Clueless beginner's AC dimmer circuit

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Is it possible to dim an AC lamp with a single resistor?

I want to dim the tiny lamp that lights the ring surrounding the on/
off switch on a small halogen table lamp.

The resistance across the (small) lamp measures 1 ohm (on multimeter
set to 200 ohms). Can I put a resistor in series to dim it, say
50-75%? How do I figure out what the resistance should be, and how
many watt resistor to use?

If this was a 12V DC circuit, I could deal with it, but AC, I dunno
what's happening there!

All the dimmer circuits I see are more complex. I don't need it to be
variable, and I don't need a delay. Nothing fancy. The light lights up
the whole bedroom at night and it drives my wife nuts. I have it
disconnected now, but it would be nice to have an indicator so I can
see the switch in the dark, but not a BEACON!

thanks to anyone who will illuminate this for me...

(I just want to see if this Omega shows up ... Ω)

 

[John Fields]

Is it possible to dim an AC lamp with a single resistor?
---
Yes.
---

I want to dim the tiny lamp that lights the ring surrounding the on/
off switch on a small halogen table lamp.
---
OK
---

The resistance across the (small) lamp measures 1 ohm (on multimeter
set to 200 ohms). Can I put a resistor in series to dim it, say
50-75%? How do I figure out what the resistance should be, and how
many watt resistor to use?

---
Well, now we've got a problem!

If that little lamp is incandescent, then it'll exhibit about a 10:1
change in resistance from when it's cold to when it's hot, so if you
measure 1 ohm cold that means it'll be about 10 ohms hot.

Then, if it's being driven by 120V mains, it'll be dissipating:

E² 120V²
P = --- = ------- = 1440 watts
R 10R

which is a little high for a pilot light, so there must be something
else going on in there.

What more can you tell us about the _entire_ lamp?

 

[John Popelish]

Is it possible to dim an AC lamp with a single resistor?

I want to dim the tiny lamp that lights the ring surrounding the on/
off switch on a small halogen table lamp.

The resistance across the (small) lamp measures 1 ohm (on multimeter
set to 200 ohms). Can I put a resistor in series to dim it, say
50-75%? How do I figure out what the resistance should be, and how
many watt resistor to use?

If this was a 12V DC circuit, I could deal with it, but AC, I dunno
what's happening there!

All the dimmer circuits I see are more complex. I don't need it to be
variable, and I don't need a delay. Nothing fancy. The light lights up
the whole bedroom at night and it drives my wife nuts. I have it
disconnected now, but it would be nice to have an indicator so I can
see the switch in the dark, but not a BEACON!

thanks to anyone who will illuminate this for me...

(I just want to see if this Omega shows up ... Ω)

The concept of RMS volts and amperes (what you measure with
your meter (if the AC is sinusoidal, not narrow pulses or
something else) were created to allow you to use the same
rules for AC as you do for DC. So the volts and amps you
measure with an AC meter can be used in Ohm's law
calculations for resistors just as if they were DC voltages
and currents.

I suspect that the 1 ohm lamp has a resistance more like 10
ohms when it is hot. Dopes the lamp have a transformer that
steps the AC line voltage down to 12 volts or something like
that?

I would start with 1 to 10 ohms in series as a trial, but
keep in mind that such a simple dimmer may give off as much
heat as the bulb it is dimming, so it might take a several
watt rated resistor.

 

[Stephen J. Rush]

Is it possible to dim an AC lamp with a single resistor?

I want to dim the tiny lamp that lights the ring surrounding the on/
off switch on a small halogen table lamp.

The resistance across the (small) lamp measures 1 ohm (on multimeter
set to 200 ohms). Can I put a resistor in series to dim it, say
50-75%? How do I figure out what the resistance should be, and how
many watt resistor to use?

If this was a 12V DC circuit, I could deal with it, but AC, I dunno
what's happening there!

All the dimmer circuits I see are more complex. I don't need it to be
variable, and I don't need a delay. Nothing fancy. The light lights up
the whole bedroom at night and it drives my wife nuts. I have it
disconnected now, but it would be nice to have an indicator so I can
see the switch in the dark, but not a BEACON!

A quick-and-dirty way to dim an incandescent lamp fed with AC is to put a
diode in series. 1-amp 50V rectifiers are easy enough to find. That
measured resistance sounds awfully low, even for the cold resistance of an
incandescent pilot lamp.

 

[PhattyMo]

A quick-and-dirty way to dim an incandescent lamp fed with AC is to put a
diode in series. 1-amp 50V rectifiers are easy enough to find. That
measured resistance sounds awfully low, even for the cold resistance of an
incandescent pilot lamp.

I would use a rectifier rated for more than 50V,if it's on the mains
side wiring.
a 1N4007 should do well (1A 1000V),and they're common and cheep.

 

[Brendan Gillatt]

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(I just want to see if this Omega shows up ... Ω)

I see it - I would think that it's a problem with some readers using a
non-UTF font or just not supporting unicode at all. Unicode support in
text boxes on the web (think google groups) is probably not great. Using
'ohms' would probably be a good idea.

- --
Brendan Gillatt
brendan {at} brendangillatt {dot} co {dot} uk
http://www.brendangillatt.co.uk
PGP Key: http://pgp.mit.edu:11371/pks/lookup?op=get&search=0xBACD7433
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