Need help understanding LM3915 Op Amp Pins

Thread Starter

mbferguson

Joined Apr 23, 2017
94

I'm working on a project for school, and have a few questions about the pins. Basically what this op amp will do, is detect the power of the signal on pin 5 and adjust the LEDs from left to right to represent how "loud" it is kind of like a volume meter. My professor said the dotted line with the capacitor is not required to make the circuit work, and neither is the R_LO on pin 4. The schematic requirements says a 100K potentiometer is required, but where would I hook this up to the op amp?

Thanks!
 

Dodgydave

Joined Jun 22, 2012
9,306

bertus

Joined Apr 5, 2008
20,650
Hello,

There are three types of these display drivers.
The LM3914, wich has a linear scale.
The LM3915, wich has a 3dB step scale.
And the LM3916, wich has a VU-meter like scale.

As @Dodgydave told you, the LM has a series of resistors and comparators inside.

LM3915_internal_schematic.png

Bertus
 

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Thread Starter

mbferguson

Joined Apr 23, 2017
94
I appreciate the help guys. I have one more question, in the schematic I posted for this circuit, at the very top the wire has a voltage of 3v to V+ (where it says V+ is also between 12v and 20v), does this mean that I can set the votlage on that wire to any value between those two? I initially thought it was automatically set by the LM3915, but that doesn't make any sense as it is not connected to the inside at all if I ignore the dotted capacitor node. *My professor said the dotted capacitor node is not required to make this circuit function.

I'm assuming that the way that this LM3915 works is that it sets the voltage on the negative side of the LED, so that it will prevent there being a voltage drop by setting certain pins to be equal to the top node. But how do I determine what the voltage on that top wire should be? Anywhere between 3v and V+ doesn't make sense to me

Best,
Michael Ferguson
 

bertus

Joined Apr 5, 2008
20,650
Hello,

The schematic you posted in directly from the datasheet.
Here is the same schematic with the formulas on it:

LM3915_typical_schematic.png
As it is used with a 10 Volts reference, the supply voltage must be at least 12 Volts.

Bertus
 

#12

Joined Nov 30, 2010
18,210
Short version: The Vled line is usually the power supply voltage, just because it's simple to do it that way.
It is impossible for Vref to be higher than Vcc, therefore you decide on Vcc first, then do the math to be sure you are setting Vref to a proper value below the power supply voltage.
I'm assuming that the way that this LM3915 works is that it sets the voltage on the negative side of the LED,
False.
The 10 pins where the LEDs are connected contain, "constant current" paths to ground, one for each pin. If the LED is off, the Vled voltage will be at the pin on the chip waiting for a signal before current flows. When the LED is supposed to be on, the chip allows current to flow to ground. Suppose you set Iled to be 10 ma. The current through each LED will be 10 ma and the chip cares not what the voltage is, except the voltage must not be so high that the chip overheats. The datasheet says Vled must be below 7 volts or the chip will overheat. Creating a 6 volt supply just for Vled is usually inconvenient, so resistors are used to dissipate the heat caused by the excess voltage and the 10 ma current through each LED.

For instance: If Vcc is 12V, that is more than 7 volts. A resistor must be placed in series with each LED to use up at least 5 volts when 10 ma is flowing. That would be 500 ohms as the minimum resistance and each resistor will dissipate 50 milliwatts of heat to save the chip from overheating. You must leave 3 volts for [the LED plus the constant current circuit] so the most voltage you can use up in the resistor is 9 volts. That means the resistor maximum is 900 ohms. Any resistor between 500 ohms and 900 ohms will be acceptable.

Now that blue LEDs have been invented, and they use about 3.3 volts, the ancient datasheet is not completely valid. In that case, you would want to allow 4.3 volts for [the LED plus the constant current circuit]. 12V - 4.3V = 7.7 volts. That changes the maximum resistor value to 770 ohms for a 10 ma current through a blue LED.

Now you have the final answer: 500 ohms to 770 ohms is the range for the resistor in series with each blue LED.
560 ohms, 680 ohms, 750 ohms? Doesn't matter. Pick one. All those values will work.
 
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