Lowest draw "power on" indicator?

--- 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.

-- 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.

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.

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.

--
Exactly! I've got one on my TEK 2465A.

--
Sorry to hear that. Perhaps if you read a little more???^)

--- 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.

-- John Fields

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.

--
John Popelish

reduce

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

How is that brainwave going to save on power?

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

Yes.

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

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

--
Steven D. Swift, novatech@eskimo.com, http://www.novatech-instr.com
NOVATECH INSTRUMENTS, INC.      P.O. Box 55997
206.301.8986, fax 206.363.4367  Seattle, Washington 98155 USA

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

--
Terry Pinnell
Hobbyist, West Sussex, UK

[snip]

Thanks, knob-head.

'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!