Today's Message Index:
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1. 07:50 AM - Cessna OV protection + DIODE across ALT side of Master Swith (racerjerry)
2. 09:03 AM - Re: Cessna OV protection + DIODE across ALT side of Master Swith (user9253)
3. 12:54 PM - Re: Re: Cessna OV protection + DIODE across ALT side of Master Swith (John MacCallum)
4. 05:15 PM - Re: Cessna OV protection + DIODE across ALT side of Master Swith (user9253)
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| Subject: | Cessna OV protection + DIODE across ALT side of Master |
Swith
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.
--------
Jerry King
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=451408#451408
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| Subject: | Re: Cessna OV protection + DIODE across ALT side of |
Master Swith
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.
--------
Joe Gores
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=451410#451410
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| Subject: | Re: Cessna OV protection + DIODE across ALT side |
of Master Swith
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
-----Original Message-----
From: owner-aeroelectric-list-server@matronics.com
[mailto:owner-aeroelectric-list-server@matronics.com] On Behalf Of user9253
Sent: Saturday, 26 December 2015 4:01 AM
Subject: AeroElectric-List: Re: Cessna OV protection + DIODE across ALT side
of Master Swith
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.
--------
Joe Gores
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=451410#451410
Message 4
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| Subject: | Re: Cessna OV protection + DIODE across ALT side of |
Master Swith
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.
--------
Joe Gores
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=451413#451413
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