text: SWITCHING CIRCUITS FOR SELECTING BETWEEN SERIES OR PARALLEL OPERATION OF VARIOUS ELECTRICAL DEVICES
  
  Many circuits exist for switching between series and parallel operation. But the selection of a particular switching circuit depends on what kinds of devices are being switched and the various constraints imposed by each kind of device.
  
  Here is a list of the various kinds of devices commonly switched, and the constraints each kind imposes on the design:
  
  • Low Voltage Incandescent or Halogen Light Bulbs
  • Resistances or impedances
  • Line Voltage Incandescent or Halogen Light Bulbs
  • Batteries
  • Guitar Pickups
  • Speaker Circuits
  
  So any switching circuit of this type must be designed for the actual use intended.
  
  Calculations for Series and Parallel Circuits
  
  The formulas for calculation of the parameters of series and parallel circuits can be found here:
  
  Calculation of series and parallel circuit parameters
  
  Comparing the Various Series and Parallel Switching Circuits
  
  The following page compares the various circuits for series/parallel switching:
  
  Comparing series/parallel circuits
  
  Series-and-parallel switching with all combinations can't be polarity-safe or neutral-safe
  
  The following page shows the proof:
  
  Proof that all-combinations switching can't be polarity-safe or neutral-safe
  
  The extra mark by each lamp (in the form of a short arc) in these diagrams indicates the lead that should be connected to the screw shell of the lamp (image at right):
  
  • In the simple circuits, the polarity of the lamp changes, and the selected lead is connected to the neutral when the lamps are in parallel.
  • In the polarity-safe neutral-safe line voltage circuits, the marked end of the lamp is always kept at a lower voltage to neutral than the other terminal of the lamp no matter how the switches are set.
  
  In the diagrams offered as proof, the extra marks are not used to show that polarity-safe and neutral-safe are impossible.
  
  Designing series-and-parallel switching circuits
  
  The following page shows the design principles:
  
  Designing series-parallel switching
  
  Simple Low Voltage Light Bulb Circuits
  
  The circuits described here are simple circuits that can be used for low voltage circuits. 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 (for safety if bulb is removed). The requirement of polarized plugs with the screw shell connected to the neutral blade makes these circuits (except SEPAR 204) not code compliant today for line-voltage circuits.
  
  The SEPAR circuit numbers used here have been used by the page author for years to catalog the various circuits.
  
  Here are the constraints incandescent loads impose on the design:
  
  1. The power source must never be short-circuited by any combination of switch positions.
  2. If screw-shell sockets are used, voltages must not be high enough to be dangerous to people if accidentally contacted.
  3. Nonshorting switches must be used.
  
  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 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 resistances, impedances and, nonpolarized heating elements.
  
  There are no constraints on circuits used to switch resistances and impedances other than those demanded by the circuit used and the circuit it is used in. But the following cautions must be observed with these circuits.
  
  1. Any power source feeding the circuit must never be short-circuited by any combination of switch positions.
  2. If live-front (e.g. knife) switches are used, the voltages must not be high enough to be dangerous to people.
  3. Nonshorting switches must be used.
  
  SIMPLE CIRCUITS SELECTING SERIES OR PARALLEL OPERATION OF LAMPS AND IMPEDANCES
  
  Advanced Line Voltage Light Bulb Circuits
  
  The circuits described here are advanced circuits that can be used for line voltage circuits. They can be used for photoflood lighting circuits, conforming to the necessity of having the screw shell of a line-voltage operated lamp on the neutral side (for safety if bulb is removed).
  
  The SEPAR circuit numbers used here have been used by the page author for years to catalog the various circuits.
  
  The SP-STD circuit numbers are new designations by the page author for new circuits.
  
  Here are the constraints line voltage loads impose on the design:
  
  1. The power must never be short-circuited by any combination of switch positions.
  2. The polarity of the power to the bulbs must never be reversed.
  3. The screw shell of the bulb must always be on the end of the filament closer to the neutral.
  4. The bulb must never be connected to hot without a circuit leading to the neutral.
  5. A nonconductive guard should surround any screw shell to prevent contact with a screw shell in contact with the socket while being inserted or removed.
  6. Nonshorting switches must be used.
  7. These circuits must never be used with compact fluorescent lamps or light emitting diode lamps.
  
  The following variations are permitted when line voltage lamps are used in series-parallel switching circuits.
  
  • One side of the lamp is connected to a 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 heating elements.
  
  These are the constraints on circuits used to switch resistances, impedances, and heaters:
  
  1. The power source must never be short-circuited by any combination of switch positions.
  2. If live-front (e.g. knife) switches are used, the voltages must not be high enough to be dangerous to people.
  3. Nonshorting switches must be used.
  
  LINE VOLTAGE CIRCUITS SELECTING SERIES OR PARALLEL OPERATION OF LAMPS AND IMPEDANCES
  
  Battery Circuits
  
  The circuits described here are circuits that can be used for switching batteries to change voltage and ampere hour capacities. They can be used for changing the configuration for charging and for using the batteries. Most of them are simple circuits.
  
  The SP-STD circuit numbers are new designations by the page author for new circuits.
  
  Here are the constraints batteries impose on the design:
  
  1. The batteries must be identical.
  2. No combination of switch positions may short out any battery.
  3. No combination of switch positions may short out the charging current.
  4. The polarity of each battery must never be reversed.
  5. If series-parallel circuits are used, each parallel group in the series must have the same number of batteries.
  6. If parallel-series circuits are used, each series group must have the same number of batteries.
  7. Nonshorting switches must be used.
  
  The following variations are permitted when batteries are used in series-parallel switching circuits.
  
  • One side of the battery is connected while the other side is open.
  
  CIRCUITS SELECTING SERIES OR PARALLEL OPERATION OF BATTERIES
  
  Guitar Pickup Circuits
  
  The circuits described here are circuits that can be used for switching guitar pickups to change the sound of the guitar.
  
  The SP-STD circuit numbers are new designations by the page author for new circuits.
  
  Here are the constraints guitar pickups impose on the design:
  
  1. The pickup polarity must not be reversed (unless this is desired for an effect).
  2. The ground end of the pickup must not be open when the live end is connected to the amp.
  3. The pickup shield must have separate wiring. It must not be tied to a pickup lead.
  4. The live lead of the amp must never be left open (causes hum).
  5. Shorting switches must be used.
  
  The following variations are permitted when guitar pickups lamps are used in series-parallel switching circuits.
  
  • A pickup may be shorted out.
  • The polarity of a pickup may be reversed for a special effect.
  
  The same circuits can be used for switching impedances and nonpolarized heating elements.
  
  CIRCUITS SELECTING SERIES OR PARALLEL OPERATION OF GUITAR PICKUPS
  
  Speaker Circuits
  
  The circuits described here are circuits that can be used for switching speakers to change the number of speakers while keeping the impedance presented to the amp near a set impedance.
  
  Here are the constraints this kind of load imposes on the design:
  
  1. The amplifier output must never be short-circuited by any combination of switch settings.
  2. The amplifier output must never be left with no load by any combination of switch settings.
  3. The polarity of a speaker must never be reversed by any combination of switch settings.
  4. Nonshorting switches are needed if moving a switch can momentarily short the amp output.
  5. Shorting switches are needed if moving a switch can momentarily leave the amp output open.
  6. The load across the amp must be kept as close to the ideal impedance as possible.
  7. Switching in another speaker should not change the volume of the other speakers.
  
  The following variations are permitted when speakers are used in series-parallel switching circuits.
  
  • Either or both ends of a speaker may be left open.
  • A speaker can be shorted out as long as the full amp output is not applied across it.
  • Another solution is the 25 volt line or the 70 volt line. Changing the load from one speaker does not affect the other speakers.
  
  The same circuits can be used for switching impedances and nonpolarized heating elements.
  
  CIRCUITS SELECTING SERIES OR PARALLEL OPERATION OF SPEAKERS
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