Today's Message Index:
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1. 12:46 PM - voltage drop across relay (Ken Ryan)
2. 01:12 PM - Re: voltage drop across relay (Charlie England)
3. 03:09 PM - Re: voltage drop across relay (Robert L. Nuckolls, III)
4. 03:32 PM - Re: voltage drop across relay (Ken Ryan)
5. 05:03 PM - Re: voltage drop across relay (Robert L. Nuckolls, III)
6. 05:07 PM - Re: voltage drop across relay (Robert L. Nuckolls, III)
Message 1
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| Subject: | voltage drop across relay |
I have scanned chapter 11 but was not able to find the answer to this
question: What would be the typical voltage drop across a relay in a 12
volt system using 10 gauge wire to carry about 20 amps?
Message 2
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| Subject: | Re: voltage drop across relay |
Should be in the range of a few millivolts, at most, if the contacts are in g
ood shape. Wire size isn't relevant to voltage drop across the relay contact
s.
Sent from my iPhone
> On Dec 15, 2014, at 2:44 PM, Ken Ryan <keninalaska@gmail.com> wrote:
>
> I have scanned chapter 11 but was not able to find the answer to this ques
tion: What would be the typical voltage drop across a relay in a 12 volt sys
tem using 10 gauge wire to carry about 20 amps?
>
>
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Message 3
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| Subject: | Re: voltage drop across relay |
At 14:44 2014-12-15, you wrote:
>I have scanned chapter 11 but was not able to find the answer to
>this question: What would be the typical voltage drop across a relay
>in a 12 volt system using 10 gauge wire to carry about 20 amps?
Charlie's right . . . but I'm curious as
to what prompted the question.
In the best of all worlds, two pieces of
metal brought together to carry current
(switch and relay contacts) would produce
a zero-ohms joint but alas, we don't live
in a perfect world.
A relay in the same 'class' as you describe
Emacs!
Emacs!
. . . has a published maximum resistance of 0.030 ohms
Emacs!
according to specs . . . a drop of 20A x 0.030 ohms = 600 millivolts
or about 1.2 watts of dissipation at the contacts. I can
guarantee that contacts of that mass dumping that much
heat would not be long for this earth.
As a practical matter, any time you measure more than
100 millivolts across any set of switch or small relay
contacts at these kinds of loads, it's worth your time
to see if the critter is getting into trouble.
Bob . . .
Message 4
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| Subject: | Re: voltage drop across relay |
The answer to "what prompted the question" as to typical voltage drop
across a 12 volt relay is:
The Rotax schematic has the regulator C wire (which measures voltage) going
through a couple of relays on its way back to the battery. The text says
that there should be no more than 0.2 volt difference between the battery
and the C terminal. I was trying to get a handle on how much drop to expect
from the two relays.
On Mon, Dec 15, 2014 at 2:08 PM, Robert L. Nuckolls, III <
nuckolls.bob@aeroelectric.com> wrote:
>
> At 14:44 2014-12-15, you wrote:
>
> I have scanned chapter 11 but was not able to find the answer to this
> question: What would be the typical voltage drop across a relay in a 12
> volt system using 10 gauge wire to carry about 20 amps?
>
>
> Charlie's right . . . but I'm curious as
> to what prompted the question.
>
> In the best of all worlds, two pieces of
> metal brought together to carry current
> (switch and relay contacts) would produce
> a zero-ohms joint but alas, we don't live
> in a perfect world.
>
> A relay in the same 'class' as you describe
>
> [image: Emacs!] [image: Emacs!]
>
> . . . has a published maximum resistance of 0.030 ohms
>
> [image: Emacs!]
>
> according to specs . . . a drop of 20A x 0.030 ohms = 600 millivolts
> or about 1.2 watts of dissipation at the contacts. I can
> guarantee that contacts of that mass dumping that much
> heat would not be long for this earth.
>
> As a practical matter, any time you measure more than
> 100 millivolts across any set of switch or small relay
> contacts at these kinds of loads, it's worth your time
> to see if the critter is getting into trouble.
>
>
> Bob . . .
>
Message 5
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| Subject: | Re: voltage drop across relay |
At 17:30 2014-12-15, you wrote:
>The answer to "what prompted the question" as to
>typical voltage drop across a 12 volt relay is:
>
>The Rotax schematic has the regulator C wire
>(which measures voltage) going through a couple
>of relays on its way back to the battery. The
>text says that there should be no more than 0.2
>volt difference between the battery and the C
>terminal. I was trying to get a handle on how
>much drop to expect from the two relays.
Oh . . . yeah . . .
Suggest you consider an architecture like
Z-16 that places no switches or relay
contacts in series with the regulator's
output (B&R) or sense lead (C) and
the bus.
Bob . . .
Message 6
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| Subject: | Re: voltage drop across relay |
At 17:30 2014-12-15, you wrote:
>The answer to "what prompted the question" as to
>typical voltage drop across a 12 volt relay is:=C2
>
>The Rotax schematic has the regulator C wire
>(which measures voltage) going through a couple
>of relays on its way back to the battery. The
>text says that there should be no more than 0.2
>volt difference between the battery and the C
>terminal. I was trying to get a handle on how
>much drop to expect from the two relays.
I'm surprised they did that. Here's an excerpt from
another Rotax document that seems to avoid that
pitfall nicely . . .
Emacs!
Bob . . .
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