SIMPLE CIRCUITS FOR SELECTING
BETWEEN SERIES OR PARALLEL OPERATION
OF LAMPS AND IMPEDANCES

Simple Low Voltage Light Bulb Circuits

The following circuits are the simple circuits that can be used for low voltage uses. They were also used for photoflood lighting circuits before the necessity was known of having the screw shell of a line-voltage operated lamp on the neutral side, in case the bulb is removed. The requirement of polarized plugs with the screw shell connected to the neutral blade makes these circuits technically illegal to use today for permanently installed line-voltage circuits. They can be used for cord and plug connected switchboxes provided all plugs are disconnected before any bulbs are changed.

Here is a list of the items commonly switched with these circuits, and the constraints each kind imposes on the design:

• Low Voltage Incandescent or Halogen Light Bulbs
  1. The power must never be short-circuited by any combination of switch positions.
  2. The voltages must not be high enough to be dangerous.
  3. Nonshorting switches must be used.
• Resistances or impedances
  1. None, other than those demanded by the circuit used.

The SEPAR circuit numbers used here have been used by the page author for years to catalog the various circuits.

The following variations are permitted when low voltage lamps are used in series-parallel switching circuits.

• Reversing the polarity of the power to the lamp as the switches are changed.
• One side of the lamp is connected to a hot or neutral lead with while the other side is open.
• The light bulb can be shorted out as long as power is not applied across it.

The same circuits can be used for switching impedances and nonpolarized heating elements.

• SIMPLE SWITCHING SYSTEMS Easy to use systems without all possibilities
• REGULAR SWITCHING MATRIX Grid systems with most possibilities
• INFINITE SWITCHING MATRIX Infinite systems can produce any circuit

SIMPLE SWITCHING
The Original Photoflood Series-Parallel Switchbox for Two Lamps. This has been used for high-color-temperature short-life photoflood lamps to conserve the lives of these bulbs since the 1930s. The lamps are dimmed while the photographer is setting up the shot. The switch is set to the dim position while the photographer sets up the shot, and the bright position during the actual photography. There is no center off position in this version. 2 switch settings. SEPAR 201 (a modification of the Original Photoflood Series-Parallel Switchbox) This is a modification of the above circuit, adding a center position on the switch to turn the lights off. The switch is set to the dim position while the photographer sets up the shot, and the bright position during the actual photography. The center position turns off the lights. 3 switch settings, all unique & useful.
SEPAR 202 (The page author's answer to a puzzle in a magazine) SWITCH LAMP A B 1 2 o o off off † o B ON ON † A o DIM DIM † A B ON off The Photoflood Series-Parallel Switching puzzle in a magazine in the 1940s showed a series-parallel switching circuit someone tried, using the same parts, but connected differently. But if the switches were set wrong, it shorted out the power line, blowing a fuse. The magazine then said there was a way to connect them that did not short out the power. This was my answer. Note that the switch position with the letter on it is the position indicated by that letter in the table. It also identifies the switch. The letter o identifies the remaining switch position. † is useful for photofloods. 4 switch settings, all unique & useful.
SEPAR 203 and SEPAR 204 (A circuit the page author built in 1984 and modified in 2004) SWITCH LAMP A B C 1 2 o o off off † o B DIM DIM † o C off ON A o ON off A B ON off A C ON ON † This Series-Parallel Switching Circuit was originally built to put two electric fans in series or parallel, or to turn on either fan alone. All possible combinations of series and parallel are possible, including off. A later version of this series-parallel switching circuit (SEPAR 204) was built to conform to the requirements of line voltage circuits. The switch positions are exactly the same, but the live connections are removed if the neutral side is opened. Note that any switch position with a letter on it is the position indicated by that letter in the table. 6 switch settings, 5 unique & useful.
SEPAR 301 (Built by page author in 1963) SWITCH LAMP A B C D 1 2 3 o o o off off off † o o D ON off off o B o DIM DIM off † o B D ON off off o C o off off off † o C D ON ON off † A o o DIM DIM DIM † A o D ON off off A B o DIM DIM off † A B D ON off off A C o off off ON A C D ON ON ON † 12 switch positions, 7 unique & useful. Designed by author while playing with SEPAR 202 diagram on paper. SEPAR 302 (1961 magazine, modified by page author) SWITCH LAMP A B C D 1 2 3 o − o − off off off † o o C o DIM DIM off † o o C D ON off off o B C o off ON off o B C D ON ON off † A o o o DIM off DIM A o o D DIM DIM DIM † A B o o off off ON A B o D off off ON A o C o DIM glim glim A o C D ON off off A B C o off ON ON A B C D ON ON ON † 16 switch positions, 11 unique & useful. The page author removed a switch, added switch A, and traded hot and neutral lines.
SEPAR 303 (Page Author, 1963) SWITCH LAMP A B 1 2 3 o o DIM DIM DIM † o B off off ON A o ON off off A B ON ON ON † Designed by author while playing with SEPAR 302 diagram on paper. 4 switch settings, all unique & useful. Can be built with one double-pole switch if only the dim and all-on positions are needed, with only 2 switch settings. SEPAR 305 (Page Author, 1965) SWITCH LAMP A B C D 1 2 3 − o − o off off off † o o o D off off off o o C D ON off off o B o o DIM DIM off † o B o D off off off o B C o off ON off o B C D ON ON off † A o o D DIM DIM DIM † A o C D ON off off A B o o DIM DIM ON A B o D off off ON A B C o off ON ON A B C D ON ON ON † The page author's attempt to remember SEPAR 302 without the documents. 16 switch settings, 10 unique & useful. SEPAR 310 (Page Author, 2013) SWITCH LAMP A B C D 1 2 3 o o o off off off † o o D ON off off o B o DIM DIM off † o B D ON off off o C o off ON off o C D ON ON off † A o o DIM DIM DIM † A o D ON off off A B o DIM DIM off † A B D ON off off A C o off ON off A C D ON ON ON † The page author added switch A and lamp 3 to SEPAR 203. 12 switch settings, 8 unique & useful.
SEPAR 306 (Simple Controls - Page Author, 2006) SWITCH LAMP B-D 2-3 1 2 3 DIM 2 DIM DIM off † DIM 3 DIM DIM DIM † BRIGHT 2 ON ON off † BRIGHT 3 ON ON ON † A two-switch way to do two or three lamps. One switch controls bright or dim. The other switch selects 2 or 3 lamps. All 4 switch settings are useful. SEPAR 307 (Simple Controls - Page Author, 2006) SWITCH LAMP A B C 1 2 3 o o o off off off † o o C off off ON o B o DIM DIM off † o B C DIM DIM DIM † A o o ON ON off † A o C ON ON ON † A B o ON off off A B C ON off off Designed by author for a simplified control panel. Turn the first switch on for bright, or the second switch for dim. The third switch controls the number of lamps. 8 switch settings, 7 unique & useful.
SEPAR 312 (Page Author, 2013) SWITCH LAMPS Settings 1 2 3 o † off off off EH ON off off BD off ON off DFH off off ON CEH † ON ON off EGH ON off ON BDFH off ON ON CEGH † ON ON ON BH † DIM DIM off F DIM off DIM ACFH off DIM DIM AFH † DIM DIM DIM ABGH ON DIM DIM CEF DIM ON DIM AEGH DIM DIM ON BFH DIM Glim Glim BGH Glim DIM Glim CFH Glim Glim DIM All possible combinations of series, parallel, parallel-series, and series-parallel can be switched with this circuit. All 18 possible output displays are shown, out of 108 switch settings.* SEPAR 402A (Page Author, 2006) SWITCH LAMPS Settings 1 2 3 4 o † off off off off AB ON off off off BCE off ON off off ACD off off ON off DE off off off ON ABCE † ON ON off off ABDE ON off off ON BCD off ON ON off ACDE off off ON ON ABCD † ON ON ON off BCDE off ON ON ON ABCDE † ON ON ON ON CE † DIM DIM off off AC off off DIM DIM AD † DIM DIM DIM off BE off DIM DIM DIM A † Glim Glim Glim Glim CD DIM DIM ON off BC off ON DIM DIM C † DIM DIM DIM DIM ADE ON DIM DIM DIM ABE DIM DIM DIM ON CDE DIM DIM ON ON ABC ON ON DIM DIM Minimal circuit for 2, 3, or 4 lamps. All settings formerly useful for photofloods shown as † All 24 possible output displays shown, out of 32 switch settings.*
SEPAR 401 SWITCH LAMPS SWITCH LAMPS Settings 1 2 3 4 Settings 1 2 3 4 o † off off off off ACGJ † DIM DIM off off AB ON off off off AEJ DIM off DIM off BCF off ON off off AHL DIM off off DIM DFJ off off ON off CEFJ off DIM DIM off JK off off off ON CFHL off DIM off DIM ABCF † ON ON off off DFL off off DIM DIM ABDFJ ON off ON off ACDJ † DIM DIM DIM off ABJK ON off off ON ACGL DIM DIM off DIM BCDFJ off ON ON off AEL DIM off DIM DIM BCFJK off ON off ON BCDK off DIM DIM DIM DFJK off off ON ON ACDL † Glim Glim Glim Glim ABCDFJ † ON ON ON off (P-S) - - - - ABCFJK ON ON off ON ACGJK DIM DIM off ON ABDFJK ON off ON ON AEJK DIM off DIM ON BCDFJK off ON ON ON ABCGK ON DIM off DIM ABCDFJK † ON ON ON ON ABEK ON off DIM DIM (S-P) - - - - BCDFL off ON DIM DIM ACEGJ DIM Glim Glim off CEFJK off DIM DIM ON ACEFJ Glim Glim DIM off ABCDFL ON ON DIM DIM ACFHL Glim Glim off DIM ACDJK DIM DIM DIM ON ADFHL Glim off Glim DIM ABCDK ON DIM DIM DIM CDFHL off Glim Glim DIM (P-S-P) - - - - ACEFL Glim Glim DIM DIM ACEFJK Glim Glim DIM ON ACEGL DIM Glim Glim DIM ACEGJK DIM Glim Glim ON ACDFHL Glim Glim Glim DIM ABCEGK ON Glim Glim DIM The page author got a plywood board with this circuit on it, made with knife switches and sockets, at a school surplus-materials auction. It provides all series combinations and all parallel combinations. All 47 possible output displays shown, out of 648 switch settings.* KEY TO TABLE ENTRIES ON   - Connected directly to power DIM   - 2 or 3 lamps in series   - series in a series-parallel Glim   - 4 or more lamps in series   - parallel in a series-parallel off   - No current through lamp −   - Switch position unimportant *   - All possible displays shown.   - Redundant combos not shown. †   - Former settings for photofloods Letters indicate closed switches. No letter for a switch, or an o for a switch, means the switch is open, or is in the unlabeled position.
SEPAR 409 (a modification of SEPAR 401) This is a modification of the above circuit, adding an extra switch and using all two-position switches. See the details on the SEPAR 409 page This is much more useful than SEPAR 401, providing two series of two lamps each. 864 connected switch settings, 56 unique & useful, out of 4096 total. (3232 settings have multiple letter positions set in the same switch ladder.) One more position for safe bulb changing.
REGULAR SWITCHING MATRIX
SEPAR 300 (The First Polarized Series-Parallel Switchbox for Three Lamps) It was the first design where the switch positions were intuitive. The user could set the switches for the desired combination of series and parallel. For example, to connect lamps 1 and 2 in series: - Set lamp 1 to FST (first). - Set lamp 1 to the 1> (to) position. - Set lamp 2 to >1 (from 1). - Set lamp 2 to LST (last). - Set lamp 3 to o (off). It can leave a lamp connected to the hot side with no neutral connection, so the circuit cannot be used with line-voltage lamps. Almost all combinations are possible. SEPAR 400 (A Four-Lamp Version of SEPAR 300) This system is expandable to SEPAR n00 for n lamps by adding these for each additional lamp: - Another column of the switches and its lamp - Another position on each rotary switch - Another horizontal bus wire 512 settings Some are redundant.
SEPAR 330 (Advanced Polarized Series-Parallel Switchbox for Three Lamps) The switch positions were intuitive. The user could set the switches for the desired combination of series and parallel by looking at the labels. For example, to connect lamps 1 and 2 in series: - Set lamp 1 to FST (first) by not selecting any < (from) positions. - Set lamp 1 to the 1> (to) position. - Set lamp 2 to <1 (from 1). - Other lamp 2 switches must not be set to a < position. - Set lamp 2 to LST (last). - Set lamp 3 to o (off). It can leave a lamp connected to the hot side with no neutral connection, so the circuit cannot be used with line-voltage lamps. All combinations are possible. SEPAR 440 (A Four-Lamp Version of SEPAR 330) This system is expandable to SEPAR nn0 for n lamps by adding these for each additional lamp: - Another column of the switches and its lamp - Another double-pole double-throw switch to each column - Another horizontal bus wire 1728 settings Some are redundant.
SEPAR 333 (Attempt to make a neutral-safe Series-Parallel Switchbox for Three Lamps) This is basically the SEPAR 330 with extra poles on the switches to disconnect the live phase if no current path to the neutral from the live phase exists. The problem is that it is easy to fool the circuit into allowing a connection to hot without a current path. It is also possible to reverse the polarity of a lamp. Since it has no advantage over SEPAR 330, it is not shown. Epilog: The page author has since proved that it is impossible to make a switching circuit that can provide all possible combinations of series and parallel and that is also polarity-safe (never reverses lamp polarity) and neutral-safe (never connects lamp to hot without a path to a neutral connection). SEPAR 444 (A Four-Lamp Version of SEPAR 333) This is not shown for the same reason. It was a dead-end project. SEPAR nnn, n000, nnnn, and nnn9 (replace n with the number of lamps) were more interlocked versions of SEPAR 333 and 444). They were also dead-end projects for the same reason.
INFINITE SWITCHING MATRIX
SEPAR I-4 (an infinitely selectable system) This is a system designed so that each end of each lamp can be connected to whatever it needs to be connected to. It uses only two-position switches. This can connect the lamps or devices in any possible network with any polarity. Number of lamps: 4 Number of interconnection buses: 3 Number of rows of switches: 5 Number of columns of switches: 8 Number of switches: 40 Number of nonequivalent combinations: 1679616 SEPAR I-n (an infinitely selectable system - replace n with number of lamps) This is the same as SEPAR I-4, but is expandable for any number of lamps. This can connect the lamps or devices in any possible network with any polarity. The extra rows and pairs of columns are added at the dashed ends. To add a lamp, add: - one lamp with its two switch columns - one switch row and bus The extra rows are lettered in reverse alphabetical order. Number of lamps: n Number of interconnection buses: n-1 Number of rows of switches: n+1 Number of columns of switches: 2n Number of switches: 2n (n+1) or 2n^2 + 2n Number of nonequivalent combinations: (n+2) ^ 2n Note that each column of n+1 two-position switches can be replaced with a single rotary switch with n+2 positions. This is SEPAR I-nZ. The panel at right is a SEPAR I-5. 390625 nonequivalent settings Some are redundant. C = center contact S = screw shell
Using SEPAR I-5 infinitely selectable system for the proof circuit This is the proof that a switching circuit with all possibilities can't be polarity safe or neutral safe. The following switches are turned on (to the named positions). 1HC - Lamp 1 center terminal is set to Hot 1ZS - Lamp 1 screw shell is set to bus Z 2HC - Lamp 2 center terminal is set to Hot 2YS - Lamp 2 screw shell is set to bus Y 3ZC - Lamp 3 center terminal is set to bus Z 3YS - Lamp 3 screw shell is set to bus Y 4ZC - Lamp 4 center terminal is set to bus Z 4NS - Lamp 4 screw shell is set to Neutral 5YC - Lamp 5 center terminal is set to bus Y 5NS - Lamp 5 screw shell is set to Neutral

LINKS:

1. SELECTING SERIES OR PARALLEL OPERATION (Main Page)
2. LIGHT BULBS, SPECTRA, and HUMAN VISION
3. Computers, Science, and Measurement