Picking out LED for Circuit?

[digi001]

Hi, When picking out a simple indicator LED for a circuit there are 2 main things to consider?

1. Using resistor to limit the current to appropriate level 10-20ma?

2. Making sure the Voltage Forward isn't any higher than the voltage of the circuit?



Example: I have a 1.8V Power Rail that I would like to use a LED to indicate when it is on. If the Voltage Forward is 2V, then I would need to find one that has a lower Voltage Forward?

 

[betwixt]

1. correct
2. correct

Brian.

 

[digi001]

It seems hard to find LEDs for 1.8V indicator.

Any suggestions of parts for this?

 

[keith1200rs]

In general the longer the wavelength, the lower the voltage. Assuming you want to see it, infra red is number use so you need to look for a deep red. A constant current driver will give a more stable brightness than a simple resistor dropper.

Keith

 

[kam1787]

I have a 1.8V Power Rail that I would like to use a LED to indicate when it is on.

Then you have 2 choices - use a DC-to-DC booster to increase the voltage

or

Find a LED with Vd < 1V8 https://www.oksolar.com/led/led_color_chart.htm https://en.wikipedia.org/wiki/Light-emitting_diode

The IR LEDs I have used do have some visible light - you might try it.

- - - Updated - - -

A constant current driver will give a more stable brightness than a simple resistor dropper.

True, although if the voltage is stable then the current will be stable. Even if the voltage does vary somewhat, the eye is nonlinear wrt light so it is not that blatantly obvious.

A constant current driver usually requires several volts (from 2V to 6V) higher than the voltage across the load to work, which can limit it's usefulness. As well as the extra cost and complexity is a concern.

 

[Darkcobra]

It's likely 1.8V isn't the only, or highest supply voltage present in your system. And you may be able to use that higher voltage to power the LED, for example through a NPN bipolar transistor; which is switched on by the 1.8V supply and therefore still gives you an indication that supply is functioning.

 

[keith1200rs]

True, although if the voltage is stable then the
current will be stable. Even if the voltage does vary
somewhat, the eye is nonlinear wrt light so it is not
that blatantly obvious. A constant current driver usually requires several
volts (from 2V to 6V) higher than the voltage
across the load to work, which can limit it's
usefulness. As well as the extra cost and
complexity is a concern.

While the supply may be stable, the LED voltage won't be, particularly with temperature.

Constant current drives can be made with less than 200mV overhead.

Keith

 

[sreekumar27]

GaAs LEDs have lower Vf about 1.2V may be you can use that.

 

[betwixt]

???? most LEDs are GaAs !!

With a 1.8V supply it may still be possible to use standard brightness indicator LEDs but high brightness types are out of the question unless a higher supply voltage is derived from somewhere. If they are just indicators the absolute brightness may not be too important so you may be able use standard parts. If they are 7-segment LEDs which use several series LED dies, you have no option but to use higher voltage. If your circuit has an on-board 1.8V regulator, you may be able to use the voltage before the regulator which in most cases will be higher.

Brian.

 

[sreekumar27]

LEDs are made of many combinations for various colors. Lesser junctions lesser breakdowns. LEDs can be GaAs, GaAsP, InP, GaSb, GaInP, SiGe, InGaN, GaN, GaP and so on... Lots of LEDs are there for different color, coherence, luminance and Vf
For low breakdown choose p-n junctions i.e. GaAs, or AlGaAs etc..

 

[keith1200rs]

It is forward voltage that matters not breakdown. Straight GaAs are infra-red. AlGaAs will be visible but still close to 1.8V. As has been suggested, an alternative voltage from the system could be used or some form of voltage pump.

Keith

 

[mikehoopes]

Ostensibly, if you're working at 1.8V, there's likely an MCU or similar in the vicinity. If you have a free digital I/O pin, a ~50 KHz or so pulse train could be applied to a Dickson Multiplier/Doubler, which is just a dual Schottky diode and two capacitors, in its simplest form. That'll get you in the range of most indicator LED forward drop levels, to include blue.

Remember also that LED intensity is only consistent part-to-part when driven within ~>10% of its maximum specified DC current rating.

Cheers,

Mike

 

[digi001]

It's likely 1.8V isn't the only, or highest supply voltage present in your system. And you may be able to use that higher voltage to power the LED, for example through a NPN bipolar transistor; which is switched on by the 1.8V supply and therefore still gives you an indication that supply is functioning.

This seems to be the winner so far. Thanks!