[sdiy] MG-1 first mods, VC res

[Scott Gravenhorst]

"jamhoney" <jmahoney at gate.net> wrote:
>> ... 35V is the MAXIMUM
>> for all 78xx and 79xx regulators, except the 7824 and 7924.
>
>Can you cascade 2 regulators? Like, transformer --> 7824 --> 7815? I
>believe it's okay, but...

Well, you *can*, but the 7815 will get pretty warm.  These regulators
are happiest with the input voltage just above the dropout voltage plus
the regulator output voltage.  How much above dropout+output depends on
several things, including ripple at the input.  I try to maintain input
about 3 volts above the regulator's output voltage.  The more input
voltage you apply, the more the regulator's shunt transistor has to
"waste" current - creating heat.

>
>Is there limit on the maximum load? Example: Given a 7824 and a 7815
>that are each rated at 1 Amp, can you squeeze a full Amp though the
>7815? I'll guess that you have to allow for some current loss at the
>second regulator.

Well, the 7824 can safely supply one ampere to the 7815, but if the
circuit requires current draw from _both_ 24v and 15v, then the sum can
be a maximum of one ampere - and don't forget that some of that 1
ampere through the 7824 is wasted in the 7815's shunt transistor (it
has to make 9 volts go away).

Frankly, a proper spec'd xfmr is a far better way.  Using cascaded
regulators like this wastes energy IMHO.

>john
>
>
>----- Original Message ----- 
>From: "harrybissell" <harrybissell at prodigy.net>
>To: "Metrophage" <c0r3dump23 at yahoo.com>
>Cc: <synth-diy at dropmix.xs4all.nl>
>Sent: Sunday, October 31, 2004 2:35 PM
>Subject: Re: [sdiy] MG-1 first mods, VC res
>
>
>> Metrophage wrote: <snipp'd>
>>
>> > These things get extremely hot! Might this be because the 58vct
>> > transformer is a but much for +12/-15 volts? I wonder if the
>regulators
>> > might be happier with something like 35vct. Opinions?
>>
>> If that is 58vct AC, the DC voltage would be just over 40V.   35V is
>the
>> MAXIMUM
>> for all 78xx and 79xx regulators, except the 7824 and 7924.  You are
>> dancing with
>> sudden death here.
>>
>> Yes, that transformer is TOO high a voltage. I'd agree with the
>35VCT
>> (actually 36vct, dual
>> 18V secondaries).  Even a 48VCT is too high for 15V supplies.
>>
>> H^) harry
>>
>> >
>> >
>> > CJ
>

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