Guest
I'm using an LED in series with a Zener (& resistor) to
get a loosely "regulated" 12.5 volts. This is part of
an expanded scale voltmeter circuit. The movement draws
200 microamps full scale. The negative side of the movement
is connected to a zener regulated 11 volts - the positive
side connects at X to the LED resistor connection:
---zener2---led--X--resistor
The circuit does what I want (I'm interested in the
voltage between 11.5 and 12.5 volts) - keeping the
meter movement from being "slammed" when the voltage
increases to absolute maximum of 15 volts. It also
lights the LED at 12.5 volts and above, reminding me
that readings over 12.5 volts are not accurate.
I could accomplish the same thing with a TL431 and
other components, but the 3 component - zener, LED
& resistor - seemed simplest. Readings between
11.5 and 12.5 are spot on accurate +/- parallax.
Essentially, I ignore the left hand first 50 uA
and the right hand lest 50 uA. Also, I switch the
expanded circuit in or out depending on whether
I want to see the full range 0 to 20 volts, or just
the expanded range 11 to 12.5 with the readings above
12.5 inaccurate.
Now to the question: are there any drawbacks lurking
in this circuit that will bite me? I haven't played
with an LED as a sort of voltage regulator before.
A couple of hours of bench testing may not reveal
issues I haven't considered. Maybe diode Vf changes
over time? By the way, I have considered temperature
compensation - I believe the circuit will be accurate
enough without it. (+/- 50 mV between 11.5 and 12.5
volts is acceptable)
get a loosely "regulated" 12.5 volts. This is part of
an expanded scale voltmeter circuit. The movement draws
200 microamps full scale. The negative side of the movement
is connected to a zener regulated 11 volts - the positive
side connects at X to the LED resistor connection:
---zener2---led--X--resistor
The circuit does what I want (I'm interested in the
voltage between 11.5 and 12.5 volts) - keeping the
meter movement from being "slammed" when the voltage
increases to absolute maximum of 15 volts. It also
lights the LED at 12.5 volts and above, reminding me
that readings over 12.5 volts are not accurate.
I could accomplish the same thing with a TL431 and
other components, but the 3 component - zener, LED
& resistor - seemed simplest. Readings between
11.5 and 12.5 are spot on accurate +/- parallax.
Essentially, I ignore the left hand first 50 uA
and the right hand lest 50 uA. Also, I switch the
expanded circuit in or out depending on whether
I want to see the full range 0 to 20 volts, or just
the expanded range 11 to 12.5 with the readings above
12.5 inaccurate.
Now to the question: are there any drawbacks lurking
in this circuit that will bite me? I haven't played
with an LED as a sort of voltage regulator before.
A couple of hours of bench testing may not reveal
issues I haven't considered. Maybe diode Vf changes
over time? By the way, I have considered temperature
compensation - I believe the circuit will be accurate
enough without it. (+/- 50 mV between 11.5 and 12.5
volts is acceptable)