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LM3915 for a volt meter

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LAN_403

Killa-X

Mon Dec 20 2010, 08:44PM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
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Posts: 1039

I just want to make sure I understand the workings of this chip before I go off trying to rig it into a 500V system.

Do these chips work like comparators? Where I can put a resistance on the reference so its 3V. Then I just make a voltage divider that takes 500V down to 3V, so when it hits 500V, the segment bar shows full?

What I want to do, is make a 0-500V segment meter for a SSTC. This way, I can estimate my voltage levels, and see voltage activity as it pulses. I just want a fancy display over a analog meter.

So if someone wouldn't mind explaining or giving me a direction...I'd be pleased to see how I can mod this schematic to take in a voltage source reference over a voltage divider.

Just want to make sure things are done right so I dont go blowing chips or segments. Thanks for your time and help as always!

Conundrum

Mon Dec 20 2010, 10:17PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

looks ok sometimes the first segment stays partially lit which is a known issue with this chip.

only thing i'd do is add a protection circuit to guard against excessive input voltage.

Killa-X

Mon Dec 20 2010, 11:41PM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
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Posts: 1039

Well this circuit here is for audio. I need to mod it for 500V input, witha voltage divider to kick it down. I wasn't sure the proper way to mod it without risking damage.

Jrz126

Tue Dec 21 2010, 01:26AM

Registered Member #242 Joined: Thu Feb 23 2006, 11:37PM
Location: Erie PA
Posts: 210

You'd probably want an LM3914 instead? It has a linear scale so each LED is 50V. Looks like the input for full scale is 5V so you'd need a 10:1 divider.

Killa-X

Tue Dec 21 2010, 03:19AM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
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Yeah I just got whatever goldmine had to be honest, woops!

But do they work the same as a comparator? Comparing the left and right side? Like, If i put resistors on the reference to make it 2.5V, then the voltage divider, 100% would be whatever hits 2.5V? Or is it simply just a 5V = 100% all lights on...

radiotech

Tue Dec 21 2010, 03:35AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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You need to restate your problem like this:

a.What wattage is the 500 volt signal going to
represent.
b. The module , as is, with a power level, of
100 watts, needs a voltage which is read on pin 5
with 5000 ohms to ground to represent full scale.
c. How can 500 V produce that same voltage at
pin 5 across 5000 ohms.
d. Why is 5000 ohms important, or is it?

Killa-X

Tue Dec 21 2010, 04:57AM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
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@Radiotech I said that schematic was a reference...

All I want to do is put a voltage divider to the signal, so 500V turns to 5V. I found a 1Mohm and 10Kohm makes it 4.95V. I did a test ealier with 12V. with a pot on the reference, and I put the signal with a votlage divider...12V to 5V. I then adjusted the POT on the reference until it filled all bars. Once it did, I loweered that 12V, and the LED segment lowered as well. This allowed me to have a visual volt meter, that just referenced "the capacitors on the coil are __% charged"

That's all im trying to show here, how much charge is in the capacitors, out of 500V, with a bargraph that shows an estimate.

Now I wonder whats the best way to link 2 in series. So I could adjust them, to act as one, for a 0-500V meter with 20 segments. Roughly estimated 1 segment == 25V

radiotech

Tue Dec 21 2010, 01:58PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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Posts: 1546

The way that bargraph display is set up, you could go from 22 volts to 500 volts.
Right now, it has approx. 3db per step and 27 db for the 9 steps.
22 volts to 500 volts is 27.1 db.

Is that what you want?

Killa-X

Tue Dec 21 2010, 04:35PM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
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Well i dont care for the db. This is a volt meter. All I'm doing it showing how much voltage is in the voltage tripler on my tesla coil. This is not for audio, so the unit decibels, doesnt fit with what im doing.

I take your saying i need to have more resistance on my reference so I can have 0-500V..?

Wolfram

Tue Dec 21 2010, 04:39PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

He's saying that you won't get a linear scale on the LEDs. The internal voltage divider chain in this chip is made with resistors values selected for a decibel-scale. If you want a linear scale, which is what you'd want in this application, you have to get an LM3914.

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