WIRING - POWER DISTRIBUTION

The following diagram has several unique features compared with the CHR provided diagrams:

• The use of clamping diodes on all contactors and relays.

• The use of fusible links on the wires running from the shunt to the AMP Meter. If either of these wires ever touched a ground, they would cause a short circuit. The links act as a fuse whereby a short causes a melted wire in a controlled and known place (i.e. inside a fiberglass and silicone sleeve). These links are also used on the Auxillary Power and Starter indicator lamp circuits since these wires lead to heavy cable and would otherwise be unfused.

• The use of ANL current limiters to catch an over-current condition on the +12 Volt bus and also to prevent the battery from shorting through the alternator in the event of an alternator diode or control circuit failure.

• Auxiliary (i.e. Ground) Power Contactor - The primary purpose of this circuit is to add over-voltage protection in the case of 24V ground power being applied and also adds a switch to enable / disable ground power without having to pull the plug. I've mounted the switch and lamp far back on the console switch panel where it is partially hidden by the center collective since it should never be required in flight.

• Alternator Contactor - Since I'm using an internally regulated alternator, I wanted a way to protect against a regulator failure. I'm led to believe that there is a way that the internal regulator can fail whereby the alternator is still generating current but there is no voltage regulation. Thus the contactor to isolate the alternator output and an over-voltage protection module in the circuit that should trip the breaker in the event of a runaway alternator that causes the +12 volt bus to rise above ~16.25 volts.

• Combined Master / Alternator Switching - The circuits that I have seen for internally regulated alternators rely on the use of a Cessna-style split master-alternator switch whereby the alternator switch can only be physically closed if the master switch is in the ON position. Since it is definately NOT ADVISEABLE to have the alternator circuit in the ON condition with the master OFF, I've used a unique twist. The "field" input is switched in the normal manner using a breaker plus switch from the +12 volt bus and also provides the +12 volts for the alternator isolation contactor. Instead of grounding the other side of the contactor, it is run to a DPDT master switch and thus the contactor should only be energized when both the master and alternator switches are in the ON position. Note that the pilot should still adhere to the practice of only turning the alternator ON when the master switch is ON.
  
  For those that aren't using an alternator isolation contactor, it is still possible to use a DPDT switch to prevent the alternator from being turned ON when the master switch is OFF. The problem is that the main bus is still being fed with power from the alternator if the engine is running even though the master contactor has disconnected the battery from the bus which is still "live". Something along the following lines could be used:
  
  

• Disconnect for gauges and electric tachs when the starter is engaged - I've added a Normally Closed relay circuit from the main power bus that is opened (i.e. disconnected) when the starter is engaged. This is to prevent the controlled circuits from being exposed to large voltage swings and possible spikes. The negative of this arrangement is for "smart" gauges that go through an initialization sequence and will be reset each time the starter is engaged. I'm also testing some transient voltage suppressors (TVS's) and have a circuit for a buck-boost regulator. If my gauges can tolerate the voltage sag during starting (or I add the regulator) then the installed relay can just be shorted (or the input from the starter contactor left disconnected) to negate it's function. Since I've installed a TVS and will be testing it, I've chosen to negate this relay for now.

WARNING: This diagram is still a work in progress for illustrative purposes only and has not been fully tested.

Notes:

• The +12V bus is fed by an 8AWG wire. The GND bus should only require 12AWG wire since the landing light has the highest current requirement and both it and the nav lights are grounded directly to the frame. It may be a bit of overkill, but I'm using 8AWG wire for the GND bus to both allow for future expansion plus lowering any potential voltage drop.

• The GND bus is shown folded just to fit on the page; its actually straight. The TIE bus GND on the solenoid board is for convenience only ... it's not necessarily good practice, but it will only be used for devices on the solenoid board.

• Although the alternator is rated at 40 Amps and thus 40 Amp ANL's are used, the provided AMP Meter is marked for 30 Amps and the shunt is a 30 Amp version. Unfortunately, Westach does not appear to offer a 40 Amp shunt and their choice is between 30 and 60 Amp models. I obtained a 40 Amp shunt from B&C Specialty which should be compatible and a drop in replacement ... the downside is that the gauge markings are for +/- 30 and I just added some labels to indicate +/- 40.

• The "always on" line to the ELT breaker (used for the remote display) and Dynon Keep Alive Power is shown as shielded wire. The shielding is not connected and is simply to provide abrasion resistance since this wire is not fused and bypasses the ANL. The wire to the TIE Bus is considerably oversized at this time to allow for any possible future expansion.

• I need to do some further testing on the alternator failed lamp once this is all operational. Unless the alternator has some special circuitry internally, it would appear that the lamp MAY not be lit under certain circumstances even when it should be. If this is the case, then I'll need to add a relay so that this lamp is always on if the Master switch is on and the alternator is not providing current.

• The indicator lamp for Starter Engaged shows a FET (Field Effect Transistor) in the circuit. This is installed to invert the signal to be consistent with the other indicator lamps whereby the input is grounded to turn the lamp ON.

• It is very easy to add a starter lockout whereby the starter cannot be engaged if there is oil pressure (i.e. engine is already running). This would be implemented by running a wire from the Normally Closed side of the oil pressure switch to the mounting frame of the starter contactor and making sure the frame is otherwise isolated. I'm refraining from adding this change until I can verify the amperage rating for the contacts in the oil pressure switch.

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Last updated: June 02, 2008