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Nick(at)Scholtes1.com
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 Posted: Mon Jan 08, 2007 1:34 pm Post subject: 5V Converter (Off Topic) |
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Hi Guys,
My "real job" is being an electrical engineer, so if I "geek out" too
much here, just slap me.......
Regarding the topic of trickle charging through a cigarette lighter plug
and having to turn the Master on and off, there is a work-around. What
you can do is put a diode in parallel with the Master switch (anode
connected to the cigarette lighter, cathode to the inboard side of the
Master switch). This makes it so that when the Master switch is turned
ON, current can flow either into the system or out of the system. But,
when the Master switch is turned OFF, current can only flow INTO the
system, hence you will only be able to trickle-charge (current flowing
IN) when the Master is off, and if you left a load plugged into the
cigarette lighter the diode would prevent it from pulling current when
the Master is off. This would alleviate the need to do anything
manually to configure breakers or switches, which I know I'd forget to
do sometime........
Regarding using a 7805, here are some thoughts. It says on the
datasheet that the 7805 can output "up to" 1 Amp. But that isn't really
the limiting factor, the heat dissipation is. The TO-220 package (which
is what the 7805 comes in) can reasonably dissipate about .75 watts if
it is not heat-sinked, and it can dissipate about 2 watts if it is
heat-sinked really well (like sitting at the bottom of Lake Michigan!)
To calculate the wattage that it dissipates, just take the voltage that
is across it, and multiply by the current through it (power = voltage *
current). Since you've got a 12V system and the 7805 outputs 5V, you've
got 7V across the 7805 (more than this really, because a 12V system
typically runs between 12.8 and 13.3 or so). So, to get .75 watts
dissipated in the device, you can pull about 0.1 amps out of the
device. As Luis stated, pulling 400 mA out of the device will be
dissipating 2.8 watts which will get it pretty darn hot without a
heatsink, and even with a very good heatsink it will still get pretty
hot, easily hot enough to burn you. Max junction temp is 125C, and 2.8
W un-heat-sinked will get it up higher than that (on a hot day with a
hot ambient temperature). Realistically, a non-heat-sinked 7805 in a
12V system will only be useful up to about 100 mA, which is useful for
many things, but not a PDA on full bright that draws 400 mA.
An alternative is an actual switching converter. You can get those at
various places like Digi-Key. I only spent a minute looking for one,
but here's one for about $15 or so, and I'm sure if you looked you could
find one quite a bit cheaper than that:
http://rocky.digikey.com/scripts/ProductInfo.dll?Site=US&V 96&M=PT78ST105H
A switching converter like this is better than 80% efficient, so only
20% of the total power consumed is dissipated in the device. So your
PDA draws 2 W (5V*.4A), which means that the converter will only
dissipate .5W, which it can handle no-problem.
The comment about using multiple 7805 in parallel is definitely a
practice to be avoided. The problem here is that the 7805, being a
power source, has a low-impedance output. If you connect two
low-impedance outputs together, and if they are not exactly matched, one
will put out just a tiny bit higher voltage than the other, and since
they are low impedance, the high one will feed a huge amount of current
to the low one. The only way to realistically "load share" using two
low-impedance outputs is to put a diode in series with each output, but
again this isn't a very good solution because the output/diode
combination that puts out the higher voltage will end up doing all the
work and it won't truly be "sharing".
Sorry for "geeking out" on you!
Best Regards,
Nick Scholtes
Time: 11:09:55 AM PST US
From: "Noel Loveys" <noelloveys(at)yahoo.ca>
Subject: RE: Re: Question for PDA GPS users: Hooking up power.
Correct you would have to leave the master and the power outlet breakers in
to trickle charge the battery...Max current 70% the amp value of the
breaker. Remember to pull the breakers on everything else.
The DC to DC adapters do consume a small (almost tiny) amount of current
even while under no load but that is why you have master switches and as I
recommend an accessory beaker between the power outlets and the main buss.
You could use several 7805 in parallel to attain the current you will need.
Keep a good heat sink on all of them. I used one on a converter I built for
a digital camera. I could get three pics before the circuit board got too
hot for my liking. The answer was to install a resistor in series with the
camera to reduce the load at the converter. I didn't mind waiting an extra
second or two for the camera to cycle. It's been working well for just
under ten years now.
Noel
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Float Flyr
Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland
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| Posted: Mon Jan 08, 2007 4:15 pm Post subject: 5V Converter (Off Topic) |
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Thanks Nick
I should have thought about the diode across the master myself. It's a nice
simple failsafe charge circuit.
I wasn't aware of the low impedance output of the 7805 chip but I can
certainly see what you are saying. The switching converter seems to be the
answer.
While you are on the line....
I have an old Icom 02-AT Handy which has a DC to DC converter built in that
allows the radio to operate over a fairly wide range of voltages including
directly off a car accessory outlet. Do you know of any reason that Icom
not install the same technology in the IC - A24?
Noel
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