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I'm trying to study an electronic schema based on a LM3914 but I can not find the same threshold voltage, can you help me?

enter image description here Source of the schematic

Components :
--------------
IC1=LM3914
D1=1N4007 or equivalent
LD1-LD10=LED
R1=22K, 1/4W
R2=3K9, 1/4W
R3=1K, 1/4W
R4=680 ohm, 1/4W
R5=18K , 1/4W
R6=1K8, 1/4W
C1, C2=10uF/63V

Here is my approach

DETERMINING SIG_IN (Pin5) VOLTAGE
R1 et R2 is a voltage divider so VSIG_IN depend of VBAT (Car Battery)
I substract the 1N4007 forward voltage (1V) I found from the 1N4007 datasheet before applying voltage divider coefficient :
VSIG_IN = (VBAT - 1) * (R2 / (R1 + R2))
VSIG_IN = (VBAT - 1) * (3900 / (22000 + 3900))
VSIG_IN = (VBAT - 1) * 0.150

DETERMINING RHI (Pin6) VOLTAGE
Voltage on RHI is the voltage across R3 plus the voltage across R4 :
VRHI = VR3 + VR4
Internal reference voltage VREF (1.25V) supply constant 1.25V across REF_OUT (Pin7) and REF_ADJ (Pin8) :
VR3 = 1.25 V
We can calculate the current into R3 :
IR3 = VR3/R3
I neglect the current going into REF_ADJ (Pin8) so voltage across R4 is :
VR4 = R4 * IR3
VR4 = 1.25 * (R4 / R3)
So we can calculate VRHI :
VRHI = 1.25 * (1 + (R4 / R3))
VRHI = 1.25 * (1 + (680 / 1000))
VRHI = 2.1 V

DETERMINING RLO (Pin4) VOLTAGE
The internal 10x1K network and R5+R6 is a voltage divider, VRLO depend on VRHI :
VRLO = VRHI * ((R5 + R6) / (10000 + R5 + R6))
VRLO = 2.1 * ((18000 + 1800) / (10000 + 18000 + 1800))
VRLO = 1.39 V

DERTMINING VOLTAGE ACCROSS INTERNAL COMPARATORS
The internal 10x1K network is a voltage divider, the voltage across is :
VRNET = VRHI - VRLO
The voltage across each resistor is :
VR = VRNET / 10
VR = (VRHI - VRLO) / 10
VR = (2.1 - 1.39) / 10
VR = 0.071 V
The voltage across positive input of first comparator (Pin1) is :
VC1 = VRLO + VR
VC1 = 1.39 + 0.071
VC1 = 1.46 V
The voltage across positive input of second comparator (Pin18) is :
VC2 = VC1 + VR
VC2 = 1.46 + 0.071
VC2 = 1.53 V
The voltage across positive input of third comparator (Pin17) is :
VC3 = VC2 + VR
VC3 = 1.53 + 0.071
VC3 = 1.60 V
And so on...

DETERMINING CORRESPONDING BATTERY VOLTAGE
Now we can retrieve the battery voltage corresponding comparators threshold :
VBAT = (VSIG_IN / 0.150) + 1
Where VSIG_IN correspond to threshold VCx
VBATx = (VCx / 0.150) + 1 Like
VBAT1 = (VC1 / 0.150) + 1
VBAT2 = (VC2 / 0.150) + 1
VBAT3 = (VC3 / 0.150) + 1
A so on...
If the voltage on VSIG_IN is upper than threshold (VCx) the corresponding LED is glowing.

PIN   VCx    VBAT  
------------------
 10   2.10   15.00
 11   2.02   14.47
 12   1.95   14.00
 13   1.88   13.53
 14   1.82   13.13
 15   1.74   12.60
 16   1.67   12.13
 17   1.60   11.66
 18   1.53   11.20
  1   1.46   10.73
------------------  
PIN  : Is the associated internal comparator  
VCx  : Is the corresponding threshold on positive input of comparator
VBAT : Is the corresponding VSIG_IN battery voltage at the threshold

As you see i found about 0.5 V upper than on the schematic and i dont understand why, can you help me ?

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  • \$\begingroup\$ Two questions for clarification in your post: (1) The +1 in your formulas: is that the diode drop? It's a bit high. 0.7 V would be better. (2) It's not clear where the table at the end comes from. Are they the results when you test the circuit with a variable voltage power supply? \$\endgroup\$
    – Transistor
    Feb 27, 2018 at 20:50
  • \$\begingroup\$ @Transistor I edit my post to take your good remarks in account, Thank you (1) I found the forward voltage of 1N4007 from datasheet and from guy who take measurement link. (2) The table at the end is the VBAT corresponding to SIG_IN (Threshold) voltage. (3) I wait for LM3914, I could not do measurment, i want to theorically understand this schematic. \$\endgroup\$
    – rom1nux
    Feb 27, 2018 at 21:08
  • 1
    \$\begingroup\$ You could replace D1 with a 1N5822 Schottky diode. Only a .5 volt drop. The LM3914 is just a string of comparators separated by equal value resistors, so the LED spacing is 'linear'. You could tie R1 before the diode. At 22 K it will limit any 12 volt line overshoot. Use 1% or .1% resistors if you can. \$\endgroup\$
    – user105652
    Feb 27, 2018 at 21:22
  • \$\begingroup\$ Thank you @Sparky256 but i try to understand why i dont find the same result as the creator of this schematic. \$\endgroup\$
    – rom1nux
    Feb 27, 2018 at 22:51
  • \$\begingroup\$ The lm3915 tag should be lm3914. They are NOT the same part. \$\endgroup\$
    – user105652
    Feb 28, 2018 at 0:27

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