Nick(at)Scholtes1.com
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 Posted: Tue Jan 09, 2007 10:06 am Post subject: LM7805 (Off Topic) |
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Ted,
You may be right about me underestimating the LM7805's practical
ability. Now that you mention it, you're probably right. But, let me
explain.
I was an analog circuit designer (this is going back 20 years or so!)
for a telecommunications company. We were making equipment that had to
be highly reliable, to telecom standards, and it had to be designed for
lots of "what if" scenario's. So, one of the scenario's was that the
air-condtioning would fail inside of the office where the equipment was
installed, and the temperature of the office would rise pretty high
(over 120F). Plus, the equipment had to last 20 years, and it had to be
de-rated such that it had an MTBF (Mean Time Between Failures) that was
very high. Plus, we were making a zillion units, so the tolerance of
different parts had to be taken into account also. So, all that said,
we had to go through this algorithm of taking the ambient temperature,
and the airflow over the part, and the thermal coefficient between the
case and the junction, and derate the junction so that it would last a
long time, etc. etc. etc. So, given all THAT criteria, that's how I
came up with my numbers. The numbers are not for the LM7805
specifically, but they are for a generic TO-220 package. I used many
many different power devices in that package over the years, and after
doing the analysis a zillion times, I came up with generic numbers,
which is what you saw in my post.
So, you're right, in a "practical" application where the ambient
temperature is 80F max and you're only making one of them, and you don't
have to derate the part to meet an MTBF, I'm sure you can run it a whole
lot closer to it's limit than what I stated. Sorry that I let my
"telecom geek" come out!
But, I still think that it's worth emphasizing that it is the POWER that
is the limit, not the current, so when you say "You can easily draw
400mA from an LM7805", that doesn't say anything about how much power
you are dissipating in the part. It's all about the power, not about
the current. If you only drop 2V across the part, then sure, you can
pull 800mA or maybe an amp without a problem, because it isn't
dissipating that much power.
Take care.
Nick
(Do Not Archive)
| Quote: | Time: 04:17:47 PM PST US
From: "TC" <flier(at)sbcglobal.net>
Subject: RE: 5V Converter (Off Topic) do not archive
Hey Nick,
You're understating the 7805's ability! You can easily draw 400mA from an
LM7805 with a small aluminum heatsink, say 2" sq. They're thermally
protected and nearly foolproof. I've never had one fail over the years and I
use them by the dozens in all kinds of supplies. They're all over my shop.
I power an iPaq 4350 and HX2715 all the time with them. If the PDA draws
less than a half amp a 7805 will work fine. If the PDA requires over an amp
(when the battery is dead or the display is cranked) then maybe another
device would be better.
Regards,
Ted
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