LM3914 Atmospheric Charge Monitor - IC1 necessary?

[joecool85]

I'd like to build this Atmospheric Charge Monitor: **broken link removed**

It measures the atmospheric electrical charge using an LM3914 with a TL071 in front. But it states that typical atmospheric charge is 100 millivolts and raises to several volts in a storm. Why do I need the TL071 then? Can't the LM3914 measure 100 millivolts and up on it's own anyway?

 

[audioguru]

The first LED (pin1) of an LM3914 lights when the input is +0.125V. Pin 1 cannot be used because the output of most opamps cannot go as low as +1.25V. The second LED (pin18) lights when the input is +0.25V.
The opamp is needed to provide voltage gain so that weak voltages are displayed.

Atmospheric charge can be positive or negative. This simple circuit does not show a negative charge.

 

[joecool85]

The first LED (pin1) of an LM3914 lights when the input is +0.125V. Pin 1 cannot be used because the output of most opamps cannot go as low as +1.25V. The second LED (pin18) lights when the input is +0.25V.
The opamp is needed to provide voltage gain so that weak voltages are displayed.

Atmospheric charge can be positive or negative. This simple circuit does not show a negative charge.

So couldn't you just omit the TL071 and then use pin 1 which would display 0.125v?

 

[joecool85]

Maybe it is less a voltage issue and more an amperage issue.

According to http://www.carnicom.com/elec1.htm "a square meter of conducting material was placed horizontally in the air, approximately .000000000003 amps would flow through that surface". I'm thinking the LM3914 wouldn't be able to measure this as it would "eat up" that tiny amount of amps.

Am I right?

 

[audioguru]

The LM3914 has a very small input bias current that is typically 25nA and is a maximum of 100nA. A 1M resistor to ground on the input is fine to give it 0V without a signal.
If you want only one LED to light when lightning is fairly close then the opamp amplifier is not needed.

 

[joecool85]

The LM3914 has a very small input bias current that is typically 25nA and is a maximum of 100nA. A 1M resistor to ground on the input is fine to give it 0V without a signal.
If you want only one LED to light when lightning is fairly close then the opamp amplifier is not needed.

So to make it more sensitive and light more LEDs, I do need the opamp. But is it due to amplifying amperage or voltage? I appreciate the help, I just like to understand whats going on.

 

[audioguru]

The opamp is amplifying the voltage. The current is almost nothing.

 

[unclejed613]

according to the block diagram, the input of the LM3914 is shunted by a resistor string totaling 10k. this would bleed charge off the sensor too fast to get any measurable voltage. the input resistance of the op amp and the sensor circuit is much higher, and so the charge does not bleed off as quickly. this is being used as an electrometer, which usually has a very high input resistance.

 

[joecool85]

according to the block diagram, the input of the LM3914 is shunted by a resistor string totaling 10k. this would bleed charge off the sensor too fast to get any measurable voltage. the input resistance of the op amp and the sensor circuit is much higher, and so the charge does not bleed off as quickly. this is being used as an electrometer, which usually has a very high input resistance.

Ah, that makes sense. So it's an impedance issue more than anything else.

 

[audioguru]

according to the block diagram, the input of the LM3914 is shunted by a resistor string totaling 10k. this would bleed charge off the sensor too fast to get any measurable voltage. the input resistance of the op amp and the sensor circuit is much higher, and so the charge does not bleed off as quickly. this is being used as an electrometer, which usually has a very high input resistance.

No.
The input resistance of the input buffer opamp of an LM3914 is very high. It works with an input to ground resistance as high as 1M ohms because its input bias current is typically only 25nA. The 10k divider provides the reference voltages for its comparators, not at its input.

The project uses an extra opamp at its input to provide voltage gain. The extra opamp might not be biased correctly since its input voltages are too low for it and its output cannot go low enough.
The buffer opamp of an LM3914 is similar to the opamps in an LM324 or LM358 that work perfectly with an input voltage as low as 0V in this circuit.

The simple circuit is missing a lowpass filter or notch filter so I think it will pickup and display lots of mains hum.

 

[ronsimpson]

Are clouds positive or negative? It looks like you project only works for positive.

 

[joecool85]

Are clouds positive or negative? It looks like you project only works for positive.

Both but it looks like primarily you will measure positive until the storm is right over you and then you measure negative. I probably won't be building this circuit now that I know more about it, but it's still very interesting and I appreciate all the excellent responses from everyone here.