# 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

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:

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

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:

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

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:

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:

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

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

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

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:

* Yes, approximately.

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

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     |
We've slightly trimmed the long signature. Click to see the full one.
Re: Step down voltage to power a single LED

"Joe Gwinn"

** 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

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"

** 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:

y phone is ringing.

A batteries.
own to about 10 volts if that helps. And the bulb still works, but phone ge
ts static. :-)

two and some resistors.

Re: Step down voltage to power a single LED
On Fri, 23 Aug 2013 14:55:19 -0700 (PDT), Andy K

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     |