nuckolls.bob(at)aeroelect
Guest
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 Posted: Mon May 11, 2009 6:20 am Post subject: Relays |
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To: "MOH" <moh(at)myconnexion.co.uk>
Subject: Re: relays
Bcc: nuckolls.bob(at)aeroelectric.com
At 11:06 AM 5/10/2009 +0100, you wrote:
| Quote: | Hi Bob,
I m really struggling to understand how to wire a relay. Whilst I
understand in principle the wiring, I m struggling to get it to work
and no amount of reading schematics is sinking into my head!
This is what I understand about the relay so far;
Pin 30 is power in
Pin 87 goes to the item to be powered - (let s say fuel pump)
Pin 86 goes to ground
Pin 85 is to the switch.
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The numbers you're referring to are artifacts of a European
terminal identification system commonly used in automobiles
and probably other vehicles for decades. It's not uncommon
today to find off-the-shelf relays with terminal markings
molded into the housings that conform to this convention.
The numbering convention is illustrated in the drawing
cited below.
Think of the relay as a big, fat, remotely operated push-
button. Continuity through the push-button in the "relaxed"
state is through terminals 30 and 87a. Power can go either
way through these contacts.
The push-button is remotely operated by a magnetic motor
with coil terminals 85 and 86. It doesn't matter how
you apply 12v to this coil . . . terminal 85 OR 86
can be (+).
When you apply power to the coil, the "push-button"
contacts move to break continuity between 30/87a
and make contact between 30/87. This state is maintained
until you remove power from the coil.
| Quote: |
At the moment this switch is wired directly to the unit and for me
that is easy to understand. The fuel pump 12v + is wired to the
switch on the panel and then into the bus, fuel pump 12v is to
airframe ground. Press the switch and it works. But put a relay
in the midst of this and nothing happens apart from frying my brain!
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Relays are used to shorten the distance you might
need to run "fat wires" and/or to add functionality.
For example, the switch needed to control the relay
is a simple single-pole, single-throw device but the
relay effects a single-pole, double-throw functionality.
| Quote: | Is pin 30 the power supply from the bus that would normally go to
the switch ? I m struggling to understand how to wire the switch
into this circuit. Whilst this might sound elementary to you Bob,
I just cannot get my head around it !!
I need an verbatim explanation on how to wire the pump to the bus,
the switch and the relay. Once I see it in practical terms, i.e.
this wire goes to this, this wire goes to that, I m sure the penny
will drop, just as it has on all other problems I ve hit!
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There's a drawing on my website at . . .
http://www.aeroelectric.com/PPS/Engine/Fuel/Fuel_Pump.pdf
. . . that describes one way to implement the
wiring you've asked about. Of course, the 10A fuse
and 18AWG wires may need to be resized for your
specific fuel pump . . .
I will invite you to join us on the AeroElectric List
to continue this and similar discussions. It's useful to
share the information with as many folks as possible.
A further benefit can be realized with membership on
the list. There are lots of technically capable folks
on the list who can offer suggestions too. You can
join at . . .
http://www.matronics.com/subscribe/
Ask your question again on the List. I check the List
several times a day and attempt to be responsive as time
will allow. Further, there are 1800 or so subscribers
that include many technically capable teachers. This
is the best venue for assisting you in your studies.
Bob . . .
Bob . . .
----------------------------------------)
( . . . a long habit of not thinking )
( a thing wrong, gives it a superficial )
( appearance of being right . . . )
( )
( -Thomas Paine 1776- )
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