6v, 24v Bulbs, trying to figure out current draw..

[momo]

I have this collection of bulbs that I want to use.
My Valvecaster project would enjoy two of these.
Sending DCv as I dont have a variable AC source..
The top ones are 6v, bottom I figure 24v since I tired them up to that.
I will be using a 14v 1A transfo that drops about 1-2 volts once loaded with these bulbs.
I know it does not say much, but I'm having difficulty metering current on this.
Some of them are identified, I see 300mA on some....
I'm loading the box with a Supreaux Deux and a ValveCaster. Not too concerned about current draw unless someone here can say they pull too much..
:-)
The little white ones bottom right are the brightest with the transfo.

[GibsonGM]

Can't you measure the current draw with your DMM?  It should work ok, if your DMM is functioning correctly...it would be good to know for your build.

[momo]

I have tried two dmm, one seemed to have the current jump around, it has an auto mode for current.
The other dimmed the bulbs and had much current going thru the setup, I know this as the small tie wires started to smoke the plastic covering....
:-)

[Rob Strand]

First up, if you apply the wrong voltage to a lamp it will burn out in a very short time.  Even testing with the wrong voltage will shorten its life.

There's two unknowns: 
- the rated voltage, and
- the current (or power).

You need to measure current to determine those.   The best idea is to work out how to use the current range on your meter.   Some meters let you turn off autoranging or force the range.   That might prevent the range jitter.

Another way to measure current is to place a resistor in series with the bulb, then measure the voltage across the added resistor.   From that you can work out the lamp current I = V/R = "Voltage across the added resistor" / R.

The added resistor will cause a voltage drop so you want it to be relatively small, say 1 ohm.  Maybe aim at 200mV to 1V drop.

I'm not 100% sure what equipment you have.
Do you have a variable DC power supply?

Procedure:
1) A starting point is to measure the DC resistance of the bulb.
   Set the meter to 200 ohm.
   Short the meter terminals together to measure the meter lead resistance, around 0.1 to 1 ohm.
   Measure the bulb resistance.
   Get the true bulb resistance by subtracting the meter lead resistance.
   I'll refer to the cold bulb resistance as Rcold.

2) The aim of the game is to adjust the voltage across the lamp so the lamp resistance
    is about 10 to 12 times the cold resistance Rcold.
    (In general for small lamps it can be anything from 7 to 13 times.)

    To measure the lamp resistance:
    - Set the voltage across the lamp to say 3V
      You can try standard voltages like 3V, 4.5V, 5V, 6.3V, 10V, 12V, 14V, 16V, 20V, 24V.
      You can skip a lot of the lower voltages by setting the voltage to something that makes
      the lamp reasonably bright.  Keep in mind many small lamps are only orange with the rated voltage.
    - measure the current through the lamp
    - Calculate the lamp resistance  Rhot = V_lamp / I_lamp
      where V_lamp is the voltage across the lamp and I_lamp is the current through the lamp.
     (If you use the 1 ohm series resistance then you need to use the method above to work out the lamp current.)
    - If Rhot is below 10 to 12 times Rcold increase the lamp voltage to the next standard voltage.
      If Rhot is too high drop the voltage.

Once you do that, from the voltage and current you can calculate the bulb power.
For the smaller bulbs on the bottom row you might expect 0.5W to 1W (extremes maybe 0.2W to 1.2W)
For the larger bulbs you might expect 2W to 3W (some can be a lot lower).

You might be able to use the power rating to get a better guess at the rated voltage.  It's quite difficult to work out the difference between a 12.6V and 13.2V rated globe since you are matching a range for Rhot.   A lower voltage is obviously safer.

Another cross-check is to work out what voltage the lamp *barely* starts glowing.  I mean barely like a small glow in a dark room.  Measuring the current and resistance at this point is also useful.

[momo]

Thanks much for all this.
It will take a bit of time to sort this out, but thanks, this is how I will learn about these bulbs.
I have a variable dc supply up to 24v. I would slowly ramp up the voltage.
My dc meters, I have 3 , they drive me nuts to test current.
Like everyone else I have to switch the probes on the meter to read current, and then of course I forget to put it back for voltage and don't realize.......
:-)

My dad has a 1974 oscilloscope, functional that I can have, the thing is though, the old caps....

[Rob Strand]

Thanks much for all this.
It will take a bit of time to sort this out, but thanks, this is how I will learn about these bulbs.
I have a variable dc supply up to 24v. I would slowly ramp up the voltage.
My dc meters, I have 3 , they drive me nuts to test current.
Like everyone else I have to switch the probes on the meter to read current, and then of course I forget to put it back for voltage and don't realize.......
:-)

My dad has a 1974 oscilloscope, functional that I can have, the thing is though, the old caps....

No problem.  What I posted before should work with that set-up.

You can make life easier using one meter to monitor the voltage across the lamp and another to measure the current (or voltage across the current measuring resistor).