> the Iled formula 12.5v/R1 term blows up, and I'm not liking that...
Relax. Around 60 ohms the regulator hits the rail, Vref near +13V.
True, that sets LED current to 200mA, which is over the usual rating...
I see that plan many places, and it is flawed. The 7-8 resistor should be fixed, the 8-2 resistor can be adjusted zero up to raise the Vadj while preserving the LED current.
Use this plan:
http://www.national.com/images/pf/LM3914/00797001.pdf
only make their R2 equal Zero. That gives 1.25V top threshold, ~~0.1V bottom threshold, 10mA LED current.... it's all good. And a close match for directly reading line levels. If you need to read larger levels, in most cases it is simpler to use a pot to reduce the source than to fiddle with the Vadj scheme.
Relax. Around 60 ohms the regulator hits the rail, Vref near +13V.
True, that sets LED current to 200mA, which is over the usual rating...
I see that plan many places, and it is flawed. The 7-8 resistor should be fixed, the 8-2 resistor can be adjusted zero up to raise the Vadj while preserving the LED current.
Use this plan:
http://www.national.com/images/pf/LM3914/00797001.pdf
only make their R2 equal Zero. That gives 1.25V top threshold, ~~0.1V bottom threshold, 10mA LED current.... it's all good. And a close match for directly reading line levels. If you need to read larger levels, in most cases it is simpler to use a pot to reduce the source than to fiddle with the Vadj scheme.
