Step down voltage to power a single LED

[Andy K]

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.

 

[Phil Allison]

"Andy K"

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

** 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

 

[Robert Baer]

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

Once phone is off hook, the voltage is so low that a neon bulb CANNOT
ionize.
The qualifier is meaningless.

 

[Andy K]

Once phone is off hook, the voltage is so low that a neon bulb CANNOT

ionize.

The qualifier is meaningless.

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.

 

[Robert Baer]

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

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.

 

[Robert Baer]

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 ?

* Yes, approximately.

 

[Phil Allison]

"Joe Gwinn"

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.

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

So 0.5mA for each LED.



... Phil

 

[Phil Allison]

<[email protected]>

Everybody is making it too complicated I think. A 250 V or 600V 0.47 uF
capacitor 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
diode across it cheap 1N4148 will do (reverse voltage rating is not
important here) - or get a bicolor LED that handles AC fine, or just put 2
LEDs back to back!

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

** At the beginning of each ring tone, the current will be much higher than
your figure and could damage the LED - so some series resistance needs to
be added, say 2.2 kohms.

To get 20 mA average for the LED means using a bridge rectifier and a cap of
about 1.5uF as well.


..... Phil

 

[Robert Baer]

Joe Gwinn wrote:

But it's an AC signal, so won't the arc tend to self-extinguish?

Yes, as long as the plasma current was not high enough to have
residual ions present when the next excitation voltage happened.