Measuring or calculating current by using light bulbs as a load
Heinrich Raman
current by using a bulb as a load
Power: 6VDC 25A
12VDC 6A
24VDC 3A
Thanx in advance
Ron Dozier
Heinrich Raman <hein...@nac.ac.za> wrote:
>Can anyone give me more information on how to measure or calculate the
>current by using a bulb as a load
You can't use a bulb as the sensing element because it is in essence a
curent dependent resistor.
To measure the I-V characteristcs of the bulb, you need to measure the
current through the device and the voltage across the device. This means
connecting two wires to each terminal of the lamp.
One pair to a voltmeter. The other pair to a resistor/ammeter in series
with a power supply. Adjust V, read V on meter, and measure I (ammeter or
from I=V/R) where V in the equation is the voltage drop across the
resistor.
Your current sensing device may be a resistor that has the availavle
power requirements. P=I^2*R or a DC ammeter. The DC ammeter has an
internal resistance which the 4 terminal method (Kelvin connections)
eliminate.
At 25 amps and low voltages, this method is mandatory.
Tom Maier
>In article <9397251...@dull.nac.ac.za>,
>Heinrich Raman <hein...@nac.ac.za> wrote:
>>Can anyone give me more information on how to measure or calculate the
>>current by using a bulb as a load
You can use a device like that as a sensor, but
it is nonlinear. More than likely data sheets on the
filiment resistance vs. heat are not available for a
specific bulb, so you need to ...
>To measure the I-V characteristcs of the bulb, you need to measure the
>current through the device and the voltage across the device. This means
>connecting two wires to each terminal of the lamp.
... collect the data about that model of bulb and create
a graph of its characteristics, like you mention above.
Then the device could be used by looking up the
data on the nonlinear chart you have created or by
making a circuit that linearizes this response to make
it have a directly readable output. Linearization can be done
either by using opamp circuits or by using a microprocessor
that reads in the value and looks it up in an internal table
that was built from the response chart that you made.
This method of not using theory to find the answer, but
rather just measuring the data and making a chart is called
an empirically derived solution. It works well with nonlinear
systems like this example.
Each model of bulb might behave differently, so you
would need to make an empirical chart of one and stick to
that model. The other thing you would have to check is to
see how wide the variation is from one bulb to the next of
this same modle number, if you are going to use this
design with more than one bulb.
Nonlinear sensors can be used, but they are an extra
pain in the butt.
Tom
Foxy
<hein...@nac.ac.za> wrote:
>Can anyone give me more information on how to measure or calculate the
>current by using a bulb as a load
>
>
Steve
>
> Can anyone give me more information on how to measure or calculate the
> current by using a bulb as a load
>
> Power: 6VDC 25A
>
> 12VDC 6A
>
> 24VDC 3A
>
> Thanx in advance
The short answer is you can't. Filament lamps have hugely varying
resistances due to temperature dependence. Measure the cold and hot
resistance by ohmmeter cold and by currents/voltage hot and see what you
get. If the lamp didn't do that it would explode immediately. Resistance
usually goes up with temperature, and in tungsten it does so immensely.
-Steve
--
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-Electronics Site!! 1000 Files/50 Dirs!! http://www.armory.com/~rstevew
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Al, N2NKB
a light bulb for measuring true RMS ac voltage.
The design takes into account the bulb's non-linearity's.
So it can be done, although quite challenging.
Perhaps a more modern way to do this is to calibrate your unit
using a lookup table.
Al
Steve wrote in message <38039F...@armory.com>...
Al, N2NKB
Beckman, scale is from 105 v to 125 v, so midscale is 115 v.
Much more practical for monitoring line voltage than an analog
0 - 150v meter.
Al
Al, N2NKB wrote in message <7u0gu4$4cr$1...@bgtnsc01.worldnet.att.net>...
James Meyer
>Heinrich Raman wrote:
>>
>> Can anyone give me more information on how to measure or calculate the
>> current by using a bulb as a load
>>
>The short answer is you can't. Filament lamps have hugely varying
>resistances due to temperature dependence. Measure the cold and hot
>resistance by ohmmeter cold and by currents/voltage hot and see what you
>get. If the lamp didn't do that it would explode immediately. Resistance
>usually goes up with temperature, and in tungsten it does so immensely.
>-Steve
In fact, over a fair voltage range and with fair accuracy, an
incandescent bulb approximates a constant current device.
Jim
Craig Hart
Hmm.. you want to know the current, but you give the current in your
figures?
since you mention the light globe, perhaps you want to know the wattage?
Now, since P=I*V (for DC)
6v25A = 150w
12v6a=72w
24v3a=72w
Craig
Heinrich Raman <hein...@nac.ac.za> wrote in message
news:9397251...@dull.nac.ac.za...
> Can anyone give me more information on how to measure or calculate the
> current by using a bulb as a load
>
>
William Janssen
doing the test. Then put a ajustable amount of power (DC) into the lamp
and adjust for the same light output.
Bill K7NOM
Steve wrote:
>
> Heinrich Raman wrote:
> >
> > Can anyone give me more information on how to measure or calculate the
> > current by using a bulb as a load
> >
> > Power: 6VDC 25A
> >
> > 12VDC 6A
> >
> > 24VDC 3A
> >
> > Thanx in advance
> -----------------------
> The short answer is you can't. Filament lamps have hugely varying
> resistances due to temperature dependence. Measure the cold and hot
> resistance by ohmmeter cold and by currents/voltage hot and see what you
> get. If the lamp didn't do that it would explode immediately. Resistance
> usually goes up with temperature, and in tungsten it does so immensely.
> -Steve
No SPAM
> Can anyone give me more information on how to measure or calculate the
> current by using a bulb as a load
> Power: 6VDC 25A
> 12VDC 6A
> 24VDC 3A
The tungsten in the filament has a positive tempco, so the resistance is
dynamic and changes with temperature and with voltage.
> Thanx in advance
Keith Wootten
>Can anyone give me more information on how to measure or calculate the
>current by using a bulb as a load
>
>
>Power: 6VDC 25A
>
> 12VDC 6A
>
> 24VDC 3A
>
>
Pass the current through a suitable DC motor. Current is proportional
to torque. Measure the torque using an appropriately sized lever with
an appropriate number of bulbs tied to the end as a load. Calculate the
current using the appropriate factors.
Cheers
--
Keith Wootten