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Lamp Bulb Resistors
April 1947 Radio Craft

April 1947 Radio-Craft

April 1947 Radio Craft Cover - RF Cafe[Table of Contents]

Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Craft, published 1929 - 1953. All copyrights are hereby acknowledged.

Neon light bulb AC vs DC (Wikipedia) - RF Cafe

Note with DC only the positive electrode glows, whereas with AC both electrodes glow. (Wikipedia)

Most people who have been in the electronics world for a while know that neon light bulbs* used to be commonly employed as a "pert-near" voltage regulator reference of between 55 and 65 volts, depending on the type. The familiar NE-2 has a turn-on voltage of 65 Vac (90 Vdc), for instance, and the voltage across the terminals remains there with little change regardless of the current through the bulb - a lot like a Zener diode. Neon bulbs are also used as non-invasive RF power detectors. Most people probably do not know, however, that incandescent bulbs also have properties that make them useful for purposes other than just lighting up a dark space. Incandescent light bulbs have been used successfully for voltage regulation and RF power measurement. They have also been used as dummy loads for transmitters. John Parchman details some of these uses in a 1947 issue of Radio-Craft magazine.

* See "Let's Use Neon Bulbs" in the July 1953 issue of QST magazine.

Lamp Bulb Resistors

By John B. Parchman

Many lamp types available - RF Cafe

Just a few of the many lamp types available.

One filament in these power measurement lamps carries r.f. - RF Cafe

One filament in these power measurement lamps carries r.f.; the other, easily measured a.c. (Photos courtesy Sylvania Electric Product). [Here is a page for the Sylvania PM8 tube]

Lamps used in dummy antenna circuits - RF Cafe

Fig. 1 - Lamps used in dummy antenna circuits.

Set-up for comparison measurements - RF Cafe

Fig. 2 - Setup for comparison measurements.

Lamp used as a high-wattage resistor - RF Cafe

Fig. 3 - Lamp used as a high wattage resistor.

Resistance in Ohms - RF Cafe

Table 1 - Resistance in Ohms.

Current in Milliamperes - RF Cafe

Table 2 - Current in Milliamperes.

Most radio men consider the incandescent lamp bulb a lowly thing useful only when it gets dark. Actually, the lowly lamp bulb can be used for many things around the radio or electric shop, such as for resistances, continuity checks, dummy antennas, and voltage or current measurement.

Most radio or electrical men have used test lamps which incorporate a lamp bulb to locate blown fuses or to check the presence of voltage in convenience outlets, and as indicating devices, etc. How many ever used one to check the presence of line voltage at the set? Many sets are dead simply because of a faulty line cord or plug which is discovered in many cases only after the switch and most of the tubes (in a series string) or the switch and transformer have been checked for continuity.

One of the greatest objections to the lamp bulbs for such uses have always been that the resistance of an incandescent filament-type lamp varies over a considerable range according to the amount of current passing through it. This property can be used to advantage in some applications. The tungsten filament in lamps is an excellent regulating filament. It is not as good as nichrome wire and some of the more complex alloy wires used in automatic ballast regulating tubes because the resistance does not remain constant for large variations of current. But when the resistances of the lamps are known (see the tables in this article), this characteristic is not serious in many applications. The voltage regulation for a given current range can be obtained from the tables. Curves can be plotted by the reader on suitable graph paper from the data given in these tables. Such curves were not included in this article because of space limitations and the difficulty of reproducing the curves in sufficient detail for accurate use.

R.F. Power Measurement

Among the common uses of light bulbs in the radio field is as dummy antennas or for measuring a transmitter's power output. The law requires some form of dummy antenna to minimize unnecessary interference. One of the cheapest methods is to use incandescent lamp bulbs coupled to the plate tank coil by means of a pickup coil or clipped directly across a few turns of the tank coil. The higher the resistance of the lamp (the lower the wattage), the greater the number of turns required for coupling. The coupling should be varied until the greatest brilliancy is obtained for a given power input. At frequencies below 15 or 20 megacycles, the lamp is practically a resistance load. At frequencies above 30 megacycles, reactance of the leads, etc., introduces loss of power. To eliminate as much loss as possible, leads should be soldered to the terminals

of the lamp instead of using a socket. Some losses may be eliminated by making the lamp load resonant with a variable condenser (see Fig. 1). Other circuits are given in various handbooks and texts.

A lamp which would light up to approximately normal brilliancy at the apparatus power input value should be chosen.

Measurement by Comparison

When more accurate check methods are not available, the brilliancy of the lamp connected to the apparatus under test may be compared with the brilliancy of a similar lamp connected to a known source of supply such as the 115-volt line (see Fig. 2). A closer comparison can be made if a colored filter is used to view the lamps. If no such filter is available, one can be made by holding a piece of glass over a smoky flame. Care should be taken to secure an even deposit on the glass. The deposit of soot or carbon black collected on the glass serves as a light filter. When viewed through such a filter, the brilliancy of the two lamps can be readily compared. They are made to match by varying the voltage supplied to the standard lamp.

A variable source of voltage can be obtained by using resistance in series with the lamp or with an autotransformer or Variac.

The voltage supplied to the standard lamp can be easily determined, after the brilliancy has been matched, with a common a.c. voltmeter, which can be found on most radio benches.

From the tables given in this article, you can find the current passing through the lamp and the resistance at the measured voltage. By the simple application of Ohm's law you can now determine the power output of the device. For example, let's consider that a voltage of 100 volts a.c. r.m.s. was measured across a 100 watt lamp bulb. This corresponds to 760 milliamperes and a resistance of 132 ohms. Ohm's law, when applied to alternating currents and voltages in a purely resistive load, is the same as for d.c. Therefore, the power output would be: power = volts times amperes or 100 x 0.760 = 76 watts.

If resistances or dissipation ratings desired cannot be found in the tables, arrangements in series, parallel, or series-parallel can be worked out to give almost any value.

Currents in circuits can also be measured with incandescent lamps when a suitable milliammeter or ammeter is not available. The lamps are connected in series with the load and the same procedure as outlined for determining power is used. The voltage drop across the lamps is measured and the value of current can be determined from the tables. For instance, with a voltage of 25 across a 200-watt lamp, the current from the tables would be 765 milliampere.

This technique can also be used to determine voltages in the absence of an a.c. voltmeter. Of course, it is necessary to calibrate the source of supply voltage for the standard lamp before beginning testing. In this case, the brilliance of the lamps is matched and the voltage read from the calibrated supply used for the standard lamp

Possibly one of the oldest uses for incandescent lamps is as an inexpensive means of obtaining voltage drops. For example, we have a piece of apparatus which uses three tubes and desire to operate it from the regular power source with a series resistor or ballast lamp. The tubes naturally should be selected to have the same current rating, 300 milliamperes, 150 milliamperes, etc. For the sake of illustration, suppose a 25L6, 25Z5, and a 6SK7 tube were to be operated from a 115-volt a.c. supply. These tubes all draw 300 ma of filament current and have a combined voltage drop of 56. Therefore, a drop of 59 or 60 volts at 300 ma must be obtained in a dropping resistor. By referring to curves drawn from the data given in the tables, it is found that a 50 watt lamp would give a resistance of 204 ohms and a current of 295 ma for a voltage drop of 60 (Fig. 3).

For accurate measurement of power output, current, and voltage, a series of power measurement devices are available on the market. Examples of these are the power measurement lamps made by Sylvania Electric (Radio Craft, March, 1944). These lamps are designated PM3, PM4, PM5, PM6, PM7, PM8, and PM9. They have two filaments in one bulb and operate on the principle (brilliance comparison) described in this article. Their resistance ranges from 36 ohms to 310 ohms. Their power ranges from 0.005 to 25 watts at frequencies from 15 to several hundred megacycles. Their voltage drops range from 0.5 to 55.



Posted May 26, 2020

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