Why won' this light bulb turn on?

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So I have an MR16 halogen light bulb like this one, 12V 75W, resistive (no LED).  I have a 12V, 10A power supply.  The bulb won't light.  I figure the bulb requires 75/12 = 6.25A when running, though somewhat more when starting up (due to lower resistance when cool).  Oddly, the cool resistance of the filament is measured to be about 24 ohms.  I say oddly because the hot resistance should be 12/6.25 = 1.9 ohms, so the cold resistance should be even lower.  At 24 ohms, the startup current should only be .5A, so the power supply should be adequate.  Nevertheless, the supply comes on for about 40 msec., rises to about 8.3V, then shuts down and repeats every second or so (see scope pic below), looking like it's being overloaded.  I've put a 6A load (as big as I have) on the power supply and it puts out 12V without skipping a beat.  What fundamental principle of electronics am I missing today?

 

Last Edited: Sat. Aug 31, 2019 - 05:22 PM
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Irrespective of the measured 24 ohms... I don't believe it. I think the bulb has an internal short.

 

You might try starting at a lower voltage and see what happens, but those are commodity bulbs: get another and see what happens to it?

 

Neil

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I've tried two bulbs with the same result.  I have one or two more.  Guess I'll try those.

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The inrush might be even higher than what your calculations come up with.  Try Neils suggestions first, THEN maybe try connecting the bulb to a car battery or a linear power supply.  The car battery obviously has the power to handle a surge, but the linear won't shut down as many do not have the protection circuit that SMPS's have.

 

 

JIm

I would rather attempt something great and fail, than attempt nothing and succeed - Fortune Cookie

 

"The critical shortage here is not stuff, but time." - Johan Ekdahl

 

"Step N is required before you can do step N+1!" - ka7ehk

 

"If you want a career with a known path - become an undertaker. Dead people don't sue!" - Kartman

"Why is there a "Highway to Hell" and only a "Stairway to Heaven"? A prediction of the expected traffic load?"  - Lee "theusch"

 

Speak sweetly. It makes your words easier to digest when at a later date you have to eat them ;-)  - Source Unknown

Please Read: Code-of-Conduct

Atmel Studio6.2/AS7, DipTrace, Quartus, MPLAB, RSLogix user

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Aah, yes.  Both bulbs work fine with a car battery.  Plus, I made a four-wire measurement of the resistance with my desktop meter (instead of my handheld as I did initially) and the cold resistance turns out to be .2 ohms, implying 60A of inrush, all of which makes much more sense.  Now, how to deal with it?  I'm leaning toward an NTC thermistor, though these are new to me and the learning curve for the specs is steep.  Still, they're a useful thing to be familiar with.  I already have a power supply that should be adequate, so I'd hate to have to buy another, especially a linear one, which would be much bigger.  Any other ideas?

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Your 24 ohms measurement for a 12V bulb is highly suspect. I think you haven't cleaned the pins - they do accumulate crud.

 

I just measured:

  • MR16 12V / 20W bulb and got about 0.6 ohm
  • MR16 12V / 55W bulb and got about 0.4 ohm

 

In fact these cold resistances are so low that the common electronic transformers supplies incorporate a soft-start circuit to (a) give the electronics a helping hand with the  high current demand,and (b) help extend the filament life by preventing a temperature shock.

 

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 I already have a power supply that should be adequate

More details needed...

 

So, if it is regulating at 12 V, and can handle the 60 A cold start, start-up current, then you don't have to do anything.

 

It will draw lots of current as it starts up, and the current will then (rather quickly) drop off to its steady state current.

 

JC

 

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It's only a 10A supply, but with a properly sized NTC thermistor I could potentially only need anywhere from a few hundred mA up to a few amps.  As the thermistor heats up, its resistance goes down, keeping the current draw within limits until the filament reaches its steady-state resistance.  

 

This is for a projector lamp (not for movies but for illuminating artwork with a very controllable beam) whose transformer has died.  Since the transformer's built into the (expensive) lamp, my plan was to bypass it with an inexpensive switching power supply, which is how I got here.  I just realized, though, that there are 75W-equivalent MR16 LED bulbs, and they probably don't have the same inrush problem.  I think I'll try one of those before going down the thermistor path.

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lautman wrote:
This is for a projector lamp (not for movies but for illuminating artwork with a very controllable beam) whose transformer has died. 

 

I have made Artwork illuminators and I use LED strips, or LED spots as I can easily control them with PWM, and in teh case of RGBW LED's the color to enhance certain items if the Artist desires.  I can then use inexpensive power supplies and no heat to worry about. 

 

THink about it before you start making your life harder/more complicated than it need be.

 

JIm

I would rather attempt something great and fail, than attempt nothing and succeed - Fortune Cookie

 

"The critical shortage here is not stuff, but time." - Johan Ekdahl

 

"Step N is required before you can do step N+1!" - ka7ehk

 

"If you want a career with a known path - become an undertaker. Dead people don't sue!" - Kartman

"Why is there a "Highway to Hell" and only a "Stairway to Heaven"? A prediction of the expected traffic load?"  - Lee "theusch"

 

Speak sweetly. It makes your words easier to digest when at a later date you have to eat them ;-)  - Source Unknown

Please Read: Code-of-Conduct

Atmel Studio6.2/AS7, DipTrace, Quartus, MPLAB, RSLogix user

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> Think about it before you start making your life harder/more complicated than it need be.

 

The optics, case, and mount already exist from the original projector.  Much easier to try to swap out the bulb or add a $1 part than to build something from scratch.

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depending on your application, the color rendering from a led might be problematic. The local home depot (translate to the local variant) should have cheapy 'electronic transformers' designed for the very application. They're like $10 in my end of the world. As mentioned they have soft start to avoid the issue of peak startup currents. As you've found, selection of power supplies is not as simple as you'd first think. Other tricky loads like motors, lamps and things with large capacitance cause problems with the average power supply.

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lautman wrote:
The optics, case, and mount already exist from the original projector.  Much easier to try to swap out the bulb or add a $1 part than to build something from scratch.

 

Understood!

 

Kartman wrote:
The local home depot (translate to the local variant) should have cheapy 'electronic transformers' designed for the very application.

 

Very True.

 

Jim

I would rather attempt something great and fail, than attempt nothing and succeed - Fortune Cookie

 

"The critical shortage here is not stuff, but time." - Johan Ekdahl

 

"Step N is required before you can do step N+1!" - ka7ehk

 

"If you want a career with a known path - become an undertaker. Dead people don't sue!" - Kartman

"Why is there a "Highway to Hell" and only a "Stairway to Heaven"? A prediction of the expected traffic load?"  - Lee "theusch"

 

Speak sweetly. It makes your words easier to digest when at a later date you have to eat them ;-)  - Source Unknown

Please Read: Code-of-Conduct

Atmel Studio6.2/AS7, DipTrace, Quartus, MPLAB, RSLogix user

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If the inrush is too high, your supply likely goes into a fold-back safety mode, maybe until power-down reset.

 

Preset your supply to 12V. Then attach the lamp & start with the current limit turned all the way down & slowly turn it up to the max.  As the current goes up, it will warm the bulb...by the time you crank up the current limit, it won't nee much & will just get to the 12V you set & draw 6 amps.

 

There's a bunch of simple FET inrush circuits you can easily build, which are mostly 1 power fet, an RC & maybe a diode.

 

You can perform some tricks, such as turning your supply on first...then attach the lamp (if the internal caps are huge , it may pass)..Or you can add massive caps and power it up first.  The charged caps act like your "battery"

....in any, case a slow ramp up (inrush limiter) is better for everyone & everything involved   

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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If you turn on the lamp with a rocker switch, replace with a 3-way rocker.
Off - 2 ohm series - 0R series.
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The lamp will warm up with the series resistance. This will give you a gentler start but it might not be gentle enough.
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Surely there are LED MR16 replacement bulbs. What is used in modern projectors?
.
David.

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Off - 2 ohm series - 0R series.

Just make sure it is a 50-100watt resistor, since there will be a brief surge, or the switch gets left in the ohms position.   A NTC (inrush limiter) would be a good choice.

 

A few years back, a friend called about his lamp setup...his relays didn't seem to work very long (he'd gone through several)!!  I found out he was switching a pretty high current bulb (no, a 10A relay won't switch a 10amp bulb) & I showed him what was happening...Then set him him up with a transistor (Mosfet) drive.  Used a current limiting mosfet, which also treats the lamp kindly.

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Sun. Sep 1, 2019 - 05:31 PM
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The spectrum below is of a 4000K 48" LED tube:  Notice the dip around 5000 nm.

 

 

The spectrum below is of a Cree RGBW emitter with the blue turned down and RG and W on full blast.  If I turn off the RGB and just use the W it looks a lot like the 4000K tube.  I turn R and G on full blast because I am using a bunch of lamps like this to illuminate my workbench and want as much light as possible, but without all the blue that is hard on the eyes.  You will notice the big dips that make RGB not render color very well.

 

 

The spectrum below is a standard halogen, probably much like the OP is using.  The green line is a 2900K black body curve.  Most of the halogens I have seen are around 2700 - 2900 K