,,my meter does not goto thousandths. (2.725volts) i only can get like 2.72 volts that will have to be close enough for me :(
but its works nontheless,,,

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Re: LM3914 advice / help

Post by dacflyer » Sun Apr 03, 2005 11:51 am

ok one last thing,,in part of this calibrating
it says..
—Trim RA for VA = 0.085V + 40 mV/°F x AMBIENT
—Example, VA = 3.285V at 80°F<p>can someone explain how this is figgured out in dummy terms? i know RA is the potientometer
and VA is supposed to be the reading i get from calibration...but i cannot get the math part.
i think the "/" means something?
i am clueless when it comes to uncommon math symbols. :(

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sofaspud
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Re: LM3914 advice / help

Post by sofaspud » Sun Apr 03, 2005 12:17 pm

The "/" just reads as "per" in that formula, i.e. "40mV per degree F of ambient temperature."
You'll need another thermometer for the calibration. Maybe a thermostat on the wall, or perhaps taking a "medicine chest thermometer" and setting it near your bench for a few minutes.
Get the temperature and multiply it by 0.04, then add 0.085. That will be the VA figure.

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Re: LM3914 advice / help

Post by dacflyer » Tue Apr 05, 2005 5:08 am

great thanks sofaspud..much easier to understand.

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