Lowest draw "power on" indicator?

[Ken]

What "power on" indicator consumes the least current?

An LED draws 20 mA. In a flasher circuit (LM3909) this can be reduce
to 0.4 mA, but at an expense of around $5 in components.

Is there anything in between?

I need an indicator for a circuit that itself only draws 20mA.


Ken
(to reply via email
remove "zz" from address)

 

[John Fields]

What "power on" indicator consumes the least current?

An LED draws 20 mA. In a flasher circuit (LM3909) this can be reduce
to 0.4 mA, but at an expense of around $5 in components.

Is there anything in between?

I need an indicator for a circuit that itself only draws 20mA.

---
High efficiency LEDs are available with a rated current of 2mA which
are clearly visible with less than 1mA going through them. High
intensity LEDs are also available which should give you a nice output
for much less than that.

Also, if you want to go mechanical, power ON-OFF switches are avaiable
with an indicator which changes color depending on whether it's ON or
OFF. Zero power required for the indicator.

 

[Clarence_A]

What "power on" indicator consumes the least current?

An LED draws 20 mA. In a flasher circuit (LM3909) this can be reduce
to 0.4 mA, but at an expense of around $5 in components.

Is there anything in between?
I need an indicator for a circuit that itself only draws 20mA.

Ken

I often use LEDs at as little as 1mA for indicators. Just aren't
as bright!

 

[Miles Harris]

you could try running the LED in series with the Power source if your
circuit does draw that much,.
you will get a slight drop in voltage how ever if that is not a problem.

How is that brainwave going to save on power?

 

[Ken]

you could try running the LED in series with the Power source if your
circuit does draw that much,.
you will get a slight drop in voltage how ever if that is not a problem.

Won't the drop be 2 volts?

Assuming I can make this work, how much power in milliwatts will the
LED actually consume?


Ken
(to reply via email
remove "zz" from address)

 

[Jonathan Kirwan]

I need an indicator for a circuit that itself only draws 20mA.

There are LEDs that will require only .5mA to 2mA with decent appearing light.
You could try those high-efficiency red types. A few discretes could achieve
the blinking (two BJTs and three or four passives come to mind.)

And in no way is the 3909 *worth* $5 -- cripes!

Jon

 

[JeffM]

if you want to go mechanical, power ON-OFF switches are avaiable

with an indicator which changes color depending on whether it's ON or
OFF. Zero power required for the indicator.
John Fields

The ones I've seen are push-on/push-off
with 2 dayglow green (or yellow) bits that come together like eyelids.

 

[Jonathan Kirwan]

Won't the drop be 2 volts?
Yes.

Assuming I can make this work, how much power in milliwatts will the
LED actually consume?

20mA * 2V or 40mW.

Probably, this series deal isn't a good idea. The use of a high-efficiency red
led is probably much better. You can tweak it's current down to say .5mA and it
will probably look okay. Another option would be to add the few discrete parts
it would take to blink the LED to further reduce the average current. (I've got
the schematic for a duplicate of the 3909, for example.)

The simplest is probably the high-efficiency red led, though.

Jon

 

[John Fields]

The ones I've seen are push-on/push-off
with 2 dayglow green (or yellow) bits that come together like eyelids.

 

[John Fields]

I often use LEDs at as little as 1mA for indicators. Just aren't
as bright!

 

[Jamie]

What "power on" indicator consumes the least current?

An LED draws 20 mA. In a flasher circuit (LM3909) this can be reduce
to 0.4 mA, but at an expense of around $5 in components.

Is there anything in between?

I need an indicator for a circuit that itself only draws 20mA.


Ken
(to reply via email
remove "zz" from address)

you could try running the LED in series with the Power source if your
circuit does draw that much,.
you will get a slight drop in voltage how ever if that is not a problem.

 

[John Popelish]

What "power on" indicator consumes the least current?

An LED draws 20 mA. In a flasher circuit (LM3909) this can be reduce
to 0.4 mA, but at an expense of around $5 in components.

Is there anything in between?

I need an indicator for a circuit that itself only draws 20mA.

Ken
(to reply via email
remove "zz" from address)

You usually do not need 20 ma to have a useful power on indicator.
And the main claim to fame of the 3909 was that it could boost a low
voltage high enough to light the LED, as well as flash it. You can
use an LMC555 as a flasher, and put the LED in series with the
discharge pin to give a flash often enough to indicate power on with a
sub milliamp average drain and much less than $5 price tag. This
assumes that you have more than about 4 volts to work with.

 

[Jamie]

Won't the drop be 2 volts?

Assuming I can make this work, how much power in milliwatts will the
LED actually consume?


Ken
(to reply via email
remove "zz" from address)

like i said, you will get a drop in voltage to your circuit, but since
i don't know your circuit totally i am only guessing that you could
recalucate for that problem.

 

[John Fields]

How is that brainwave going to save on power?

---
Well, my boy, first consider:


+V
|
+------+--E1
| |
| [RS]
| |
[RL] +--Eled
| |
| [LED]
| |
+------+
|
GND

E1² (E1 - Eled)
Pd = ----- + E1 * -------------
RL RS


Assuming that +V is 10V, that RL is the load, (which draws 20mA at
10V) that Eled is 2V, and that the LED draws 20mA, if we plug in those
numbers we'll have:


10² (10V - 2V)
Pd = ------ + 10V * ------------ = 0.4W
500R 400R




Then, if our circuit becomes:


+V
|
+--------E1
|
[RL]
|
+--------Eled
|
[LED]
|
GND


We'll have:


E1 - Eled
Pd = E1 * -------------
RL


Now, keeping everything as it was before and assuming that the
circuitry represented by RL can work with 8V across it, if we plug in
the numbers, this time we'll have:


10V - 2V
Pd = 10V * ---------- = 0.16W
500R



So, Einstein, since 0.16W < 0.4W, there's your answer.

 

[Steven Swift]

Also, if you want to go mechanical, power ON-OFF switches are avaiable
with an indicator which changes color depending on whether it's ON or
OFF. Zero power required for the indicator.

For safety reason, I think mechanical indication is required on line powered
equipment.

 

[Terry Pinnell]

Or a simple toggle labeled ON / OFF. (Optional extra: colour the
labels.)

 

[Miles Harris]

On Mon, 10 Jan 2005 17:44:20 -0600, John Fields

[snip]

So, Einstein, since 0.16W < 0.4W, there's your answer.

Thanks, knob-head.

 

[Joel Kolstad]

For safety reason, I think mechanical indication is required on line
powered
equipment.

'Required?' There's tons of common household equipment that uses pushbutton
switches that have no mecahnical indication of their powered/unpowered
state. (In fact, I have a monitor where this is actually kinda annoying...
the power LED changes color depending on the mode, and it can be hard to
tell red's 'off' from orange's 'on, but in power save mode' at times!)

The original poster might want to go a Google news groups search for the
thread about ultra-low power LED flasher circuits some months back.
Winfield Hill had plenty to contribute to it, which is a pretty good
indication that the results were high quality and robust!