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Subject: Re: High Voltage Input into 78xx
Date: Sat, 02 Nov 2002 03:30:23 -0800
X-Mailer: Mozilla 4.78 [en] (Win98; U)
> Shaun Davy wrote:
> > Just wondering if anyone could point me in the direction of how i could go
> > about running a +5V regulator off a 35V DC input (24VAC) and draw around the
> > 250mA mark and if possable for future projects upto .5A or 1A.
> > Should i just use a normal 7805 and strap a big heatsink to it to dissapate
> > all that heat or is there a better more efficient way?
> > Thanks
> > Shaun
There are lots of better and/or more efficient ways, eg any switcher.
There are a bunch of issues to be considered.
7805 is a generic term like band-aid. There are variants. I'm using the info
from the motorola MC7805 spec.
Is your peak voltage 35VDC? If it's unregulated derived from 24VAC, it may be
depending on line conditions. The maximum input spec on a 7805 is 35V.
You don't want to be anywhere near that number if you expect reliability.
If the regulator shorts, you'll smoke whatever you're driving.
Available current goes way down as you approach 35Vin.
You may not be able to get enough heat out even with a perfect heat
on which package you choose.
It doesn't show up on this spec sheet, but I'd worry a little about second
effects when running near the maximum input voltage.
Fewest parts: Use a BIG zener in the supply line to the 7805.
Or make a crummy zener out of a small zener and a big transistor.
Or use a PNP pass transistor and put a small zener in the drive to reduce
the volts on the 7805.
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