Come on Olin, now your letting me down.

78XX positive voltage reg
79XX negative voltage reg

lm7824 = three terminal linear reg 24V

Peter Cousens
cousens@biscit.biz

> -----Original Message-----
> From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU] On
> Behalf Of Olin Lathrop
> Sent: Saturday, August 16, 2003 7:25 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [EE:] 24vdc from a 15vdc psu
>
> > I need 24v@300mA, and 12v@2A. I've just been off and measured it all
> > up again and this is the worst-case power consumption, so I'd better
plan
> > for it.
> >
> > I'm using a 16f628 for this one, and I had thought of using the PWM
> > function to drive a diode-cap voltage doubler, but I'm not sure how
much
> > juice I can squeeze out of it.
>
> 300mA is a tall order for a diode pump.  The obvious answer seems like
a
> boost converter to me.
>
> > I could use a second 15v supply, then hook it up in series to an
> > lm7824(?), and then branch out the 15v rails to a beefy 12v reg.
>
> I'm not sure what you mean here, and I'm not going to look up what an
LM7824
> is just to answer a question.  If you've got two 15V supplies, you can
hook
> them in series to get two voltages, 15V and 30V.
>
> How accurate does the 12V output need to be?  Is using the 15V input
> acceptable?  If this is to power something meant for a car, then it
should
> run fine from a 15V supply with maybe a single power diode in series
to drop
> the voltage to about 14.3V.  Any car device will be able to handle
14.3V
> indefinitely because that's close the the charging voltage of a "12V"
> lead-acid battery.
>
> If you really need 12.0V out, note that a linear regulator will
dissipate
> 6W.  Less if you can adjust the 15V supply down a bit.
>
> If you can get a second 15V power supply, why not a 12V power supply
> instead?  Why not two 12V supplies?
>
> A lot of your comments don't seem to be consistent with each other, or
> there's a lot you're not telling us.  You've finally given us the
current
> spec for the two outputs, but what about the voltage tolerance?  What
kind
> of devices will be using this power?  Where does the original input
power
> come from?  If you can just "get another 15V supply", why can't you
just get
> exactly the supplies you need in the first place?
>
>
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