Incandescent Bulb Question

[MatthewD]

Hi,

My apologies for an off topic question, hopefully the mods will give me a bit of latitude with this one.

My background is I have built a few pedals successfully, but I am a rank beginner with electronics theory knowledge.

My question is... I have a DC power supply hooked up to an incandescent bulb. If I had it set to 30 volts and 1 amp (30 watts), would it produce the same brightness as 3 volts at 10 amps (also 30 watts)

Any info would be appreciated to help me learn, I know the basic calculations for watts etc, I did try to look into this but I found it hard to get a definitive answer via google. For example an incandescent bulbs resistance changes (a lot) as it heats up.

[antonis]

What is bulb's voltage rating..??

Usually, bulbs have marked their voltage and power rating..
(e.g. 230V / 60W or 12V / 5W, etc..)

[Rob Strand]

My question is... I have a DC power supply hooked up to an incandescent bulb. If I had it set to 30 volts and 1 amp (30 watts), would it produce the same brightness as 3 volts at 10 amps (also 30 watts)

In simple terms keeping the bulb power constant is a reasonable approximation for the same brightness. The assumptions here are: same bulb technology and operating at the rated voltage.

However, not all bulbs are made equal.    Different bulbs will operate at different temperatures at their rated voltage and that affects the light output and the light spectrum.   In some instances the bulbs are operated at lower temperatures to increase bulb life.

In addition, small bulbs are often made differently to the mains type bulbs.  The smaller bulbs are largely evacuated whereas the mains bulbs some amount of have noble gas.   Higher power bulbs tend to operate at higher temperatures to squeeze more light out.

Any info would be appreciated to help me learn, I know the basic calculations for watts etc, I did try to look into this but I found it hard to get a definitive answer via google. For example an incandescent bulbs resistance changes (a lot) as it heats up.

When you apply different voltages to a bulb the bulb no longer operates at the same temperature.   The resistance change.  The light output changes.   The light spectrum changes.   It's a whole new ball game.   In short if you have a 12V 100W bulb and reduce the voltage so the power is 30W the light output will be completely different nature to a 30W rated bulb operating at 30W.   In fact if you had a 10000W bulb and operated it at 30W it would not even glow it would be just like a resistor getting warm but no detectable light output to the eye,   

Here's a plot of what happens when you change the voltage from the rated voltage, you can see some vast differences here.




Some time back I posted this info on how to workout what bulb you have.  You probably won't find that info anywhere.

https://www.diystompboxes.com/smfforum/index.php?topic=130433.0

[MatthewD]

Thanks for the replies, for the sake of brevity I left some information out of my question, so I owe you all an elaboration.

When I bought my DC power supply and first went to use it I got a suprise. I had assumed that the supply would have independent control of Volts and Amps. Unfortunately it is rigged so if you adjust one the other changes as well. This is not what I wanted.

So I hooked up a 12v 21w bulb and moved the controls, when it got to 12v it was indeed reading 21 watts.

However one of the reasons I got the supply was to do some amateur experiments with A1 Kanthal wire 24gauge (I chose the length of the filament carefully using an online calculator). I was hoping to answer the question myself by experimenting with different voltage and amps combinations and seeing the results for myself. But because my unit doesn't give me full independent control I am unable to do so.

This is frustrating because when I apply current to my filament and get it up to exactly 9.5amps, every time I do so the voltage is different, 5.7v, 5.6v, 5.3v, it changes each time. I have to believe that this means my unit is inconsistent due to not being able to independently control voltage.

So if I understand the responses because the voltage is different each time the bulb will be different temperatures.

Thanks again, and Rob thanks for the article on checking bulbs.

[PRR]

When I bought my DC power supply and first went to use it I got a suprise.

What exactly did you get and where did you get it?

There ARE supplies with interdependent V and I, and they sometimes look like "bargains" because they aren't much good outside their intended application.

[Rob Strand]

This is frustrating because when I apply current to my filament and get it up to exactly 9.5amps, every time I do so the voltage is different, 5.7v, 5.6v, 5.3v, it changes each time. I have to believe that this means my unit is inconsistent due to not being able to independently control voltage.

So if I understand the responses because the voltage is different each time the bulb will be different temperatures

With a current like 9.5A and inconsistencies of 0.4V, you only need contact resistances varying 0.4V/9.5 = 42 milliohms!     This type of fluctuation can be very normal even for good connectors.  For things like bulb connectors things could be a lot worse.  I'd really be thinking it's that more than the power supply.   You can also get issues where the contacts heat-up and the contact resistance changes.

What people do in this type of circumstance is use Kelvin connections to do the test.   This tends to work better with a constant current source instead of a constant voltage source.   With a constant current source the current source takes-up the extra/varying voltage drops due to the contact resistances.   You measure the bulb/load voltage with a Kelvin connection.  With a voltage source the fluctuating resistances necessarily changes the current so all the numbers change, even with a Kelvin connection - you can see from  I = (V - I*Rcontact) / R_load that a varying Rcontact will mess with I when V is constant.

If you directly measure the PSU terminals under load without the meter points being affected by the load current you might find that the PSU is stable.

These types of problems are very common with sensors.   How to set-up the experiment is a big part of getting the correct results.

Another thing is a power supply will have an output resistance.   If you can characterize the load regulation you can factor in the resistance.   However, if the contact resistance is varying you don't know what it really is and it cannot be factored out.    (If you want to be very clever you can put the sense wire for the PSU's feedback at the load and that will compensate for any resistances - but the PSU can also become unstable.   Look up "remote sense" for power supplies. eg.
https://www.edn.com/power-supply-remote-sense-mistakes-remedies/
)

[antonis]

When I bought my DC power supply and first went to use it I got a suprise. I had assumed that the supply would have independent control of Volts and Amps. Unfortunately it is rigged so if you adjust one the other changes as well.

That's the way of the vast majority variable voltage/current power supplies work..
(current limiting develops at the expense of voltage..)

[antonis]

The smaller bulbs are largely evacuated whereas the mains bulbs some amount of have Nobel gas.

Do gases earn a prize..??

[amptramp]

If you look at various HP audio oscillators including their first piece of test equipment, the Model 200ab, they has a lamp in the cathode of the Wein bridge oscillator to stabilize the output.  Subsequent models like the 200cd had two lamps used for stabilization, connected to the 6SH7 grid to stabilize the output.  These can be found on bama.edebris.com which is a mirror site of Boat Anchor Manual Archive, where boat anchor is a common term for amateur radio receivers, since they tend to be heavy.  Lots of amateur and test equipment data there.

Knight-Kit also used lamps as output stage current limiters in one of their audio amplifiers.  Normally, the lamp resistance was low enough to ignore but if you shorted an output, the lamp would come on and limit the current.

Lamps have a lot of uses that are not associated with the production of light and are sometimes the easiest way to do things.

[ElectricDruid]

This is frustrating because when I apply current to my filament and get it up to exactly 9.5amps, every time I do so the voltage is different, 5.7v, 5.6v, 5.3v, it changes each time. I have to believe that this means my unit is inconsistent due to not being able to independently control voltage.

So if I understand the responses because the voltage is different each time the bulb will be different temperatures.

Yes, and if you keep the *current* the same (as you have done) but not necessarily the temperature, the *voltage* will be different (as you've seen).

As Rob said, the effect you're seeing isn't huge and certainly nothing outside what you'd expect. There's nothing wrong with your power supply.

[Phend]

E=IR or E/I=R  30/1=30  3/10=0.3

[antonis]

E=IR or E/I=R  30/1=30  3/10=0.3

   

[Phend]

[Rob Strand]

Do gases earn a prize..?

I'm sure some of mine could.