LED impedance

I want to add a red LED directly in series with a phone to show when it is on use. The current is about 15mA and is not subject to change, it is an internal phone system. But how about the LED impedance, is it low enough not to disturb the 600 ohm phone impedance? Or a capacitor in parallel is absolutely necessary?

Thanks! Dan

Reply to
D. G.
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It won't bother the voice stuff, but it might affect the ring voltage, and the AC ring voltage could certainly blow the LED. I'd drive it through a bridge rectifier. It will get *very* bright during ring, maybe, depending on the phone ringer load.

John

Reply to
John Larkin

The bridge rectifier is a good idea. It will allow the led to light even if the phone line reverses -- which can happen if the telco rewires something. If you do this, remember that the AC connections of the bridge (where the anodes and cathodes come together) need to go in series with the phone(s). If you don't use a bridge rectifier then you must put some reverse protection across the led (like a diode), else if you hook it up backwards it will become a ded (dark-emitting diode).

The ringing period, in itself, won't hurt the led. The impedance of a old-style mechanical ringer (iirc) is about 8Kohms@20Hz. The newer ones are even higher impedance. So, even 100VAC/8Kohms = 12mA.

The problem may be during (what's called) ring trip. This is when the user goes offhook during a ring cycle (as the phone is ringing). The current, then, can be very high (maybe 100mA). It doesn't last long but it's still a lot of current.

You could play some tricks to insure the max current of the led is never exceeded, but it may be more trouble than it's worth.

Bob

Reply to
Bob

I forgot to mention there is no ringing on that line. Let's say it is a "one way" phone! So it is really safe for the LED as far as I can see. But adding the bridge is a good idea as my line connection could be reversed.

Thanks for the feedback! Dan

Reply to
D. G.

ohm

You could use a 2-pin bicolor LED (red/green) in series with the line.

Inactive it is just white. Ringing gives a mixed color, off-hook is one color (preferrably red), when the other side answers the line reverses polarity so you get the other color (preferrably green). No need for a rectifier bridge, or cap in parallel.

--
Arie de Muynck
Reply to
Arie de Muynck

I've been doing this for years now. The original use was to catch programs from 'phoning home' without my knowledge. (So I also added a line cut-out switch).

Ordinary LED with bridge rectifier in series with phone line. It has little effect on the phone either ringing or talking. Both the ring current and the talk current are withing the 'survival' range of my cheapo LED.

Luhan

Reply to
Luhan

^^^^^^^^^^^^^^^^^^^^^^

Maybe if we asked the OP if his internal system has ringer voltage on it...

In answer to the OP's question, since you're in control of the power supply, I don't see any particular problem with it - it shouldn't even need a cap, since it's in a current loop. And yes, your

15 mA supply will need another 1.2V or so of compliance. :-)

Cheers! Rich

Reply to
Rich Grise

A couple points:

  1. Most red LEDs at 15 mA drop 1.6 volts (for "original formula" GaAsP-on-GaAs) to 1.9-2.05 volts (for many InGaAlP types). But then again, if the LED was in series with a phone on a phone line where the impedance is often said to be 600 ohms and the open circuit voltage (for a POTS line) is 40 or 48 volts or whatever, this should not make a significant difference.

  1. I just did some measurements with my POTS line.

On-hook: Voltage across phone 44V Current through phone less than .1 mA

Off-hook: Voltage across phone 7.2V Current through phone 22 mA

Phone DC resistance: 33 ohms (approx.) Line DC impedance: 1670 ohms (approx.)

Voltage drop across red LEDs at 22 mA: Mostly ranges from 1.6 to 2.1 volts, depending mostly on chemistry. Some red and/or reddish orange LEDs could be considered non-defective if dropping as high as 2.6 possibly 2.8 volts at 20 mA, although 2.3 is really high and 1.5 is really low.

In my experience with a bit of high current pulsing of 5 mm and 3 mm (T1-3/4 and T1) LEDs, I have seen two components to the voltage drop:

  1. A resistance of mostly about 10-20 ohms

  1. The diode drop - whose voltage drop changes by .1 volt or maybe sometimes to maybe often some larger fraction of a volt per factor-of-10 change in current. In a few cases of imperfection, the diode drop may be found to have resistance of a few megohms or 100K's of ohms in parallel. Static-damaged GaN or InGaN LEDs could have parallel resistance in or near the the 100-ohm - 1,000 ohm ballpark.

With voltage change unrelated to resistance unlikely to change by .1 volt or more as current changes from 10 to 20 mA (or vice versa), which is a dynamic impedance of 10 ohms or less, plus resistance of 10-20 ohms, I would guess that an red LED is typically effectively a 1.4-2 volt voltage drop in series with a resistance anywhere from 15 to 30 ohms.

  1. If you put an LED in series with a telephone on a POTS ine, consider ringing conditions:

a) Line impedance may be the "traditional" 600 ohms and I have yet to rule out less, with phone impedance possibly well below 330 ohms due to the ringer being contacted. Peak voltage is traditionally 80 volts, and that may be under load of who knows what impedance. I did try myself, and peak current through a ringing on-hook phone surprisingly to me appeared to be less than 22 mA in my particular case. But I fear much more could be possible! I may want an LED in series with a phone to be a "Luxeon" or similar one rated at least 350 mA just to be safe!

b) I have known the peak-to-peak ring signal voltage to exceed twice the DC on-hook voltage. An LED in series with a phone should have a reverse diode across the LED. Some LEDs have damage mechanisms beyond merely overheating if reverse breakdown occurs, with many GaN and InGaN ones being especially intolerant of any reverse breakdown at all! Also, wondering if the phone line has any intolerance of especially unequal conductivity between one direction and the other - could be a good reason to put an antiparallel diode (or LED) across any LED that is in series with the phone!

- Don Klipstein ( snipped-for-privacy@misty.com)

Reply to
Don Klipstein

...

...

That's not applicable to a system where he owns his own battery. ;-)

Cheers! Rich

Reply to
Rich Grise

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