Re: Determine Resistance of a Halogen Light

[Jeff Strickland]

Why do you even bother reinventing the wheel, or the light bulb? Use a set
of aligator clips and connect the lamps yo have to a car battery, if they
light they will work, if they don't light they won't work.

Any 120V light will not work in a 12V circuit. I recently had a home owner
that installed a standard light bulb in a low-voltage system, and called me
because the light did not come on. Among various breaks in the wires, I
found the offending bulb. It turns out they (whoever "they" is) make a 12V
bulb for RV applications that use a standard base. Since the bulb she had
fit, she put it in.

 

[James Sweet]

It's more complex than this. A cold filament has a MUCH lower resistance
than a hot filament, so if you measure the lamp with an ohm meter the
results will be nowhere close to actual behavior.

Look at the bulb type in your halogen lights, often it's a 55W H7
halogen lamp, but others are available. If you want to know the exact
draw, wire one up to 12V and measure the draw with the DC Amps range on
your multimeter.

 

[Jeff Strickland]

It's more complex than this. A cold filament has a MUCH lower resistance
than a hot filament, so if you measure the lamp with an ohm meter the
results will be nowhere close to actual behavior.

Look at the bulb type in your halogen lights, often it's a 55W H7 halogen
lamp, but others are available. If you want to know the exact draw, wire
one up to 12V and measure the draw with the DC Amps range on your
multimeter.

It's more fundemental than that. The OP stated he has some halogen lamps,
and INFERS they are household type lamps, but he wants to install them in an
automotive application.

Beyond the inability to accurately evaluate the resistance or current draw
of a cold filiment, one is never able to light a household filiment (120V)
with an automotive (12V) power supply.

 

[Jeff Strickland]

It's more fundemental than that. The OP stated he has some halogen lamps,
and INFERS they are household type lamps, but he wants to install them in
an automotive application.

MY APOLOGIES.
I went back to re-read the original post, and there is no such inference.

 

[James Sweet]

It's more fundemental than that. The OP stated he has some halogen
lamps, and INFERS they are household type lamps, but he wants to install
them in an automotive application.

Beyond the inability to accurately evaluate the resistance or current
draw of a cold filiment, one is never able to light a household filiment
(120V) with an automotive (12V) power supply.

You didn't read the original post very well, he said they were offroad
type lamps, those have been halogen for decades. Headlamps, fog lamps,
driving lamps, they're all halogen, I didn't see anything suggesting in
any way that they are household mains voltage lamps. They're very likely
either 55 or 100W H7 or H1 12V halogen bulbs.

 

[Jeff Strickland]

You didn't read the original post very well, he said they were offroad
type lamps, those have been halogen for decades. Headlamps, fog lamps,
driving lamps, they're all halogen, I didn't see anything suggesting in
any way that they are household mains voltage lamps. They're very likely
either 55 or 100W H7 or H1 12V halogen bulbs.

I agree. Odds favor the 55W variety, but the choices are 55 or 100. I think
they will be H1 or H4, but it doesn't really matter. The housing will define
the lamp type ...

 

[Roy]

From: [email protected] (James Sweet)
Newbie Disclaimer: I am not an electrical engineer, but I am very
interested in the field, so forgive the naive question.
I have a set of halogen lights, like off-road lights for a car. I know
nothing about these lights. I would like to figure out the amperage
they'll put off when connected to a 12V DC electrical system.
So I was thinking all I need is to determine the resistance and then I
could do the I = V/R formula and poof there I go.
I don't have the lights at home with me, but for a quick proof of
concept I grabbed a household light bulb, 120V 60W, regular old bulb. So
I figured out the amperage for that with I = P/V (60W / 120V = 0.5
amps). So to double check those numbers I grabbed a DMM, switched the
range to selector to the OHM symbol, and touched the leads to the
contacts on the light bulb, it finally gets a steady reading of 17.5
OHM. My calculation says that for the above formula to be correct you
would need to have a resistance of 240 OHM.
Is there something that I'm not aware of in all of this? Is a digital
multimeter not the right tool for this? What is the best way to
determine the amperage for these lights? I want to choose the correct
switches, relays and wire gauge for all this.
Thanks for any advice in advance.
Nate

It's more complex than this. A cold filament has a MUCH lower resistance
than a hot filament, so if you measure the lamp with an ohm meter the
results will be nowhere close to actual behavior.

I Don't Think So...Resistance drops as heat increases, hence R rises as
heat decreases/=>

Look at the bulb type in your halogen lights, often it's a 55W H7
halogen lamp, but others are available. If you want to know the exact
draw, wire one up to 12V and measure the draw with the DC Amps range on
your multimeter.
<=\

Roy Q.T. ~ US/NCU ~ E.E. Technician
[have tools, will travel]

 

[[email protected]]

| Beyond the inability to accurately evaluate the resistance or current draw
| of a cold filiment, one is never able to light a household filiment (120V)
| with an automotive (12V) power supply.

Never is a long long time. OTOH, if you reverse the situation, it will light
for a very very short time.

 

[Palindrome]

| Beyond the inability to accurately evaluate the resistance or current draw
| of a cold filiment, one is never able to light a household filiment (120V)
| with an automotive (12V) power supply.

Never is a long long time. OTOH, if you reverse the situation, it will light
for a very very short time.

In this case, "never" is < 7 minutes. Which would be about the time it
would take to take an inverter to the car, together with a household
filament table lamp.

 

[ItsASecretDummy]

If you want to know the exact
draw, wire one up to 12V and measure the draw with the DC Amps range on
your multimeter.
<=\

You sure make it easy for us to "draw" a picture of your capacity and
aptitude for electrical engineering.

Ever heard of a precision shunt, dipshit?

 

[Roy]

If you see responses with long strange email addresses & scary inflamed
names Don't listen to them ....

It's just some escaped Monkey, he may sound smart but he is 57 years
old, neurotic and is only mimcking his human masters.

Oh' We believe he's a racists too.