# LM3914 unexpected results, help?

Discussion in 'The Projects Forum' started by bummer101, May 28, 2012.

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1. ### bummer101 Thread Starter New Member

May 28, 2012
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0
Hi,

I am working on my first 3914 circuit to construct a fuel-gauge. I have done my calculations based on datasheets and on other peoples projects and I thought everything should run smoothly but I had some strange results.

I've searched the forums for answers to my questions to no avail.

Can you tell me which voltages this setup is supposed to measure? How many volts to light 1st LED and how many volts to light 10th LED? And how many mWs should each LED be getting?
I'm curious to see if my calculations are correct but just doesn't work or if the circuit is doing what it should and the fault lies with me.

Help and answers are greatly appreciated!

Edit: I'll post my actual readings on the circuit after someone can say what it SHOULD read.

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2. ### MrChips Moderator

Oct 2, 2009
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Why don't you do it the other way around?
You provide your readings and we will tell you if this is correct or not.

3. ### absf Senior Member

Dec 29, 2010
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I attached the block diagram for reference. According to the datasheet, The reference voltage out is

1.25(1+R2/R1) = 1.25(1+4.8k/2.5k) = 3.65V

So the highest LED should be around 3.65V. Is that right? As for the lowest LED, I'll let you calculate based on the block diagram.

Allen

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4. ### Ron H AAC Fanatic!

Apr 14, 2005
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The adjust pin current is typically 75uA, which flows through R2. This adds 75uA*4.8k=360mV to the output, for a total of about 4V. The reference divider can be anywhere between 8kΩ and 17kΩ. Taking 4V as Rhi, the voltage at Rlo can be anywhere between 348mV and 674mV, but the typical value will be 558mV. Of course, it could be even outside those limits, because we used a typical value for the reference output. In actuality, the internal reference can be between 1.2V and 1.34V, and the adj pin current can be as high as 120uA, with no guaranteed lower limit.

5. ### djsfantasi AAC Fanatic!

Apr 11, 2010
3,519
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Fuel gauge for what?

6. ### bummer101 Thread Starter New Member

May 28, 2012
2
0
Thank you! 3.65 was exactly was I was aiming for! The attached block diagram was way to grainy, couldn't read a letter but the datasheet has the same one.

I was aiming for 1st LED lighting up at ~0.51V and the 10th one lighting up at 3.65V. And 5mW for each LED so the total should be 0.5W power dissipation, well within the 3914's range so it wouldn't get so hot when all LEDs are lit.

My actual readings are 1st LED lights up at around 0.8V and the 10th one lights up at around 3.9V and the LEDs seem alot brighter than if they were running at 5mW, plus the IC gets really hot. I even tried with a heatsink, it still gets hot.

Well, this seems like the answer to my confusions. Strange that I haven't read that before? Must have missed something crucial. How should I apply this in a formula to calculate my resistors?

Should I measure up the inner resistance of pin 4? To which pin to I measure? Should I measure the voltage between pin 7 and ground? Or can anyone give me the values for my resistors so that the LEDs get 5mW each and measures between 0.51V and 3.65V?

It is for a Volvo 740 -88. I have measured the tank sensor and it gives 0.51V when empty and about 3.77V when full, and I lowered the "full-voltage" a bit so that the 10th LED is lit a while longer rather than only when the tank is superfull.

Apr 16, 2010
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8. ### Wendy Moderator

Mar 24, 2008
21,351
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The owners of All About Circuits has elected not to host discussions of automotive electrical system modifications/enhancements due to safety concerns, the potential of legal ramifications and the possible circumvention of vehicle regulations at the state and federal level.

This thread is against the AAC forum rules, Chapter 6, as seen here:

Automotive modifications of any kind are strictly forbidden. Therefore, this thread will be closed.

Please try to understand the reasons behind this action, and feel free to browse and use the forums.

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