Stan Prevost
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Reged: 06/02/04
Posts: 773
Loc: Huntsville, Alabama
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I was browsing the maintenance manual for my '99 Saratoga II TC and ran across an interesting little detail. See the attached schematic of the radio master switch wiring.
Assume the radio master is OFF, as is the usual case before starting the engine. Turning on the Battery Master switch applies battery power to the Electrical Bus. OK, that's normal. What is interesting is that the electrical bus is connected to the avionics bus through the normally closed contacts of the radio master relay, which was initially deenergized. So turning on the battery master also applies power to the avionics bus until the radio master relay pulls in, disconnecting the two buses. I don't know how fast that relay is, I would guess about 15 milliseconds, might be 40, but the 24V ones are faster than the 12V ones.
The same thing happens when external power is connected and the external power contactor is energized, which applies external power to the electrical bus.
I always thought the avionics bus was completely disconnected from the master electrical bus until the avionics master was turned on. Turns out that is not entirely true, and accounts for some transient behavior on the avionics panel I have observed at battery master switch turn-on (need to verify this next time at the airplane).
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Best Regards,
Stan
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Jeff Hartmann (EQY/SVH)
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Reged: 05/18/04
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Loc: SVH
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Not uncommon, This way if the radio master fails...pull the avionics breaker...no power to the relay..Avionics on!
All the King Airs are this way, when the bat switch is turned on you hear a short pop in the speakers.
(Edit).. When you use a GPU with this set up you should have the Bat switch on before putting the ground power on, to avoid spikes into the avionics.
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Jeff
Man's biggest mistake is to believe that he is working for someone else. (fortune cookie)
Edited by Jeff Hartmann (INT/SVH) (02/26/07 02:07 PM)
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Joe Budge (W29)
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Interesting. The diagram for the older Toga's isn't as clearly labelled but it looks like it works the same way. Thanks for the point out!
Regards,
Joe
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Stan Prevost
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Loc: Huntsville, Alabama
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Quote:
The diagram for the older Toga's isn't as clearly labelled but it looks like it works the same way.
The 1980 'toga I was in before had a manual emergency bus switch under the panel to get power to the avionics in case of a radio master switch circuit failure.
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Best Regards,
Stan
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Stan Prevost
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Reged: 06/02/04
Posts: 773
Loc: Huntsville, Alabama
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Quote:
Not uncommon, This way if the radio master fails...pull the avionics breaker...no power to the relay..Avionics on!
As you can see from the wiring diagram I posted, there is no avionics breaker involved in the avionics master relay circuit. Power to the relay comes directly from the main electrical bus, and the circuit is completed through a ground applied by the radio master switch.
Quote:
All the King Airs are this way, when the bat switch is turned on you hear a short pop in the speakers.
That is what I notice in this Saratoga.
Quote:
(Edit).. When you use a GPU with this set up you should have the Bat switch on before putting the ground power on, to avoid spikes into the avionics.
What spikes? The battery master spikes the avionics. Not with an overvoltage spike, just a short-duration application of power. When external power is connected, it is not applied to the electrical bus until the external power contactor is energized by externally connecting the contactor coil to the external power positive. Having the battery master just applies an uncontrolled surge to the ship's battery, if it is lower than the external power, which could stress the master solenoid contacts. If the external power is so dirty that it requires ship's battery to be connected for stabilization, seems to me that it should not be used. Anyway, the POH says to leave the battery master and alternator switch OFF when external power is connected.
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Best Regards,
Stan
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Joe Budge (W29)
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<< The 1980 'toga I was in before had a manual emergency bus switch under the panel to get power to the avionics in case of a radio master switch circuit failure. >>
As does mine ('88). It looks like that was a standard part of the installation if you had an avionics master in the '80-'93 planes (no other drawings in the Maintenance Manual.) Must not have needed it very much to have dropped it in the '94's. I don't think I've ever heard of anyone needing to use the emergency bus switch.
Regards,
Joe
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Jeff Hartmann (EQY/SVH)
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Stan,
I answered before I looked at the schematic...
The King Air set up is different..(attached) still relies on a relay, power off relay on...
As far as the GPU...I have seen some wild power from gas powered GPUs. The King Air models/years are different. Some require the master on to close the External power relay, some have a separate switch for Ground power, but all have the caution to have the battery on line to absorb "transients".
edit:...goes to prove you can't mix Pipers and Beeches..<G>
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Jeff
Man's biggest mistake is to believe that he is working for someone else. (fortune cookie)
Edited by Jeff Hartmann (INT/SVH) (02/27/07 03:06 AM)
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Stan Prevost
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Loc: Huntsville, Alabama
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Quote:
The King Air set up is different..(attached) still relies on a relay, power off relay on...
Jeff, it looks pretty much the same, with the addition of a circuit breaker in the power line to the avionics master relays, and a subpanel, and breakers for the avionics busses themselves. But as you said, the same idea.
I really don't understand either the Beech or the Piper circuit. In the '80 Saratoga, I believe the emergency avionics bus switch would apply power to the avionics bus directly from the battery, regardless of whether the battery master was on or not. It could be used to power the avionics on the ground, or in the air if the battery master had to be turned off in an electrical fire emergency. These other circuits we are looking at do....what? All I can see is that, in the Beech, if the avionics master relay coil developed a short in the coil or in the wiring to the coil, so that it smoked the relay or wires, either the circuit breaker to the relays would trip or the pilot could pull it. It also seems very misleading to label a breaker "Avionics Master Power", when pulling the breaker restores power <g>. In the Piper, I don't see any purpose for the circuit arrangement at all, as opposed to a more direct and obvious circuit that pulls in the relay and applies power to the avionics bus when the radio master switch is turned on. What does it protect against? All I can see that it does is glitches the avionics when the battery master is turned on.
Quote:
As far as the GPU...I have seen some wild power from gas powered GPUs. The King Air models/years are different. Some require the master on to close the External power relay, some have a separate switch for Ground power, but all have the caution to have the battery on line to absorb "transients".
I have little experience with external power units, but I could imagine a lot of transients as they are started up and are stabilizing. But one would hope that the airplane external power relay would not be closed during external power unit startup.
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goes to prove you can't mix Pipers and Beeches..<G>
and that you can't trust external power units!
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Best Regards,
Stan
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Ron Koyich
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Reged: 04/28/04
Posts: 1467
Loc: YWVA
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Our Baron left the factory with a similar system, Stan. It was wired so the
relay coil failing would keep the bus energized. What about the contact
points, I asked?
It's now got two manual switch-breakers in parallel (for redundancy, should
one fail open), which connect the avionics bus to the main bus. I don't have
plans at this point for a separate emergency avionics bus, but it would be
easy enough to incorporate.
I'm not concerned about transients on the avionics at starting, as some folks
seem to be, either, but I couldn't stand the thought of a little bitty relay
giving me problems down the road.
Ron (who never did like relays)
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Ray Tackett
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Reged: 04/30/04
Posts: 6067
Loc: Philadelphia, USA
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Ron,
Wassamatta U? Not liking relays, indeed! One of the first pieces of test
equipment I worked on was entirely relay logic and about half the size of a
local telephone exchange. It was normal to burnish contacts or to replace a
relay a couple of times a day. It kept six people gainfully employed full
time. What's wrong with that? It was also one of the last machines I saw
where I could actually identify all the components (relays, resistors,
capacitors, diodes, etc.) at a glance. <BSEG>
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Ron Koyich
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Reged: 04/28/04
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Loc: YWVA
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Maybe I just didn't ever like burnishing contacts or troubleshooting at the
24 pin sockets, etc., Ray.
Whatever - I'm happy they're mostly a thing of the past.
Ron
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Ray Tackett
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Reged: 04/30/04
Posts: 6067
Loc: Philadelphia, USA
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Ron,
That whole note was very tongue in cheek and labeled as such. I was
astonished that the machine could exist in a world where transistors were
quite good and the transition to the first small TTL chips was under way.
Its input was punched paper tape. Output was a Friden Flexowriter, a typebar
typewriter designed to be driven electrically.
The machine's purpose was to test backplane wiring for shorts and continuity.
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