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HAWK21M
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L Recirc Fan & Light Dimmer - B757

Thu Jan 10, 2013 7:26 am

On the B757 What is the connection electrically.....that with the left recirculation fan Unpowered, the Light dim function of the Indicator lights gets Inoperative.
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fr8mech
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RE: L Recirc Fan & Light Dimmer - B757

Thu Jan 10, 2013 8:23 am

Are you saying that when the L RECIRC Fan is turned off the master dim function on the flight deck is inop? Or just the INOP light on the switch?
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fr8mech
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RE: L Recirc Fan & Light Dimmer - B757

Thu Jan 10, 2013 8:51 am

Hmmm, I never saw the connection before. I guess it's because the L RECIRC fan is always running.

A quick reading of the AMM:

Under normal conditions, 28v dc from the dim control circuit breaker energizes a power control relay which sends 28v dc to the dim control relays. The dim control relays are energized by the DIM position of the control switch providing a ground.
When air conditioning airflow sensors or rack overheat sensors detect insufficient airflow for equipment cooling, or power is removed from the recirculation fan, a ground is removed from the power control relay by the air conditioning trim valve controller. The power control relay then removes 28v dc from the dim control relays and disables the automatic dimming function. This ensures all warning and caution annunciations are visible even if cooling is lost.

[Edited 2013-01-10 00:59:03]
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HAWK21M
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RE: L Recirc Fan & Light Dimmer - B757

Sat Jan 12, 2013 6:38 pm

Quoting fr8mech (Reply 1):
Are you saying that when the L RECIRC Fan is turned off the master dim function on the flight deck is inop? Or just the INOP light on the switch?

No just power supply off thru CB.Then lights dont get dimmed selectable.

Quoting fr8mech (Reply 2):
The dim control relays are energized by the DIM position of the control switch providing a ground.

Thanks.
Any reason for this circuitry
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fr8mech
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RE: L Recirc Fan & Light Dimmer - B757

Sun Jan 13, 2013 2:41 pm

Quoting HAWK21M (Reply 3):
Any reason for this circuitry

The AMM says:

Quoting fr8mech (Reply 2):
This ensures all warning and caution annunciations are visible even if cooling is lost.
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boeing767mech
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RE: L Recirc Fan & Light Dimmer - B757

Sun Jan 13, 2013 3:15 pm

Quoting fr8mech (Reply 4):
Quoting HAWK21M (Reply 3):Any reason for this circuitry
The AMM says:

Quoting fr8mech (Reply 2):This ensures all warning and caution annunciations are visible even if cooling is lost.

I have been told with smoke in the cockpit you turn off the Left Hand Recirc fan, that is a reason why the warning lights go bright when turned off
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