12 V Lamp On 240 V

"Bob Monsen" <rcmonsen@gmail.com> wrote in message
news:KK%1j.24352$JD.21058@newssvr21.news.prodigy.net...
"ian field" <dai.ode@ntlworld.com> wrote in message
news:IeX1j.31839$dN2.17700@newsfe6-gui.ntli.net...
A LED lends itself to the "wattless dropper" trick, at the mains
frequency the impedance of a capacitor is much higher than the load so it
acts similar to a constant current source. The capacitor must be rated
for mains - at

He can do this with his 12V incandescent. Use a 0.68uF cap and a 10 ohm 5W
resistor in series with the 240V and the bulb. As you pointed out, the cap
must be rated for 400VAC, which means it won't be cheap.

If he uses an LED, he'll need another diode in parallel, backwards, and
probably a zener in parallel to prevent overvoltage. The incandescent
doesn't care about this, so it may be a better choice.

As others have pointed out, the best and cheapest choice may be a glue gun
and a neon indicator bulb which is rated for 240VAC.
Click to expand...
There's a simple formula for driving filaments that more or less holds true
for lower voltages - one of the UK TV makers used the "wattless dropper" for
the 300mA heater chain, the capacitor was 4.3uF which by dividing the 300mA
down to the required current and then dividing the capacitance by the same
amount gives the value required.
 
On Sat, 24 Nov 2007 04:15:52 -0800, Dave.H wrote:

I need to operate a 12 volt AC/DC filament indicator lamp on 230/250
volt mains. I'm guessing a resistor would do the trick. If so, what
value?
Click to expand...
Use a transformer. Or a 240V lamp.

Using a resistor will result in a 20-fold increase in power consumption.
Also, if the lamp fails open (which it will eventually), it will have to
withstand the entire 240V.
 
"Nobody" <nobody@nowhere.com> wrote in message
news:pan.2007.11.24.21.59.36.171897@nowhere.com...
On Sat, 24 Nov 2007 04:15:52 -0800, Dave.H wrote:

I need to operate a 12 volt AC/DC filament indicator lamp on 230/250
volt mains. I'm guessing a resistor would do the trick. If so, what
value?

Use a transformer. Or a 240V lamp.

Using a resistor will result in a 20-fold increase in power consumption.
Also, if the lamp fails open (which it will eventually), it will have to
withstand the entire 240V.
Click to expand...
That never bothered the radio manufacturers who made AC/DC sets with series
heater chains and the dial lamps in series with the heater chains.
 
ian field wrote:
"Nobody" <nobody@nowhere.com> wrote in message
news:pan.2007.11.24.21.59.36.171897@nowhere.com...
On Sat, 24 Nov 2007 04:15:52 -0800, Dave.H wrote:

I need to operate a 12 volt AC/DC filament indicator lamp on 230/250
volt mains. I'm guessing a resistor would do the trick. If so, what
value?

Use a transformer. Or a 240V lamp.

Using a resistor will result in a 20-fold increase in power consumption.
Also, if the lamp fails open (which it will eventually), it will have to
withstand the entire 240V.


That never bothered the radio manufacturers who made AC/DC sets with series
heater chains and the dial lamps in series with the heater chains.
Click to expand...

????? The rectifier tubes were designed with a tap for a #47 lamp.
They didn't use a string of resistors simply to light the pilot lamp.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida
 
"Nobody" <nobody@nowhere.com> schreef in bericht
news:pan.2007.11.24.21.59.36.171897@nowhere.com...
On Sat, 24 Nov 2007 04:15:52 -0800, Dave.H wrote:

I need to operate a 12 volt AC/DC filament indicator lamp on 230/250
volt mains. I'm guessing a resistor would do the trick. If so, what
value?

Use a transformer. Or a 240V lamp.

Using a resistor will result in a 20-fold increase in power consumption.
Also, if the lamp fails open (which it will eventually), it will have to
withstand the entire 240V.
Click to expand...
Hmm... That's a misunderstanding. As long as the lamp is good, the resistor
has to withstand almost the entire 240Vac. To be precise, 240-12=228Vac. It
also has to dissipate the 12W mentioned by me and others. Once the lamp
fails open (as incandescents usually do) the resistor feels neither voltage
nor current.

petrus bitbyter
 
The current neon bulb style is NE-2. I can buy these from Dick Smith
for around 45 c, and a 270k resistor for 6 c
.. I can probably borrow a glue gun from my father. I think thats the
simplest and cheapest way to do it.
 
"Dave.H" wrote:

I need to operate a 12 volt AC/DC filament indicator lamp on 230/250
volt mains. I'm guessing a resistor would do the trick. If so, what
value?
Click to expand...
A resistor would be a dreadful choice for a whole host of reasons, not least the
fact that you'll create 20W of waste heat for every 1W of lamp rating.

Graham
 
On Sun, 25 Nov 2007 02:09:56 +0100, petrus bitbyter wrote:

I need to operate a 12 volt AC/DC filament indicator lamp on 230/250
volt mains. I'm guessing a resistor would do the trick. If so, what
value?

Use a transformer. Or a 240V lamp.

Using a resistor will result in a 20-fold increase in power consumption.
Also, if the lamp fails open (which it will eventually), it will have to
withstand the entire 240V.

Hmm... That's a misunderstanding. As long as the lamp is good, the resistor
has to withstand almost the entire 240Vac. To be precise, 240-12=228Vac. It
also has to dissipate the 12W mentioned by me and others. Once the lamp
fails open (as incandescents usually do) the resistor feels neither voltage
nor current.
Click to expand...
I meant that the (12V) lamp will have to withstand 240V, as the resistor
won't be dropping any of it if the lamp isn't drawing any current.

This may not be a problem, but there's no easy way to tell. Lamps aren't
designed to be used this way, so the only voltage figure likely to be
given is the one which produces the desired power. There isn't likely to
be a separate insulation breakdown voltage quoted for the (unlikely) case
where an open filament results in a much higher voltage.
 
Dave.H wrote:
Thanks. Found a 4.7k 5 watt, wirewound, ceramic cased resistor on the
Dick Smith website.

Read my post properly. You need a resistor of more than twice that power.
Click to expand...

Rob.
 
As I stated in my previous post, I'm sticking with neon lamps. The
existing resistor has testing fine, so now I just have to pay a whole
44 cents for an NE-2 lamp. Cheap and easy enough.
 
"Nobody" <nobody@nowhere.com> wrote in message
news:pan.2007.11.25.09.22.09.316507@nowhere.com...
I meant that the (12V) lamp will have to withstand 240V, as the resistor
won't be dropping any of it if the lamp isn't drawing any current.
Click to expand...
If the lamp filament opens there will be no current flowing.
The resistor is in series with the lamp, so no current means
no voltage drop across the resistor.

This may not be a problem, but there's no easy way to tell. Lamps aren't
designed to be used this way, so the only voltage figure likely to be
given is the one which produces the desired power. There isn't likely to
be a separate insulation breakdown voltage quoted for the (unlikely) case
where an open filament results in a much higher voltage.
Click to expand...
Yes, there will be the the full 240V at the lamp socket when the
lamp opens or it is removed. This could be a shock hazard
depending upon the design of the lamp socket.

Lamps are designed to run at a specified voltage and current.
No more, no less. How one achieves the correct operating
conditions is down to practicality, safety, and common sense.
 
"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:4748C755.39908708@earthlink.net...
ian field wrote:

"Nobody" <nobody@nowhere.com> wrote in message
news:pan.2007.11.24.21.59.36.171897@nowhere.com...
On Sat, 24 Nov 2007 04:15:52 -0800, Dave.H wrote:

I need to operate a 12 volt AC/DC filament indicator lamp on 230/250
volt mains. I'm guessing a resistor would do the trick. If so, what
value?

Use a transformer. Or a 240V lamp.

Using a resistor will result in a 20-fold increase in power
consumption.
Also, if the lamp fails open (which it will eventually), it will have
to
withstand the entire 240V.


That never bothered the radio manufacturers who made AC/DC sets with
series
heater chains and the dial lamps in series with the heater chains.


????? The rectifier tubes were designed with a tap for a #47 lamp.
They didn't use a string of resistors simply to light the pilot lamp.
Click to expand...
In Europe it was common practice to save the cost and weight of a
transformer by having all the tubes with equal current rating instead of
equal voltage so they could be wired all in series, if a panel lamp was
required this would also be added to the series chain with the same current
rating as all the tube heaters. These were generally known as AC/DC sets and
were easy to spot when first switched on, as the bulb filament had much
lower thermal inertia than the tube heaters it would light overbright for a
split second as the heaters got up to temperature - so lamp failure was far
more common than heater failure and the blown lamp would have the full
240VRMS across it.
--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida
Click to expand...
 
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