Wannabe EE
New Member
I found numerous queries in the internet involving people trying to connect multiple LM3914s. Each time, I was optimistic that they would include the answers to my questions. They did not.
I posted my own query on this forum with similar results. However, I did get a valuable tool from Eric Gibbs, that was the first major piece of my answering my own questions. Eric's VB5 LM3914 simulation program was key to my deciphering the mysteries of the LM3914.
On another forum, I found a couple more keys, and now that I have unraveled the LM3914, I am posting a summary of the inner workings of the LM3914 - essentially answering MY questions for everyone else.
There are a lot of things about the datasheet that were unclear to me, and some of that was because the datasheet showed so many variations in how to set up the reference voltages. Also, the interaction between the LED brightness and reference voltage adjustment confused me so that I assumed there were more interactions than there really are.
So the next piece of the puzzle that made a big difference for me, was discovering (on a different forum) that the reference voltage source (pins 7 and 8) is basically the same thing as an LM317 voltage regulator. As a matter of fact, they both use the same equations to determine their voltage.
The final key for me was understanding that the IC's internal voltage divider is a stack of resistors that is accessed between pin 4 and pin6 and is independent of the voltage reference source.
Depending upon what you are measuring, you can determine your reference from the voltages required.
The LEDs I selected are pretty bright and will therefore need 6 ma to 7 ma current.
Keep in mind that the reference voltage regulator needs 1.5 volts headroom from the supply voltage.
If your reference voltage is to be the nominal 1.25 of the voltage reference source, the adj. pin (pin 8) can be connected to ground (zero ohms resistance).
You now know the resistor values to start with.
I used six cascaded LM3914s. The resistor values determined in the previous step are what are used on the last (most significant) IC, with some modification....
View attachment 62267
Here you see that R1 is 4.02 kilohms and R2 is 7.18 kilohms. Closer examination reveals that pin 7 is at a potential of 3.48 volts in relation to ground (that voltage drop will show across R1 and R2 combined). It is directly connected to pin 6 (RHi), the high end of the internal voltage divider. Since I have six LM3914s, I want to divide the 3.48 volts into six equal steps - one for each chip.
The total resistance of R1 and R2 is 11.2 kilohms. So six equal steps will be 11.2 divided by 6 or 1.87 Kilohms. And you'll have something like this.
View attachment 62268
But what about the connection to pin 8? The simulator tells us we need 4.02 Kilohms between pin 7 and pin 8. So we do this.
View attachment 62269
Now there is another thing to take into account. Inside each LM3914, there is a stack of resistors for it's own internal voltage divider. It turns out, these resistors are in parallel with the resistors we just assembled. Since parallel resistors change the overall resistance, we need to change the value of our resistors to make up for this.
There are six LM3914s (with their internal resistors) in parallel with our 11.2 kilohm stack. The nominal resistance of the LM3914's resistors is 10 kilohms. This is what we get.
View attachment 62271
We actually have 60 kilohms in parallel with 11.2 kilohms. This will only give us a total of 9.44 kilohms where we need 11.2 kilohms. So we need to recalculate what value will give us the right resistance. I am lazy, so I just used one of the online parallel resistance calculators for this. So we will need 13.77 kilohms total resistance in our voltage divider, to give us 11.2 kilohms when the parallel resistance is included. This is a 23% increase in the resistance.
So we multiply the resistor values by 1.23 and we get this.
View attachment 62273
Open the LM3914 simulator again. Set the slider for R2 to zero ohms. Now adjust the slider for R1 to deliver the appropriate current to the LEDs.
View attachment 62274
For my application, R1 is 2.37 kilohms. The lower five ICs will need 2.37 kilohms between pin 7 and pin 8, and pin 8 will be grounded.
View attachment 62276
The datasheet gives the tolerance for the voltage reference source (between pin 7 and pin8) as anywhere from 1.2 volts to 1.34 volts.
This can have a significant effect on the results of the above calculations, since they are based on a nominal voltage of 1.25 volts.
I measured the voltage of my chip and made the appropriate adjustments in my circuit. This variation is part of the reason that trimmer pots are shown in the datasheet's schematics. You may rather add trimmer pots and go through a calibration process instead of recalculating everything after measuring the actual value.
Another issue could be the tolerance of the resistance of the internal voltage divider stack. The datasheet specifies from 8 kilohms to 17 kilohms. This is a huge potential variation!
Since I have seventy-five LM3914s to choose from, I measured the resistance and voltage to select six ICs that were a close match. You may not have this luxury, so trimmer pots may be the easiest solution....
-EDIT-
If the voltage divider resistors inside the chip are matched, or their resistance is close enough to provide acceptable accuracy, the stack of external resistors are unnecessary. The only resistors needed are between pin 7 and pin 8, and pin 8 to ground on the top LM3914. These are to set the LED current and reference voltage for the circuit. Be sure to include the stack of internal resistors in parallel when calculating the values. If the IC's internal reference voltage source is not used, and an external reference is provided, the LED current for all six LM3914s can be set as shown in Step 5 above.
The purpose of the external resistors is to provide a method of correcting or adjusting the inequalities of the internal resistor stack from chip to chip. This can be done by including trimmer pots in the design, or by measuring each chip's resistance (between pin 4 and pin 6), and then calculating the appropriate value for the external resistance (of that chip) that normalizes the resistance difference.
When I originally wrote this guide, I had some tunnel vision (from part of the datasheet that explained this resistor stack), and didn't see (until Jeff illuminated it in post #4) that it was for normalizing the variation between ICs.
Since I have selected six LM3914s with resistance between 11.47k and 11.52k, the normalization is unnecessary. Thanks Jeff!
I posted my own query on this forum with similar results. However, I did get a valuable tool from Eric Gibbs, that was the first major piece of my answering my own questions. Eric's VB5 LM3914 simulation program was key to my deciphering the mysteries of the LM3914.
On another forum, I found a couple more keys, and now that I have unraveled the LM3914, I am posting a summary of the inner workings of the LM3914 - essentially answering MY questions for everyone else.
There are a few things I then discovered that helped me see the big picture....
There are a lot of things about the datasheet that were unclear to me, and some of that was because the datasheet showed so many variations in how to set up the reference voltages. Also, the interaction between the LED brightness and reference voltage adjustment confused me so that I assumed there were more interactions than there really are.
So the next piece of the puzzle that made a big difference for me, was discovering (on a different forum) that the reference voltage source (pins 7 and 8) is basically the same thing as an LM317 voltage regulator. As a matter of fact, they both use the same equations to determine their voltage.
The final key for me was understanding that the IC's internal voltage divider is a stack of resistors that is accessed between pin 4 and pin6 and is independent of the voltage reference source.
Since I'm going to make around 24 different bargraph displays with something like 10 different input sources (temp. sensors, pressure transducers, analog position sensors, etc.), I plan to make a general circuit and condition the input with an op amp. This means I can pick whatever reference I want and adjust the range with the op amp.
Depending upon what you are measuring, you can determine your reference from the voltages required.
The LEDs I selected are pretty bright and will therefore need 6 ma to 7 ma current.
Start up the LM3914 simulator program (for this, you can use the single LM3914 version) and setup the supply voltage. Then play with the sliders for R1 and R2 to get the LED current and reference voltage you selected.
Keep in mind that the reference voltage regulator needs 1.5 volts headroom from the supply voltage.
If your reference voltage is to be the nominal 1.25 of the voltage reference source, the adj. pin (pin 8) can be connected to ground (zero ohms resistance).
You now know the resistor values to start with.
I used six cascaded LM3914s. The resistor values determined in the previous step are what are used on the last (most significant) IC, with some modification....
I selected 3.48 volts as my reference voltage and 6.3 ma to drive the LEDs (as shown).
View attachment 62267
Here you see that R1 is 4.02 kilohms and R2 is 7.18 kilohms. Closer examination reveals that pin 7 is at a potential of 3.48 volts in relation to ground (that voltage drop will show across R1 and R2 combined). It is directly connected to pin 6 (RHi), the high end of the internal voltage divider. Since I have six LM3914s, I want to divide the 3.48 volts into six equal steps - one for each chip.
The total resistance of R1 and R2 is 11.2 kilohms. So six equal steps will be 11.2 divided by 6 or 1.87 Kilohms. And you'll have something like this.
View attachment 62268
But what about the connection to pin 8? The simulator tells us we need 4.02 Kilohms between pin 7 and pin 8. So we do this.
View attachment 62269
Now there is another thing to take into account. Inside each LM3914, there is a stack of resistors for it's own internal voltage divider. It turns out, these resistors are in parallel with the resistors we just assembled. Since parallel resistors change the overall resistance, we need to change the value of our resistors to make up for this.
There are six LM3914s (with their internal resistors) in parallel with our 11.2 kilohm stack. The nominal resistance of the LM3914's resistors is 10 kilohms. This is what we get.
View attachment 62271
We actually have 60 kilohms in parallel with 11.2 kilohms. This will only give us a total of 9.44 kilohms where we need 11.2 kilohms. So we need to recalculate what value will give us the right resistance. I am lazy, so I just used one of the online parallel resistance calculators for this. So we will need 13.77 kilohms total resistance in our voltage divider, to give us 11.2 kilohms when the parallel resistance is included. This is a 23% increase in the resistance.
So we multiply the resistor values by 1.23 and we get this.
View attachment 62273
We now have the resistor values to set up the reference voltage to each LM3914, plus we have set the LED current for only the top IC. Now we need the set the current for the remaining five chips.
Open the LM3914 simulator again. Set the slider for R2 to zero ohms. Now adjust the slider for R1 to deliver the appropriate current to the LEDs.
View attachment 62274
For my application, R1 is 2.37 kilohms. The lower five ICs will need 2.37 kilohms between pin 7 and pin 8, and pin 8 will be grounded.
View attachment 62276
The datasheet gives the tolerance for the voltage reference source (between pin 7 and pin8) as anywhere from 1.2 volts to 1.34 volts.
This can have a significant effect on the results of the above calculations, since they are based on a nominal voltage of 1.25 volts.
I measured the voltage of my chip and made the appropriate adjustments in my circuit. This variation is part of the reason that trimmer pots are shown in the datasheet's schematics. You may rather add trimmer pots and go through a calibration process instead of recalculating everything after measuring the actual value.
Another issue could be the tolerance of the resistance of the internal voltage divider stack. The datasheet specifies from 8 kilohms to 17 kilohms. This is a huge potential variation!
Since I have seventy-five LM3914s to choose from, I measured the resistance and voltage to select six ICs that were a close match. You may not have this luxury, so trimmer pots may be the easiest solution....
-EDIT-
If the voltage divider resistors inside the chip are matched, or their resistance is close enough to provide acceptable accuracy, the stack of external resistors are unnecessary. The only resistors needed are between pin 7 and pin 8, and pin 8 to ground on the top LM3914. These are to set the LED current and reference voltage for the circuit. Be sure to include the stack of internal resistors in parallel when calculating the values. If the IC's internal reference voltage source is not used, and an external reference is provided, the LED current for all six LM3914s can be set as shown in Step 5 above.
The purpose of the external resistors is to provide a method of correcting or adjusting the inequalities of the internal resistor stack from chip to chip. This can be done by including trimmer pots in the design, or by measuring each chip's resistance (between pin 4 and pin 6), and then calculating the appropriate value for the external resistance (of that chip) that normalizes the resistance difference.
When I originally wrote this guide, I had some tunnel vision (from part of the datasheet that explained this resistor stack), and didn't see (until Jeff illuminated it in post #4) that it was for normalizing the variation between ICs.
Since I have selected six LM3914s with resistance between 11.47k and 11.52k, the normalization is unnecessary. Thanks Jeff!
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