Low Voltage Low Wattage Incandescent bulb needed.

Thread Starter

Wendy

Joined Mar 24, 2008
22,145
What is the lowest voltage lowest current bulb you know that can be bought? Just looking around, and figured some people might have some good ideas on the subject.

I'm looking at making a very low power Wien Bridge oscillator for a published project, it needs to be extremely available.

So far this is the best I've found...

http://www.bgmicro.com/LIT1017.aspx
 
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GopherT

Joined Nov 23, 2012
8,012
@Wendy

The LM386 datasheet suggests the ELDEMA size bulb (near left margin). 3V at 15 mA.. If you need higher voltage, see the on in the op amp version of Wein bridge. There are smaller but these two bulb sizes seem more accessible. I got some at my local Batteries Plus store. RadioShack used to sell these.

image.jpg image.jpg
 

Thread Starter

Wendy

Joined Mar 24, 2008
22,145
Yeah, Radio Shack was handy for reference parts if nothing else. I had also thought of grain of wheat bulbs as I was going to sleep. Thanks for the input guys.
 

GopherT

Joined Nov 23, 2012
8,012
Why not use LEDs?
The negative temp coefficient of an incandescent bulb is a trick used to automatically control the gain of the sine wave oscillator. The bulb does not generally even light up, just enough resistance to keep everything in the perfect state of dis-equilibrium.
 

bertus

Joined Apr 5, 2008
20,564
Hello,

The wienbridge oscilator uses the NTC-like properties of the lamp for stabalizing the oscillator.
Leds will not work, also because they are direction dependend.

Have a look at chapter 15.7.1 of the attached PDF.

Bertus
 

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bertus

Joined Apr 5, 2008
20,564
Hello,

I stand corrected. I found this in the posted PDF about the behaviour of the lamp:

The current heats up the filament and the resistance increases, lowering the gain.

Bertus
 

cmartinez

Joined Jan 17, 2007
6,992
Incandescent lamps have a positive temperature coefficient of resistance, not negative.
Another source for cheaply available lights would be from old Christmas ornaments ... you could easily disassemble several series and get those bulbs by the thousands. I'm not so sure about the low voltage requirement, though.
 

OBW0549

Joined Mar 2, 2015
3,437
One potential "gotcha" to watch out for when using low-current incandescent lamps in a Wien bridge oscillator circuit: distortion at low frequencies. The lower the oscillator frequency, the more the lamp filament tends to heat and cool in step with the instantaneous value of the output voltage. And the more it heats and cools, the more its resistance varies over the course of each cycle, thus modulating the loop gain of the amplifier and causing harmonic distortion.

Something to consider...
 

cmartinez

Joined Jan 17, 2007
6,992
One potential "gotcha" to watch out for when using low-current incandescent lamps in a Wien bridge oscillator circuit: distortion at low frequencies. The lower the oscillator frequency, the more the lamp filament tends to heat and cool in step with the instantaneous value of the output voltage. And the more it heats and cools, the more its resistance varies over the course of each cycle, thus modulating the loop gain of the amplifier and causing harmonic distortion.

Something to consider...
Would that cause a resonant effect in the circuit as well? Or is harmonic distortion a result of exactly that sort of thing?
 

OBW0549

Joined Mar 2, 2015
3,437
Would that cause a resonant effect in the circuit as well? Or is harmonic distortion a result of exactly that sort of thing?
I don't think it would cause a resonance, but rather the amplitude control loop would become unstable below some critical frequency. I think. It would "blurp" instead of oscillate. Or maybe "bloop"...
 

cmartinez

Joined Jan 17, 2007
6,992
I don't think it would cause a resonance, but rather the amplitude control loop would become unstable below some critical frequency. I think. It would "blurp" instead of oscillate. Or maybe "bloop"...
So after accumulating charge in some component of the circuit it would perform some sort of unpredictable electronic burp?
 

ian field

Joined Oct 27, 2012
6,539
What is the lowest voltage lowest current bulb you know that can be bought? Just looking around, and figured some people might have some good ideas on the subject.

I'm looking at making a very low power Wien Bridge oscillator for a published project, it needs to be extremely available.

So far this is the best I've found...

http://www.bgmicro.com/LIT1017.aspx
The smallest is probably a "grain of wheat" bulb that were once common in cheap LCD wristwatches. Even though they only operated when you press one of the buttons, they had to be low power to not kill the single button cell.

48V wedge-end telecoms bulbs enjoyed a period of popularity in wein bridge oscillators. AFAICR: HP used a 117V bulb rated somewhere around 7W or so, but voltage wasn't much of an issue in those old tube designs.

Some designs specified an RA-53 thermistor, which is also peculiar to telecoms equipment.
 

OBW0549

Joined Mar 2, 2015
3,437
So after accumulating charge in some component of the circuit it would perform some sort of unpredictable electronic burp?
There is no accumulation of charge; rather, the heating and cooling of the lamp modulates the loop gain of the oscillator drastically over the course of each cycle, and at some point it would just become unstable and start-stop-start-stop, etc. (what I used to call "motorboating").
 

ian field

Joined Oct 27, 2012
6,539
There is no accumulation of charge; rather, the heating and cooling of the lamp modulates the loop gain of the oscillator drastically over the course of each cycle, and at some point it would just become unstable and start-stop-start-stop, etc. (what I used to call "motorboating").
If the frequency of oscillation is slow enough; it can modulate the temperature of the thermistor (bulb) just the same as amplitude variations.
 

OBW0549

Joined Mar 2, 2015
3,437
FWIW, I don't think you really need to go out of your way to find a very low voltage lamp.

A tungsten filament is going to begin to glow at somewhere around 10%-20% of its rated voltage, and from measurements I just took on a #7219 lamp (12V, 60mA) it appears the point of greatest control sensitivity (that is, the highest dR/dV) occurs just below the point of incandescence, with about 810 mV across the lamp. Above that, the rate of resistance change with voltage drops off, I assume due to thermal conduction and radiation.

So from this, I would conclude that any lamp with a 12V rating or lower should suffice.
 
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OBW0549

Joined Mar 2, 2015
3,437
If the frequency of oscillation is slow enough; it can modulate the temperature of the thermistor (bulb) just the same as amplitude variations.
That's exactly the point I was making in post #11, though we're saying it in slightly different ways.
 

#12

Joined Nov 30, 2010
18,210
Same thing in different words, again:

Every physical mass has a thermal time constant. A thermal time constant for a thermometer is defined as the time required for the temperature of the thermometer's mass to change by 50% of the difference in temperature it is exposed to. It's a sort of successive approximation where 5 or 10 time constants is considered, "close enough". I have a stainless steel thermometer with a time constant of about 20 seconds. I have a type K thermocouple with a time constant of about 3 seconds. How fast does a light bulb come on after you flip the switch? When the energy changes of an oscillator get very slow, the temperature of the mass can change detectably in 1/2 cycle.

In this case, the temperature of the filament doesn't have to move very far to change the gain of the amplifier. Even if the time constant of the filament is as slow as 1/10th of a second, the change in temperature starts measurably affecting the gain in less than a tenth of a second. If you are trying to work with audio frequencies, which allegedly include 20 Hz, you are in the danger zone, so to speak. If you are measuring audio distortion in the range of 0.01% (which modern amplifiers can achieve) this kind of oscillator will be easily detected.
 
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