DESIGNING SWITCHING SYSTEMS
FOR SELECTING BETWEEN VARIOUS
SERIES AND/OR PARALLEL CIRCUITS
FOR VARIOUS ELECTRICAL DEVICES

Many times a system of switches is needed for the purpose of connecting various devices together in various configurations of series, parallel, series-parallel, parallel-series, or other connections. This is a guide to designing this kind of circuit while avoiding the following problems:

• Shorting out power sources
• Exposing people to dangerous voltages
• Reversing polarities of sensitive loads
• Violating requirements of specific devices

The following kinds of devices can be switched:

• Low voltage incandescent lights
• Line voltage incandescent and halogen lights*
• Resistances and impedances
• Electronic components
• Batteries*
• Guitar pickups*
• Loudspeakers*
• *   Special restrictions apply.

DESIGN ELEMENTS

The following are principal elements of the design of these circuits:

• Polarity:
  • Batteries have + and - polarities which must not ordinarily be reversed.
  • Battery charging circuits have + and - polarities which must not be reversed.
  • Unbalanced audio signals have hot and ground terminals. Reversing them causes hum.
  • Balanced audio signals have hot and cold terminals. Reversing them reverses the signal phase.
  • The phase of a speaker must not be reversed unless this is needed for a special purpose
  • Line voltage lamps have hot and neutral connections for safety purposes - This is the basis for neutral-safe switching
• Shorting and Loading:
  • Batteries must not be shorted out.
  • Battery charging circuits must not be shorted out
  • Guitar pickups can be shorted out. Changes in loading can change their sounds
  • Microphones and phono pickups can be shorted out. Changes in loading can change their sounds
  • Unbalanced audio signal sources should not ordinarily be shorted out
  • Balanced audio signal sources should not ordinarily be shorted out
  • Unbalanced audio signal inputs must not be left unloaded or hum will result
  • Balanced audio signal inputs should not be left unloaded or hum will result
  • Power amplifier outputs must not be shorted out, but must be loaded
  • Line voltages must not be shorted out
  • Power supply voltages must not be shorted out.
• Switch Nomenclature
  • Switches are identified by the number of poles and the number of throws e.g. SPDT.

    - SPST is Single Pole Single Throw.

    - DPDTCO is Double Pole Double Throw Center Off.

  • The pole is the connection to the moving armature of the switch.
  • Multiple poles are linked mechanically (ganged), but not electrically connected.
  • The throw is the contact the switch connects to when it is thrown that way.
  • A single throw switch has OFF as the other switch position.
  • CO after the switch code indicates that an extra centered position with no connection (center off).

    - The CO position is not included in the throw number.

  • Numbers of poles and throws larger than 2 are represented by numbers e.g. 3P4T.
  • Switches with numbers of throws larger than 2 are usually rotary switches.
  • Switches are also designated as shorting or nonshorting

    - Nonshorting switches break one connection before making the next connection.

    - Shorting switches make the next connection before breaking the previous connection.

  • All switches used in lamp circuits must be nonshorting.
  • Microphone and guitar pickup switches should be shorting.
• Switch Requirements for series-parallel switching systems
  • To prevent shorting out a power supply, the pole of a switch should always be connected to one of the following:

    - A load

    - A throw of another switch with its pole connected to a load

    - A throw of another switch with its pole connected to another such throw

    An exception is a master power switch for all power to the circuit. The pole must face the hot feed by code.

  • The pole of a switch should never be connected to any of the following:

    - A power source or a ground

    - Another pole of another switch

  • A chain of n-1 SPDT switches can be cascaded into a switch ladder to make the equivalent of a rotary switch with n throws:

    - This allows the use of standard NEMA wiring devices where a rotary switch would otherwise be used.

    - Only one of the switches in the ladder would be thrown to the active position as a time.

  • Multiple-pole switches are used with neutral-safe, polarity-safe, battery, guitar pickup, and speaker circuits to make sure the correct number of units are always connected and polarity is maintained.
• Basic Circuit Configurations
  • There are several kinds of basic circuit configurations used in these circuits:

    - A basic series circuit with switches providing the other combinations

    - Basic controls easy to use for photofloods

    - Multipole switching for special devices

    - Neutral-safe polarity-safe configuration

    - Regular switching matrix (all lamp channels alike)

    - Infinite switching matrix (all lamp channels alike)

  • Configuration provides for the needs of photoflood switching.

    - Switch positions are provided to provide bright and dim settings.

    - Settings exist for 2, 3, and (if lamp provided) 4 lamps.

    - Settings include 2 series, 2 parallel, 3 series, 3 parallel, 4 series-parallel (or parallel-series), and 4 parallel.

• Other Circuit Requirements
  • There are three kinds of interconnection buses in these circuits:

    - The hot power bus

    - The neutral power bus

    - An interconnection bus

    Buses are usually connected to the throw terminals of switches.
  • All screw shells should be connected so that when the lamp is in a parallel connection, the screw shell is connected to the neutral bus.

DESIGN EXAMPLES

The following are examples of designs made using these principles:

• SEPAR 302
  • A basic series circuit with switches providing other combinations
  • All switch poles are connected to lamp terminals.
  • All lamps are connected so that the screw shell is switched to the neutral bus when switches are set for lamps in parallel.
  • A special setting switches all screw shells to the neutral bus while disconnecting all hot bus connections.
  • All switch connections to the hot bus are throw terminals.
  • All switch connections to the neutral bus are throw terminals.
  • There are no interconnection buses.
  • There are no switch ladders.
  • This system is not polarity-safe and not neutral-safe.
  • Provides needed settings for 2 and 3 lamp photoflood switching.
  • The arc next to the lamp circle denotes the screw shell.
• SEPAR 409
  • A basic series circuit with switches providing other combinations
  • All switch poles are connected to lamp terminals or cascaded to lamp terminals.
  • One exception is the master power switch P, which must have the pole on the hot feed.
  • All lamps are connected so that the screw shell is switched to the neutral bus when switches are set for lamps in parallel.
  • A special setting switches all screw shells to the neutral bus while disconnecting all hot bus connections.
  • All switch connections to the hot bus are throw terminals.
  • All switch connections to the neutral bus are throw terminals.
  • All switch connections to interconnection buses are throw terminals.
  • There are 2 interconnection buses.
  • There are 4 switch ladders. D-E, F-G-H, I-J, and K-L.
  • Uses standard NEMA switches and parts (photo at right).
  • This system is not polarity-safe and not neutral-safe.
  • Provides needed settings for 2, 3, and 4 lamp photoflood switching.
  • Provides all 4 single-lamp-on settings, plus off.
  • Provides all 11 possible parallel settings.
  • Provides all 11 possible series settings.
  • Provides 13 series-parallel, 10 parallel-series, and 5 other settings.
  • Page author's unit is in use controlling infrared lamp output.
• SP-STD 3
  • A neutral-safe polarity-safe configuration
  • SP-STD 3 is two SP-STD 2 circuits cascaded. The SP-STD 2 is the basic unit for these.
  • All lamps are connected so that the screw shell is always switched to the neutral bus when switches are set for lamps in parallel.
  • Not all switch connections to the hot bus are throw terminals.
  • All switch connections to the neutral bus are throw terminals.
  • There are no interconnection buses.
  • Double-pole switches are used to enforce polarity-safety and neutral-safety.
  • There are no switch ladders.
  • Provides needed settings for 2 and 3 lamp photoflood switching.
  • Provides all 3 single-lamp-on settings.
  • Provides all 4 possible parallel settings.
  • Provides all 4 possible series settings.
  • Cannot provide all series-parallel or parallel-series circuits.
  • This neutral-safe system cannot provide bridge circuits.
• SEPAR 330
  • A regular switching matrix
  • All switch poles are connected to lamp terminals or cascaded to lamp terminals.
  • All lamps are connected so that the screw shell is always switched to the neutral bus when switches are set for lamps in parallel.
  • All switch connections to the hot bus are throw terminals.
  • All switch connections to the neutral bus are throw terminals.
  • All switch connections to interconnection buses are throw terminals.
  • The switch ladders are alternate splitting versions using double-pole switches.
  • This system is not polarity-safe and not neutral-safe.
  • All lamp channels are alike.
  • Provides (but hard-to-find) needed settings for 2 and 3 lamp photoflood switching.
  • Provides all 3 single-lamp-on settings, plus off.
  • Provides all 4 possible parallel settings.
  • Provides all 4 possible series settings.
  • Provides all possible series-parallel and parallel-series settings.
  • This regular system cannot provide bridge circuits.
• SEPAR I-4
  • An infinite switching matrix
  • All switch poles are connected to lamp terminals or cascaded to lamp terminals.
  • Lamps are NOT always connected so that the screw shell is switched to the neutral bus when switches are set for lamps in parallel.
  • All switch connections to the hot bus are throw terminals.
  • All switch connections to the neutral bus are throw terminals.
  • All switch connections to interconnection buses are throw terminals.
  • There are 3 interconnection buses (one less than the number of lamps).
  • All switches are in switch ladders. Each column is a ladder.
  • This system is not polarity-safe and not neutral-safe.
  • All lamp channels are alike.
  • Provides (but hard-to-find) needed settings for 2, 3, and 4 lamp photoflood switching.
  • The polarities of all lamps can be reversed, multiplying numbers of settings.
  • Provides all 8 single-lamp-on settings, plus off.
  • Provides all 72 possible parallel setting permutations.
  • Provides all 624 possible series setting permutations.
  • Provides all possible series-parallel and parallel-series settings.
  • This infinite system provides no bridge circuits because they require at least 5 lamps.
  • The 40 switches can be set to 1,099,511,627,776 positions, but only 1,679,616 are not ladder redundant.
• SEPAR I-5
  • An infinite switching matrix
  • All switch poles are connected to lamp terminals or cascaded to lamp terminals.
  • Lamps are NOT always connected so that the screw shell is switched to the neutral bus when switches are set for lamps in parallel.
  • All switch connections to the hot bus are throw terminals.
  • All switch connections to the neutral bus are throw terminals.
  • All switch connections to interconnection buses are throw terminals.
  • There are 4 interconnection buses (one less than the number of lamps).
  • All switches are in switch ladders. Each column is a ladder.
  • This system is not polarity-safe and not neutral-safe.
  • All lamp channels are alike.
  • This version uses standard NEMA switches and parts (image at right).
  • Provides (but hard-to-find) needed settings for 2, 3, 4, and 5 lamp photoflood switching.
  • The polarities of all lamps can be reversed, multiplying numbers of settings.
  • Provides all 10 single-lamp-on settings, plus off.
  • Provides all 232 possible parallel setting permutations.
  • Provides all 6320 possible series setting permutations.
  • Provides all possible series-parallel and parallel-series setting permutations.
  • This infinite system provides all possible bridge circuits.
  • Used for the proof that a switching circuit with all possibilities can't be polarity safe or neutral safe.
  • The 60 switches can be set to 1,152,921,504,606,846,976 positions, but only 282,475,249 are not ladder redundant.
• SEPAR I-n
  • An infinite switching matrix
  • All switch poles are connected to lamp terminals or cascaded to lamp terminals.
  • Lamps are NOT always connected so that the screw shell is switched to the neutral bus when switches are set for lamps in parallel.
  • All switch connections to the hot bus are throw terminals.
  • All switch connections to the neutral bus are throw terminals.
  • All switch connections to interconnection buses are throw terminals.
  • There are n-1 interconnection buses (one less than the number of lamps).
  • All lamp channels are alike.
  • All switches are in 2n switch ladders of n+1 switches each. Each column is a ladder.
  • Each switch ladder can be replaced with a rotary switch with n+2 positions (throws).
  • Each lamp channel has 2 switch ladders of n+1 switches each.
  • One switch ladder feeds the lamp's center terminal. The other feeds its screw shell.
  • Each switch ladder can be replaced with a rotary switch with n+2 positions (throws).
  • This system is not polarity-safe and not neutral-safe.
  • This version uses standard NEMA switches and parts (image above right).
  • Provides (but hard-to-find) needed settings for up to n lamp photoflood switching.
  • The polarities of all lamps can be reversed, multiplying numbers of settings.
  • Provides all 2n single-lamp-on settings, plus off.
  • Provides all possible parallel setting permutations (see table).
  • Provides all possible series setting permutations (see table.)
  • Provides all possible series-parallel and parallel-series setting permutations.
  • This infinite system provides all possible bridge circuits.
  • See the table to find number of switches, number of switch permutations, and number of positions that are not ladder-redundant.
  • The SEPAR I series can be expanded to any number of lamps by adding one transfer bus and one lamp with its two switch ladders for each extra lamp.

  # Lamp	Rotary Sws	Sw Pos / Rot Sw	Sw Pos Rotary Tot	# Lads	Sws / Lad	Sw Pos / Lad	# Lad Sws	Sw Pos Lad Cal	Sw Pos Ladders Total	Good Pos/Lad	Good Pos Lad Cal	Good Pos Ladders Tot	Series Min	Parallel Min	Series Total	Parallel Total
  n	2n	n+2	(n+2) 2n	2n	n+1	2 n+1	2n (n+1)	2 2n (n+1)	2 2n (n+1)	n+2	(n+2) 2n	(n+2) 2n	2 n - n - 1	2 n - n - 1	n   ∑  (P(n,r) 2r)   r=0	n   ∑  (C(n,r) 2r)   r=0
  2	4	4	256	4	3	8	12	2 12	4096	4	4 4	256	1	1	8	4
  3	6	5	15625	6	4	16	24	2 24	16777216	5	5 6	15625	4	4	72	20
  4	8	6	1679616	8	5	32	40	2 40	1099511627776	6	6 8	1679616	11	11	624	72
  5	10	7	282475249	10	6	64	60	2 60	1152921504606846976	7	7 10	282475249	26	26	6320	232

• SP BAT 3
  • Multipole switching for special devices
  • All switch poles are connected to loads when charging.
  • All switch connections to the hot bus are throw terminals.
  • All switch connections to the neutral bus are throw terminals.
  • There are no interconnection buses.
  • There are no switch ladders.
  • This system is polarity-safe, but not neutral-safe.
  • Provides no settings for photoflood switching.
  • Has only one switch for foolproof operation.