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racerjerry
Joined: 15 Dec 2009
Posts: 202
Location: Deer Park, NY
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 Posted: Fri Dec 25, 2015 7:48 am Post subject: Cessna OV protection + DIODE across ALT side of Master Swith |
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On 12 volt models of Cessna 172, it appears that when the Over-Voltage Sensor was added (172 L and M models), that Cessna removed the DIODE previously found on the ALT side of the master switch [output] contact. The diode goes to ground and apparently protects the ALT side master switch contacts from arcing.
I would like to install the Cessna over-voltage sensor in an earlier 172 without over-voltage protection. I do not need or desire the over-voltage warning light. I would prefer interrupting the field circuit (Cessna) over crowbar type protection for a standard externally regulated Ford type alternator.
Can anyone venture a good guess as to why this diode was removed? If the diode was left installed, would it interfere with the over-voltage sensor operation?
Thanks to you all, and may you all enjoy this Holiday season.
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Jerry King |
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user9253
Joined: 28 Mar 2008
Posts: 1908
Location: Riley TWP Michigan
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| Posted: Fri Dec 25, 2015 9:01 am Post subject: Re: Cessna OV protection + DIODE across ALT side of Master S |
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Voltage spike protection diodes are not needed if replaced with crowbar type over-voltage protection because the crowbar shorts out high voltage. But you said it was not the crowbar type. Regardless, I do not know how a reversed biased diode could interfere with any aircraft electrical circuit. If it could, someone please explain how.
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Joe Gores |
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maca2790
Joined: 26 Jan 2011
Posts: 59
Location: Australia
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| Posted: Fri Dec 25, 2015 12:54 pm Post subject: Cessna OV protection + DIODE across ALT side of Master Swith |
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It's a reverse Voltage spike that the Protection Diodes on relays and
switches protect against not a positive spike which is what the Over Voltage
Crowbar is for. The reverse Voltage Spike occurs because of current being
drawn in the circuit abruptly stops when the switch or relay opens. The
resulting Magnetic Field Collapse in the wiring on that circuit causes a
Negative going Voltage spike that can amount to voltage well in excess of
what some equipment can handle. So although I believe nearly all modern
Avionics would be protected against a reverse spike it does not hurt to be
careful. The Protection Diodes short to frame ground a negative going spike
that can damage Avionics that are not protected with their own Suppression
devices and reverse protection Diodes also help to preserve the switch
contacts on switches and relays.
Yours
John MacCallum
--
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user9253
Joined: 28 Mar 2008
Posts: 1908
Location: Riley TWP Michigan
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| Posted: Fri Dec 25, 2015 5:13 pm Post subject: Re: Cessna OV protection + DIODE across ALT side of Master S |
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I had assumed that reason voltage spike protection diodes are not used to protect the ALT switch in Bob's Z diagrams is because there is crowbar over-voltage protection. But maybe the real reason for no diode is because the alternator field does not produce a voltage spike when it is shut down. Bob?
Diodes should be connected across inductive loads (coils). Connecting a reversed biased diode between the load side of the ALT switch and ground should not hurt anything.
A diode should be connected directly across a coil rather than some distance away. In the case of a master contactor that is controlled by a switch to ground, a diode connected from the coil to ground will be reversed biased BOTH during normal operation and while an inductive spike is produced by the collapsing magnetic field. The reason being that when current to an inductor is interrupted, the collapsing magnetic field tries to keep current flowing in the SAME direction. I agree with John that induced voltage will be seen as reversed polarity to other loads connected in parallel with the coil.
Without a diode across a coil, most of the coil induced energy will be dissipated across the opening switch contacts and will shorten switch life.
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Joe Gores |
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racerjerry
Joined: 15 Dec 2009
Posts: 202
Location: Deer Park, NY
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| Posted: Sat Dec 26, 2015 5:35 am Post subject: Re: Cessna OV protection + DIODE across ALT side of Master S |
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Does anyone have any info regarding the internal circuitry of the Cessna over-voltage sensor? I am wondering if this sensor's internal transistorized circuit contains the equivalent of the removed diode and therefore a separate diode is no longer needed for arc suppression at the ALT side of the master switch. Hmmm.
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