DO TUNGSTEN LAMPS BURN OUT WHEN YOU TURN THEM ON??

Years ago, I ran a test to see how many times a lamp could be turned on/off before failure. A computer turned the lamp on for one second and then off for one second, and repeated the sequence until the lamp failed (determined with a sensor). When the computer was connected to a mechanical relay, which most closely modeled a regular light switch, the lamp would fail after a few hundred cycles to a few thousand cycles; however, when the computer was connected to a SSR (solid state relay) with zero voltage crossing detection, the lamp lasted longer than the test (several days).

The test seemed to prove that if you turned a lamp on exactly during the crossover part of the A.C. sine wave. Remember that in an A.C. circuit, the voltage starts at zero volts, climbs to about 190 volts positive (120 volts A.C. is the root mean square of the voltage, not the peak voltage), drops to zero volts, continues dropping to about 190 volts negative and then climbs again. In any case, the sine wave crosses the zero point 120 times per second for those of us who have 60 cycle power. and 100 times per second for those who have 50 cycle power. Using a device, such as a solid state relay or dimmer control that has zero voltage crossover detection should give you the maximum life of the bulb, all other things being equal.

Mike

- Please excuse my imcomplete sentence. An old brain tends to lose track of the important parts of a sentence.

that the lamp lasted much longer than if the lamp were turned on randomly during the A.C.sine wave.

On 10/18/2003 12:06 PM Richards spake thus:

Do you happen to know the circuit one would use to implement this? I wonder if this could be done cheaply enough with simple components (e.g., triacs, SCRs, whatever) to make it worthwhile to build a little box for each bulb whose live one wishes to extend.

There are several low-tech approaches:

1.) Just stick a suitable rheostat or potentiometer in series with the bulb.

2.) Stick a big diode in series with the bulb, and short out the diode when full brightness is required.

3.) Fancy switch to put the bulbs in series when full output is not required and in parallel when full output is required.

4.) Bite the bullet and use Quartz-Halogen bulbs. Easier to color correct for color anyhow, I suspect.

If you want to go the higher tech route, you can drive the bulb from a constant current source, where the current is about the amount required to give it normal full power. This protects the bulb from the starting surge.

A low-tech version of this high-tech route is to find a ferroresonant "constant voltage" transformer whose output rating is equal to or a bit higher than the set of bulbs you want to power. These are large and heavy; they weight about 70 pounds per 1000 W of output. They used to be expensive, but I've found a number of them selling for very little in surplus places. They were typically used to power expensive server computers, and those have largely gone the way of the dodo, surplusing the power conditioning box.

Anyway, constant voltage transformers have a couple of interesting behaviours. When the input line voltage varies +-15% from its nominal value, the output of the transformer varies only 2-3%, keeping your lights at constant brightness and colour temperature. (That's where the "constant voltage" label comes from.)

They *also* have inherent current limiting. No matter how heavy a load you connect to their output, including a dead short, the output current is limited to about twice the nominal rating of the transformer. The transformer will hum loudly, but it won't burn up or be damaged by a short of any reasonable duration.

A 500 W transformer is rated to deliver about 4 A at normal voltage, and it will never put out more than about 8-10 A. This is considerably less than a 500 W halogen lamp will draw when cold, so if the lamp is plugged into a 500 W CV transformer, the transformer will provide automatic current limiting that pampers the lamp.

Now, the CV transformer I'm talking about has no active parts other than a transformer and one or more capacitors. There are no electronics. There's another type of voltage-regulating transformer that uses electronics to switch among several different transformer taps as the line voltage changes. This protects computers agains brownouts too, but it's not very useful for powering lamps. It regulates voltage only in large steps, and it has no inherent current limiting. You don't want a tap-switching regulator for this.

Dave

Seriously OT, but these devices are of considerable conceptual interest. They are in essence "parametric oscillators" (at least some of them, the ones I know about are).

The core of the device is not actually a transformer, but rather a big inductance that looks much like and is physically made like a big iron transformer. This inductance, which would be essentially the output winding of the transformer if it were a transformer, is paired with an equally big capacitance to make an LC circuit resonant at 60 Hz (for U.S. versions anyway).

The input windings on the transformer are arranged such that they induce no voltage in the output winding, for example by counter-winding two series-connected input coils on the input leg of the transformer so they cancel each other. Instead of inducing an output voltage, the input current merely saturates the iron underneath these coils, thus time-modulating the L value in the resonant circuit -- and note that this happens at twice the input frequency, i.e. at 120 rather than 60 Hz, since current in either direction tends to saturate the iron.

So, you have an LC circuit resonant at 60 Hz, with one of its reactive elements modulated in value at 120 Hz, and that's the essential condition to have a so-called "degenerate parametric oscillator". If the modulation is strong enough compared to the losses in the circuit, it will break into parametric oscillation at 60 Hz, and that's what drives the load. If you put on too much load -- including a dead short

-- that kills the oscillation, thus giving you complete short-circuit or overload protection.

This device thus operates on exactly the same basic parametric principle that governs how a child pumps a swing; radio-frequency "varactor diode" amplifiers (variable C instead of L) that used to be used as very low noise RF receivers a few decades back; and optical parametric amplifiers using nonlinear crystals that split "pump" photons into "signal" and "idler" photons and are routinely used to generate tunable optical signals at wavelengths where tunable lasers are not readily available. It's a neat invention.

A guy named Kent Wanlass (one of two Wanlass brothers) may have invented it, or at least made and sold these several decades ago.

The device you go on to describe is not what I'm familiar with. The constant voltage transformers I referred to are made by Sola, Superior Electric, and Hammond among others.

As I understand it, they are indeed transformers. The simplest version has fairly ordinary-looking primary and secondary windings wound on the centre leg of a shell-type transformer core. Unlike a normal transformer, where the primary is wound over the secondary (or vise versa), the primary and secondary windings are physically separated. Magnetic shunts (chunks of transformer steel) are inserted between the centre and outside legs of the core at a point between the primary and secondary winding. These magnetic shunts provide a flux path around the primary that bypasses the secondary winding, producing lots of leakage inductance. This is what limits the current when the secondary is shorted.

Meanwhile, the secondary winding is parallelled by a capacitor, chosen to make the secondary resonant at 60 Hz. The resonance drives the portion of the core inside the secondary winding into saturation, which limits the amplitude of the secondary voltage. Changes in primary voltage have almost no effect on secondary voltage over the regulating range.

Now, the above is actually a simplification. In real CV transformers, the secondary actually has enough turns to step up the voltage by a factor of several, so the capacitor is operating at several times line voltage. This allows the capacitor to be lower capacitance for resonance, which is physically smaller and cheaper than what you'd need at 115 V. The actual output voltage is obtained from a tap on the secondary where the voltage is 115 V or so.

Also, the transformer I've described so far outputs a pretty square waveform. That's great for the input stage of a DC power supply, but not for some AC loads. The commercial CV transformers I see use a "harmonic neutralized" design that gives an output closer to a sine wave. Instead of one secondary winding, there are two, with another pair of magnetic shunts between the two secondaries. The capacitor is connected across the two secondary windings in series. The output voltage is taken from just the "middle" secondary winding. In the Sola transformer, there's also an air gap in the centre leg of the core, at the end where the 3rd winding is. I don't understand how the extra winding and shunt cancel some of the 3rd harmonic output, but they do.

Dave

I don't have extensive experience with constant voltage transformers, but did play quite a bit with one of the "parametric oscillator" type commercial power supplies a few decades ago out of conceptual interest and as a classroom demonstration of the basic phenomena.

The experimental stigmata you could look for to see if your devices fall within my description are:

1) Existence of an oscillation threshold as input pump power is increased: Put a variac on the input and a medium load on the output, turn the input voltage up slowly from zero, see if the output voltage "turns on" or suddenly jumps from low value up to near full value as the input voltage approaches the specified input value. 2) Sudden cessation of oscillation if output is overloaded. Gradually increase the output current drawn from the secondary with fixed input voltage, e.g, by adding more and more small light bulbs in parallel across the output. As you add more load the output voltage may gradually droop, but if at some point a small increase in load causes the output to suddenly drop to essentially zero, that argues for the parametric type behavior.

Of the CV transformers I have around to play with, the Sola does seem to behave like an oscillator. As you slowly increase the input voltage, the output voltage also slowly increases for a while (no CV action). Then, suddenly, the transformer starts its characteristic humming, and the output voltage jumps up near its standard output value. After that point, increasing the input voltage further produces small changes in output voltage. In addition, if you take the input voltage back down, the transformer drops out of oscillation at a lower voltage than that needed to start it. So there is some hysteresis in the oscillation threshold.

All of the above describes what happens with no load on the output. I think that under load, you get the same behaviour but it takes a higher input voltage to produce oscillation.

The other two (Superior Electric and Hammond) do not suddenly start oscillating in this way. As you raise the input voltage from 0 to about

70 V, the output voltage is just 1.5 times the input voltage. Then the voltage out curve begins to flatten out, and by about 90 or 95 V in the curve is quite flat but still positive slope. As input voltage goes from 100 to 120 V, output voltage rises by only 2-3 V. There is no point at which the transformer suddenly starts humming, and no hysteresis in the voltage in/out curve.

I haven't tried that. The transformers are currently buried in the basement, so it's not easy to make new measurements.

Dave

On the theory of ferroresonant transformers:

Several years ago I was filing a patent (04356433) on a controllable HID power supply.

The attorney mentioned he had represented Sola in a patent infringement suit against [Federal/National/Superior??] Electric.

At the conclusion of the trial he was convinced that no one at either firm had the foggiest idea of how the device really worked.

The effect was first discovered by accident and the transformer variant was developed by mucking around on the bench.

Search the patent literature for further confusion.

BTW: Be aware that a ferro draws a hefty amount of power even with no load: be sure to turn the transformer off at the conclusion of the darkroom session or the electric bill will rise by $100/year -- A 100 Watt load left on for a month costs $7.30 at 10 cents/KW/Hour.

Also a ferro has quite an overshoot when turned on -- don't turn the thing on with the lamp connected to the output, i.e. don't control the lamp by connecting the timer to the input to the ferro.

in article RbVkb.6081$ snipped-for-privacy@newsread2.news.atl.earthlink.net, Nicholas O. Lindan at snipped-for-privacy@ix.netcom.com wrote on 10/20/03 10:46 AM:

At one time, I was working at a company that used Wabash transformers for charging capacitor banks. The idea was that you could run them into a short circuit (discharged capacitors) and then not over charge when you hit the constant output voltage. They ran HOT! The power factor was not all that great either. It probably averaged about 40% over the charging cycle. I looked at the waveform of current in an external capacitor supplied with the transformer. It was not a pretty sight. It looked like, IIRC, an asymmetrical sawtooth. Although I read up on ferroresonance, it never made sense.

One solution I tried was to get a current limiting (high leakage inductance) transformer. A capacitor was put in parallel with it. That caused the power factor to change from leading to lagging during the charging cycle. This gave a reasonable average power factor.

Another interesting item is that you would get rectified current flowing into the capacitors only while the instantaneous ac voltage exceeded the bank voltage. This meant that the charge rate slowed down as the bank charged. That is, you only charged near the peak of the sinusoidal inpaut. To get over that, I used a much higher open circuit voltage than what was otherwise necessary. To preven accidental overvoltage, I used two voltage measuring circuits to detect when proper voltage was obtained.

To get around this problem, I devised the monocyclic transformer described here in an earlier post.

Bill