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Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
Then either you don't have an LM3915, or there is something weird with your chip. That's not how the LM3915 is supposed to work, at least not according to the datasheet. From reading the datasheet, it seems that 3V would have 6 LEDs lit, 6V would have 8 LEDs lit and 9V would have 9 LEDs lit.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
The chip is very simple inside. It's just one comparator for each LED. All of the + inputs are connected to the input signal pin, and the - inputs are tapped down a string of resistors built into the chip.
Anyway... Both ends of the resistor chain are brought out, so you can use two (or more) LM3914s to make a bigger display by just connecting the resistor chains in series.
Reading the datasheet would have told you all of this and saved my time.
I saw a mention of fake LM391x on Rod Elliot's fake semiconductors page, maybe this explains the log/linear discrepancy.
Registered Member #1643
Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039
I see...So really then I can just hook it like I would any old comparator, by using just pin 6 and 5, since those go to the comparators inside...
So pin 7 and 8 go to a "Reference Voltage Source" But this isnt vitally needed right? I can JUST use 6 and 5? I don't need to mess with LED brightness as I'm feeding it 12V with a 470ohm resistor.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
You need the two resistors around pins 6, 7 and 8 to set the reference voltage. The reference is adjustable, it works the same as a LM317 type regulator.
Just wire it up as shown in page 2 of the datasheet, and connect the output from your voltage divider to pin 5.
The resistor values shown on page 2 give a 5V reference, so make your divider 100:1 (say, 4k7 from pin 5 to ground, 470k from pin 5 to HV supply) and the meter will show 0 to 500V.
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LM3915 for a volt meter
