power supply and 7805 reg confusion ??

[mark krawczuk]

hi, just looking up some specs on the 7805 regulator , one site says a max
of 35vdc input and this says 25vdc max input
(http://store.americanmicrosemiconductor.com/7805.html) ,,,,,, i`m pretty
sure the max is 35vdc ?

what is correct ?

also i have nearly finished building a fixed and adj power supply all in
one , and after the filter cap i have a reading of 35.9vdc , now i`m
relying on the max being 35 vdc for the reg , so how can i EASILY knock
the voltage down to about 25 volts or so ?


thanks,'

mark k

 

[David L. Jones]

On Dec 2, 2:51 pm, "mark krawczuk" <krawc...@adam.com.au> wrote:

hi, just looking up some specs on the 7805 regulator , one site says a max
of 35vdc input and this says 25vdc max input
(http://store.americanmicrosemiconductor.com/7805.html) ,,,,,, i`m pretty
sure the max is 35vdc ?

what is correct ?

Whatever the datasheet says for your brand of 7805 device.

also i have nearly finished building a fixed and adj power supply all in
one , and after the filter cap i have a reading of 35.9vdc , now i`m
relying on the max being 35 vdc for the reg , so how can i EASILY knock
the voltage down to about 25 volts or so ?

Have you done your thermal calculations on your required heatsink?
What is your max output current?
A typical 1A capable output will dissipate 20W in the 7805 at max
current with a 25V input.
Whatever you use to drop the 10V will also dissipate 10W @ 1A.

Dave.

 

[Wayne.]

spot the fake wrote:

On Tue, 2 Dec 2008 15:21:50 +1030, "mark krawczuk" <krawczuk@adam.com.au> wrote:

hi, just looking up some specs on the 7805 regulator , one site says a max
of 35vdc input and this says 25vdc max input
(http://store.americanmicrosemiconductor.com/7805.html) ,,,,,, i`m pretty
sure the max is 35vdc ?

what is correct ?

also i have nearly finished building a fixed and adj power supply all in
one , and after the filter cap i have a reading of 35.9vdc , now i`m
relying on the max being 35 vdc for the reg , so how can i EASILY knock
the voltage down to about 25 volts or so ?

No transformer taps available?

If you're not bothered by the prospect of the AC input doing a +10% excursion,
then just chuck an extra diode into the bridge output.

I wonder if a zener and a SCR might do the trick on the supply side.

 

[spot the fake]

On Tue, 2 Dec 2008 15:21:50 +1030, "mark krawczuk" <krawczuk@adam.com.au> wrote:

hi, just looking up some specs on the 7805 regulator , one site says a max
of 35vdc input and this says 25vdc max input
(http://store.americanmicrosemiconductor.com/7805.html) ,,,,,, i`m pretty
sure the max is 35vdc ?

what is correct ?

also i have nearly finished building a fixed and adj power supply all in
one , and after the filter cap i have a reading of 35.9vdc , now i`m
relying on the max being 35 vdc for the reg , so how can i EASILY knock
the voltage down to about 25 volts or so ?

No transformer taps available?

If you're not bothered by the prospect of the AC input doing a +10% excursion,
then just chuck an extra diode into the bridge output.

 

[kreed]

On Dec 2, 2:51 pm, "mark krawczuk" <krawc...@adam.com.au> wrote:

hi, just looking up some specs on the 7805 regulator , one site says  a max
of 35vdc input and this says  25vdc max input
(http://store.americanmicrosemiconductor.com/7805.html) ,,,,,, i`m pretty
sure the max is 35vdc ?

what is correct ?

also i have  nearly finished building a fixed and adj power supply all in
one   , and after the filter cap i have a reading of 35.9vdc , now i`m
relying on the max being 35 vdc  for the reg , so how can i   EASILY   knock
the voltage  down to about 25  volts or so ?

thanks,'

mark k

I always believed that it was 35v actually, but this may vary between
manufacturers.

The simplest way is this item from Farnell, though note that its only
0.5A output:

http://www.datel.com/data/meters/dms-78xxsr.pdf
http://au.farnell.com/murata-power-solutions/7805sr-c/switching-regulator-ic/dp/1278561

(I have used this sort of device before to make a +5 V rail where only
28VDC was available, the only
drawback these devices may have is they wont let you put a diode from
the "Adj" pin to 0v to raise the
output voltage. The ADJ pin must always go direct to the 0v rail.)


Alternatively:
Unless you are only going to be supplying a small output current on
the 7805, (say up to 100ma) then the only proper way is to reduce this
35v at the source, either by removing windings from the transformer,
or installing an extra transformer to supply the lower voltage to your
regulator setup. Something like 9-12V DC would be a pretty good
choice for an input to a 7805, while allowing plenty of headroom to
keep the +5v stable if the mains were to drop or fluctuate.
A transformer like DSE.com.au part no M6672 would be a good choice.
Add a rectifier and filter cap and its ready to go.

You can use a large resistor or pre-regulator (7815,7824) to drop it,
but as said by David L Jones you will be dissipating lots of heat. For
example if you plan on pulling 1a from the system, you will be
dissipating 30 w (30v x 1A). (The 30v figure is the 35v in less the
5v out). Whether the 30w is dissipated in one or 2 regulator
devices or in a HUGE resistor and a regulator, its still a lot of heat
you will need to remove from the case, (and pay for on your
electricity bill) probably will probably also need a fan to do this.

 

[Trevor Wilson]

"mark krawczuk" <krawczuk@adam.com.au> wrote in message
news:gKWdncqvlJpxI6nUnZ2dnUVZ_ojinZ2d@adnap.net.au...

hi, just looking up some specs on the 7805 regulator , one site says a
max of 35vdc input and this says 25vdc max input
(http://store.americanmicrosemiconductor.com/7805.html) ,,,,,, i`m pretty
sure the max is 35vdc ?

what is correct ?

also i have nearly finished building a fixed and adj power supply all in
one , and after the filter cap i have a reading of 35.9vdc , now i`m
relying on the max being 35 vdc for the reg , so how can i EASILY
knock the voltage down to about 25 volts or so ?

**Depends on the application. You could use a suitably sized resistor.
Better, would be to use a zener/series pass transistor ahead of the
regulator. You could use a cheap, high power transistor to reduce the
regulator input to 9 Volts or so, reducing the dissipation of the regulator.
With a cap from Base to earth, you could gain some extra regulation too.

Read David's post. There are important questions/points there.

--
Trevor Wilson
www.rageaudio.com.au

 

[geoff]

Trevor Wilson wrote:

"mark krawczuk" <krawczuk@adam.com.au> wrote in message
news:gKWdncqvlJpxI6nUnZ2dnUVZ_ojinZ2d@adnap.net.au...
hi, just looking up some specs on the 7805 regulator , one site says
a max of 35vdc input and this says 25vdc max input
(http://store.americanmicrosemiconductor.com/7805.html) ,,,,,, i`m
pretty sure the max is 35vdc ?

what is correct ?

also i have nearly finished building a fixed and adj power supply
all in one , and after the filter cap i have a reading of 35.9vdc
, now i`m relying on the max being 35 vdc for the reg , so how can
i EASILY knock the voltage down to about 25 volts or so ?


**Depends on the application. You could use a suitably sized resistor.
Better, would be to use a zener/series pass transistor ahead of the
regulator. You could use a cheap, high power transistor to reduce the
regulator input to 9 Volts or so, reducing the dissipation of the
regulator. With a cap from Base to earth, you could gain some extra
regulation too.

Whether a transistor, series diodes, resistor, or a pre-regulator is used,
the excess power *will* be dissapted as heat directly related to the voltage
to be 'lost' and current..

If the current is significant, then an additional (or different) transformer
is the easiest approach.

geoff

 

[Trevor Wilson]

"geoff" <geoff@nospam-paf.co.nz> wrote in message
news:gsmdnRt3Q8ZcOqjUnZ2dnUVZ_q_inZ2d@giganews.com...

Trevor Wilson wrote:
"mark krawczuk" <krawczuk@adam.com.au> wrote in message
news:gKWdncqvlJpxI6nUnZ2dnUVZ_ojinZ2d@adnap.net.au...
hi, just looking up some specs on the 7805 regulator , one site says
a max of 35vdc input and this says 25vdc max input
(http://store.americanmicrosemiconductor.com/7805.html) ,,,,,, i`m
pretty sure the max is 35vdc ?

what is correct ?

also i have nearly finished building a fixed and adj power supply
all in one , and after the filter cap i have a reading of 35.9vdc
, now i`m relying on the max being 35 vdc for the reg , so how can
i EASILY knock the voltage down to about 25 volts or so ?


**Depends on the application. You could use a suitably sized resistor.
Better, would be to use a zener/series pass transistor ahead of the
regulator. You could use a cheap, high power transistor to reduce the
regulator input to 9 Volts or so, reducing the dissipation of the
regulator. With a cap from Base to earth, you could gain some extra
regulation too.

Whether a transistor, series diodes, resistor, or a pre-regulator is used,
the excess power *will* be dissapted as heat directly related to the
voltage to be 'lost' and current..

If the current is significant, then an additional (or different)
transformer is the easiest approach.

**Of course. However the history of the OP's ideas suggests that common
sense is in short supply. He will likely take the most circuitous and
inconvenient route to achieve his aim.


--
Trevor Wilson
www.rageaudio.com.au