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mark(at)metalcrafters.ph
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 Posted: Mon Jun 04, 2007 6:55 pm Post subject: Dropping Voltage to 5V from 12V |
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Hi There Folks
I need to supply a small device in the cockpit with 5V from a 12V aircraft
supply. One way to do this is with a transformer I would guess. Is there a
way to do it with resistors. The current requirement is 200-300 milliamps.
Appreciate any suggestions and recommendations
Regards
Mark Cupitt
RP-C883
Philippines
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aadamson(at)highrf.com
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| Posted: Mon Jun 04, 2007 7:16 pm Post subject: Dropping Voltage to 5V from 12V |
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Boy we are onto this topic tonight aren't we... Let's see for 5v it's a
7805, for 9v it's a 7809, to get to 12v from higher, it's a 7812. They even
make a 7810 for 10v... Negative voltage versions are 79xx 
Try LM7805 with a small number of additional components. All the major
mailorder places carry them, but can't tell you where to go in the
Philippines.
Alan
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Tim Olson
Joined: 25 Jan 2007
Posts: 2872
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| Posted: Mon Jun 04, 2007 9:29 pm Post subject: Dropping Voltage to 5V from 12V |
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I used a DC-DC converter for powering 2 5V devices from ship power.
Works really well for that. I tried LM7805's and didn't have good
luck because they couldn't deliver nearly the current well enough.
It depends on your draw, but I was using hard drive type video
ipod type devices and they just didn't do a great job and they
got really hot. The DC-DC converters can handle more load if you
buy the right one, and they'll do a great job.
Tim Olson - RV-10 N104CD - Flying
do not archive
Alan Adamson wrote:
[quote]
Boy we are onto this topic tonight aren't we... Let's see for 5v it's a
7805, for 9v it's a 7809, to get to 12v from higher, it's a 7812. They even
make a 7810 for 10v... Negative voltage versions are 79xx
Try LM7805 with a small number of additional components. All the major
mailorder places carry them, but can't tell you where to go in the
Philippines.
Alan
--
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john(at)ballofshame.com
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| Posted: Mon Jun 04, 2007 10:37 pm Post subject: Dropping Voltage to 5V from 12V |
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From 12volts, if you have any sort of significant current you really
need a heatsink. To get an idea of how much heat you'll be giving off,
if you're dropping 7 volts (12->5) at 750mA, you're dissapating 7V*.750A
= 5.25 watts. That's a lot for a tiny little package.
If you're at 200ma, it's 7*.2=3.5watts....which is still gonna get
pretty warm I would think. It'll be worse at 13.8V.
So the two things to do are:
a) drive the device with as much voltage as you can....check the specs
to see what it can take....maybe it's okay up to 8 volts, and you can
drive it safely at 7.5 with the linear regulator instead....LM317 if I
remember right (requires a couple of extra resistors to make it
work...no big deal). With the right heat sink, this can safely drive >
1A....Off the top of my head I remember 1.5A as being the rated
current. The 7805 is significantly less (.5A, or thereabouts). Just
remember that the less it has to drop from 13.8, the less heat you'll
need to dissipate.
b) order the heat sink for it. It attaches with a 2-56 or 4-40 screw
(can't remember at the moment)...piece of cake.
re: resistors....you can make a voltage divider, or something like that,
but you won't get 5V out the other end. You'll get some function of the
bus voltage (which will vary between 12.x Volts and 14.x Volts).
Ok, someone double check my math...I'm half asleep over here!
-John
Tim Olson wrote:
[quote]
I used a DC-DC converter for powering 2 5V devices from ship power.
Works really well for that. I tried LM7805's and didn't have good
luck because they couldn't deliver nearly the current well enough.
It depends on your draw, but I was using hard drive type video
ipod type devices and they just didn't do a great job and they
got really hot. The DC-DC converters can handle more load if you
buy the right one, and they'll do a great job.
Tim Olson - RV-10 N104CD - Flying
do not archive
Alan Adamson wrote:
>
> <aadamson(at)highrf.com>
>
> Boy we are onto this topic tonight aren't we... Let's see for 5v it's a
> 7805, for 9v it's a 7809, to get to 12v from higher, it's a 7812.
> They even
> make a 7810 for 10v... Negative voltage versions are 79xx
>
> Try LM7805 with a small number of additional components. All the major
> mailorder places carry them, but can't tell you where to go in the
> Philippines.
>
> Alan
> --
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henryroden
Joined: 09 Jan 2006
Posts: 13
Location: Albuquerque
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| Posted: Tue Jun 05, 2007 12:23 pm Post subject: Dropping Voltage to 5V from 12V |
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One idea Mark mentions is using a transformer! I was always told that when you put DC, as in 12V aircraft supply, in a transformer the output is smoke!
All the suggestions were for DC.
Henry
Time: 07:55:03 PM PST US
From: "Mark Cupitt" <[url=http://webmaila.juno.com/webmail/new/8?folder=Inbox&msgNum=0000Pp00:0016PGtD00001QK^&block=1&msgNature=all&msgStatus=all&count=1181074100&content=central#]mark(at)metalcrafters.ph[/url]>
Subject: AeroElectric-List: Dropping Voltage to 5V from 12V
Hi There Folks
I need to supply a small device in the cockpit with 5V from a 12V aircraft
supply. One way to do this is with a transformer I would guess. Is there a
way to do it with resistors. The current requirement is 200-300 milliamps.
Appreciate any suggestions and recommendations
Regards
Mark Cupitt
RP-C883
Philippines
[quote][b]
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wrmaxwell(at)bigpond.com
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| Posted: Tue Jun 05, 2007 6:51 pm Post subject: Dropping Voltage to 5V from 12V |
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Yes, there is no way a transformer will step DC voltage either up or down. You need to first convert it to AC before you can have any transformer action. The transformer primary winding will present as a dead short, or a very small resistance at best, to a DC voltage.
Resistors will do it but the unrequired 7 volts will generate heat. The current required is not huge, so I would be inclined to use a 3-terminal regulator, probably the readily available and cheap 7805 - less than $1 , mounted on a suitabe heatsink and with by-pass capacitors across both input and output. It will still generate heat but nothing that cant be catered for and you will get much better voltage regulation.
Bill
[quote] ---
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