Today's Message Index:
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1. 06:05 AM - Will a 12v component work in a 24v environment? (donjohnston)
2. 06:40 AM - Re: Will a 12v component work in a 24v environment? (C&K)
3. 12:29 PM - Re: Will a 12v component work in a 24v environment? (Robert L. Nuckolls, III)
4. 01:12 PM - Re: Relays (Robert L. Nuckolls, III)
Message 1
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| Subject: | Will a 12v component work in a 24v environment? |
The recent thread on relays got me wondering about something:
Is there something about a 12v relay or switch (basically mechanical components)
that precludes it's use in a 24v system of the same or less current loads?
Or put another way, a wire suited for 8 amps at 12v will be okay with 8 amps at
24v. So it would that logic also apply to a switch or relay?
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=439978#439978
Message 2
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| Subject: | Re: Will a 12v component work in a 24v environment? |
For the switch and for relay contacts, 12volts or 24 volts are
interchangeable for practical purposes.
But not for a 12 volt relay coil. It's current is controlled by its
resistance on our direct current systems. So twice the voltage means
twice the current and four times the power (Power is volts time current
which is heat). It will get hot and have a short life at twice the rated
voltage.
Ken
On 28/03/2015 9:04 AM, donjohnston wrote:
>
> The recent thread on relays got me wondering about something:
>
> Is there something about a 12v relay or switch (basically mechanical components)
that precludes it's use in a 24v system of the same or less current loads?
>
> Or put another way, a wire suited for 8 amps at 12v will be okay with 8 amps
at 24v. So it would that logic also apply to a switch or relay?
>
>
> Read this topic online here:
>
> http://forums.matronics.com/viewtopic.php?p=439978#439978
>
>
Message 3
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| Subject: | Re: Will a 12v component work in a 24v environment? |
At 08:04 2015-03-28, you wrote:
>
>The recent thread on relays got me wondering about something:
>
>Is there something about a 12v relay or switch (basically mechanical
>components) that precludes it's use in a 24v system of the same or
>less current loads?
>
>Or put another way, a wire suited for 8 amps at 12v will be okay
>with 8 amps at 24v. So it would that logic also apply to a switch or relay?
There are TWO ratings that describe
a relay's function. CONTACTS and COIL.
Generally speaking ANY switch or relay
with published CURRENT ratings for 120VAC will
have ratings in the same ballpark at
14VDC.
The coil is another matter. As others have
pointed out, operating a relay at 2x its
rated coil voltage will QUADRUPLE the power
it dissipates . . . guaranteed destruction.
Bob . . .
Message 4
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OOPS! I didn't get all the downloads on the thread . . .
You've had some excellent feedback already . . .
The only thing I would add is to not obsess over
contact ratings. If you pull the engineering data
sheets on any of the Digikey offerings, you'll find
that each product's ratings are accompanied with
a service life figure in the tens of thousands of
cycles.
Given that you'll probably fly your airplane no
more than 100 hours/year . . . how many years or
hours of flight are required to impress say 5000
cycles on any of your switches/relays?
Bottom line is that I've replaced far more such devices
that suffered from environmental effects of age and
DISUSE than for failure to observe the manufacturer's
published ratings.
The relay is for the e-bus battery feed on a 14 volt system. The
load is currently 12 amps, but I have been challenged to better
define my electrical design objects and possibly reduce the load down
to 8 amps.
I don't think that will change the relay selection if I were to use a
design of 5-20 amps.
Use anything you can find that is in-expensive and
easy to replace. Suggest you install something like
this.
http://tinyurl.com/qagvybn
I've purchased similar devices for as little as
$2.00 each in hundreds. Given that this is the
ALTERNATE feed path relay, it will get cycled ONCE
per flight so maybe a couple hundred times per year.
Further, its functionality is PREFLIGHT tested thus
limiting its ability to become an undetected latent
failure.
I selected TE Connectivity 1432793-1 Automotive relay SPDT 30A 12V.
Here is the link
<http://www.digikey.com/product-detail/en/1432793-1/PB1773-ND/1236843>
It is non latching with a diode across the coil. Some of the specs,
however, have me confused.
The Coil voltage is 12VDC but the Switching voltage is 14 VDC -
Nom. Is this an issue?
Also the Turn On Voltage (Max) is 7.8 VDC and Turn Off Voltage (Min)
1.2 Volts. Does this mean that the coil requires 7.8 volts to latch
and it will un-latch if voltage drops below 1.2 volts?
Pretty much.
I see that most of the automotive relays have high ohm resistors
across the coil, or nothing across the coil. When would you select a
resistor, diode or nothing?
Either resistor or diode is fine . . . I prefer
the diode but again . . . for a few hundred
cycles per YEAR . . . the precise suppression
method is exceedingly un-critical.
I have another need for a relay in my airplane. My EFIS remote
annunciation mute requires a contact to be close via 2 terminals via
a momentary switch. The vendor does not specify a current
requirement for the switch, and when I contacted them they said any
switch would work is is a fraction of an amp. I would mute the EFIS
with one of the buttons on my control stick, however my control stick
only has a common ground, so all switches close to ground. This would
not work for the remote mute.
Is there any reason why a relay wouldn't work?
Would the relay above work? It probably is not the best choice, but
it is only $4 and it would make for common spare parts, so if it
would work it could be my 'best' choice.
This is not a flight-critical functionality. If
I were spec'ing the relay into a TC aircraft,
I would pick something with established reliability
(just to keep the paper shufflers happy). For our
purposes, any "telecom" rated (low current contacts)
like this would do just fine.
http://www.digikey.com/product-detail/en/1461070-5/1461070-5-ND/1427501
The logistical problem with these relays is how
to bolt he little buggers to an airplane. Unlike
the one I cited above, these critters are designed
to solder to an etched circuit board and don't feature
mounting ears of any type.
Probably the easiest thing to do is solder 22AWG
lead wires to the proper 4 pins . . . perhaps install
a coil-suppression diode right on the pins. Pot the
assembly into a 'bolt-able' housing with long-set
epoxy with the wires hanging out. Alternatively,
mount to a perf-board and bring vibration supported
wires off the board. There's no graceful way to do
this. From a production perspective, I've often
considered offering a relay something like this
Emacs!
http://www.digikey.com/product-detail/en/TQ2SL-12V/255-2853-ND/649407
mounted within one of our stock d-sub housings sorta
like this . . .
Emacs!
This is one way to incorporate such relays in
a manner that looks more business-like for airplanes.
Also much less prone to failure by reason
of installation shortfall . . .
It IS a bit of a conundrum . . . but know that ratings for
these relays are the least of the issues.
Bob . . .
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