Step down voltage to power a single LED

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I have a single LED that I would like to use as a visual indicator that my phone is ringing.

It came off a string for a store display that used either a 9V or a few AA batteries.

Voltage to be reduced is around 40 volts D.C.

Hooking up to a red 12 volt "instrument type light" brought the voltage down to about 10 volts if that helps. And the bulb still works, but phone gets static. :-)

What would it take to step down the voltage so I can use the bulb ?

It's been a lot of years, but I think I remember needing a zener diode or two and some resistors.

Thanks.




Re: Step down voltage to power a single LED

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If you are connecting to a standard telephone line, the ring voltage is
about 100 volts.  A NE-2 neon tube in series with a resistor string and
inverse-parallel LED could work.  When the phone is answered, the
voltage drops to maybe 10 volts, and the NE-2 deionizes (so long as the
series resistance is adequate to prevent an arc) and the LED load
vanishes.

Joe Gwinn

Re: Step down voltage to power a single LED
Joe Gwinn wrote:
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   Once phone is off hook, the voltage is so low that a neon bulb CANNOT  
ionize.
   The qualifier is meaningless.


Re: Step down voltage to power a single LED

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Don't forget the corner case - the on-hook voltage is 48 volts, and a
hook flash can get you there.  So, it's best to ensure that the NE-2
will always extinguish on 48 volts DC.

Compared to a zener, the advantage of of a NE-2 is that there is zero
leakage, so you don't trigger the central-office tests of leaky lines
requiring service attention.

Joe Gwinn

Re: Step down voltage to power a single LED
Joe Gwinn wrote:
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   Use the Chem Rubber Handbook of Chemistry and Physics; the FIRST  
ionization potential of neon is about 63V, equal to what one would  
measure at nominal NE-2 currents.


Re: Step down voltage to power a single LED

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The issue is if the neon is already ionized, say by the ring current,
or by a line impulse, and transitions to an arc (from a glow).  If the
circuit isn't designed to be self-extinguishing, this will happen all
the time.

Joe Gwinn

Re: Step down voltage to power a single LED
On Sunday, August 25, 2013 4:23:57 PM UTC-4, Joe Gwinn wrote:
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But it's an AC signal, so won't the arc tend to self-extinguish?


Re: Step down voltage to power a single LED
On Friday, August 23, 2013 11:51:26 PM UTC-5, Robert Baer wrote:
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I think you understand.

Someone said that the voltage is 100 volts when it's ringing.

Is that the voltage it takes to ring the bell in the phone ?

???

If so I may hook up my coffee maker when I don't need my phone. Just kidding.  

I worked repairing lab instruments for 21 years, so I think I can diagnose and study problems.


Re: Step down voltage to power a single LED

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Yes.  It's about 100 volts at 20 or 25 Hz riding on top of the 48-volt
on-hook voltage.

<http://en.wikipedia.org/wiki/Tip_and_ring


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Well, if the ring were continuously on, and not sourced through a
thousand ohms, it could work.  But a ringer takes far less power.


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How about hooking an oscilloscope up to the phone line and looking?

Note that one wire will be near ground, and the other wire rests at -48
volts.  This is for DC; for AC, it's a balanced twisted-pair
transmission line.


Joe Gwinn

Re: Step down voltage to power a single LED
Andy K wrote:
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* Yes, approximately.

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Re: Step down voltage to power a single LED
On Sun, 25 Aug 2013 13:17:38 -0800, Robert Baer

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I found, while making my spam-call killer, that most modern phones
with piezo ringers, will ring aggressively on 24V peak-to-peak.
        
                                        ...Jim Thompson
--  
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
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Re: Step down voltage to power a single LED

"Joe Gwinn"

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** An NE-2 can reliably pass only about 1mA.

   So 0.5mA for each LED.



   ...   Phil




Re: Step down voltage to power a single LED

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That's the long-term continuous-duty limit.  A NE-2 will pass far more,
to the point of melting, if one allows it.  This is true of all gas
tubes.

Joe Gwinn

Re: Step down voltage to power a single LED

"Andy K"

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** A 56V 400mW zener and a 3.9 kohm 1W resistor should get you there.

 But you must feed the circuit via a bridge rectifier first as the ring tone  
is AC.


...   Phil



Re: Step down voltage to power a single LED
Everybody is making it too complicated I think.  A 250 V or 600V 0.47 uF ca
pacitor would about do it.  (100V would probably work for a while, but why  
tempt fate?)  To protect your LED from back voltage, put a reverse-biased d
iode across it cheap 1N4148 will do (reverse voltage rating is not importan
t here) - or get a bicolor LED that handles AC fine, or just put 2 LEDs bac
k to back!

The series capacitor will limit current to about 5 mA, max, and will only p
ass the 25 Hz AC ringing voltage.

On Friday, August 23, 2013 5:55:19 PM UTC-4, Andy K wrote:
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y phone is ringing.
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A batteries.
own to about 10 volts if that helps. And the bulb still works, but phone ge
ts static. :-)
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 two and some resistors.
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Re: Step down voltage to power a single LED
On Fri, 23 Aug 2013 14:55:19 -0700 (PDT), Andy K

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Why not something as simple as this...

Newsgroups: alt.binaries.schematics.electronic
Subject: Step down voltage to power a single LED (from S.E.D) -
Ring_Detector.png
Date: Tue, 27 Aug 2013 10:47:18 -0700
        
                                        ...Jim Thompson
--  
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
We've slightly trimmed the long signature. Click to see the full one.

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