decreasing votage to regulators ?

M

mark krawczuk

Guest
HI, i am in the process of building up a power supply using lm 7805 regs
etc.....
but the voltage before its had a capacitor across it , staright of the
rectifier is about 37 volts dc, which is too much for the reg, plus with the
cap across it would increase it abit.

is there a simple way to reduce the voltage to the regualtor ?
thanks, mark k



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mark krawczuk wrote:
HI, i am in the process of building up a power supply using lm 7805 regs
etc.....
but the voltage before its had a capacitor across it , staright of the
rectifier is about 37 volts dc, which is too much for the reg, plus with the
cap across it would increase it abit.

is there a simple way to reduce the voltage to the regualtor ?
thanks, mark k
Click to expand...
A different transformer (if practicable).
A high-power zener-diode (eg. 12V) in series with the regulator.

How much current do you need to use at 5V ?
 
HI, i am in the process of building up a power supply using lm 7805 regs
etc.....
but the voltage before its had a capacitor across it , staright of the
rectifier is about 37 volts dc, which is too much for the reg, plus with
the
cap across it would increase it abit.

is there a simple way to reduce the voltage to the regualtor ?
thanks, mark k
 
"mark krawczuk" <krawczuk@dodo.com.au> wrote in message
news:4168c01b@news.comindico.com.au...
HI, i am in the process of building up a power supply using lm
7805 regs
etc.....
but the voltage before its had a capacitor across it , staright of
the
rectifier is about 37 volts dc, which is too much for the reg, plus
with
the
cap across it would increase it abit.

is there a simple way to reduce the voltage to the regualtor ?
thanks, mark k

32 volts is a big waste of power unless you only need a small amount
Click to expand...
and some other circuit needs the 37 volts.
A smaller transformer seems to be the way to go.
But then you did not tell us everything about you requirements.
--
John G

Wot's Your Real Problem?
 
hi, only a maximum of 1 amp.
thanks, mark k

--



"MC" <MC@nonexistant.place> wrote in message
news:ckab14$4u3$1@news-02.connect.com.au...
mark krawczuk wrote:
HI, i am in the process of building up a power supply using lm 7805
regs
etc.....
but the voltage before its had a capacitor across it , staright of the
rectifier is about 37 volts dc, which is too much for the reg, plus with
the
cap across it would increase it abit.

is there a simple way to reduce the voltage to the regualtor ?
thanks, mark k

A different transformer (if practicable).
A high-power zener-diode (eg. 12V) in series with the regulator.

How much current do you need to use at 5V ?
Click to expand...
 
hi, its just that i got the transformers really cheap so.....
mark k

--

"John G" <Greentest@ozemail.com.au> wrote in message
news:du3ad.334$7n.17790@nnrp1.ozemail.com.au...
"mark krawczuk" <krawczuk@dodo.com.au> wrote in message
news:4168c01b@news.comindico.com.au...

HI, i am in the process of building up a power supply using lm
7805 regs
etc.....
but the voltage before its had a capacitor across it , staright of
the
rectifier is about 37 volts dc, which is too much for the reg, plus
with
the
cap across it would increase it abit.

is there a simple way to reduce the voltage to the regualtor ?
thanks, mark k

32 volts is a big waste of power unless you only need a small amount
and some other circuit needs the 37 volts.
A smaller transformer seems to be the way to go.
But then you did not tell us everything about you requirements.
--
John G

Wot's Your Real Problem?
Click to expand...
 
"mark krawczuk" <krawczuk@dodo.com.au> wrote in message
news:4169234d@news.comindico.com.au...
hi, its just that i got the transformers really cheap so.....
mark k

You're going to be dissipating about 32W in the heatsink attached to your
Click to expand...
regulator at 37V, 1A. Using a 1.4 degree C/W heatsink ($16.75 @ Jaycar)
you're going to get a temperature rise of about 44 degrees, or a painful 80
degrees C when the air temperature gets to 36. A multitapped 0-15V 15VA
transformer from the same supplier will set you back $10.95. Using our
cheap trnsformers is likely to be false economy.

Cheers,
Alf
 
"mark krawczuk" <krawczuk@dodo.com.au> wrote in message
news:4169231c@news.comindico.com.au...
hi, only a maximum of 1 amp.
thanks, mark k
Click to expand...
**ONLY a maximum of 1 Amp! Yikes! Do the math. Far too much for a TO220 3
term reg. Here is what I would do, if I were you:

* Use a series pre-regulator, with a 2N3055 (with heatsinking) and a zener
(say: 12Volts).
* Use either a current booster with your 7815, or an LM338K (with
heatsinking).

The back of your Dick Smith catalogue should contain all you need. As does
the National Semiconductor data sheets.

Your requirement is a VERY inefficient use of materials. MUCH better to
lower the input Voltage to a more reasonable level.


--
Trevor Wilson
www.rageaudio.com.au
 
"Unbeliever" <alfkatz@remove.the.bleedin.obvious.ieee.org> wrote in message
news:41692c95$0$4310$afc38c87@news.optusnet.com.au...
"mark krawczuk" <krawczuk@dodo.com.au> wrote in message
news:4169234d@news.comindico.com.au...
hi, its just that i got the transformers really cheap so.....
mark k

You're going to be dissipating about 32W in the heatsink attached to your
regulator at 37V, 1A. Using a 1.4 degree C/W heatsink ($16.75 @ Jaycar)
you're going to get a temperature rise of about 44 degrees, or a painful
80
degrees C when the air temperature gets to 36. A multitapped 0-15V 15VA
transformer from the same supplier will set you back $10.95. Using our
cheap trnsformers is likely to be false economy.
Click to expand...
**Actually, the situation is worse. Much worse. The junction to case thermal
resistance of the 7805T is around 4oC/Watt. Even with your 1.4oC/Watt
heatsink, the temperature rise will cause the regulator to shut down. A
pre-regulator is the best way out, IMO. A TO3 regulator MIGHT survive.


--
Trevor Wilson
www.rageaudio.com.au
 
no its not, LOOK at the data for the lm 78 XX regulators, they do handle 1
amp.........
MARK K

--


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"Trevor Wilson" <trevor@SPAMBLOCKrageaudio.com.au> wrote in message
news:4169960d$1@news.comindico.com.au...
"mark krawczuk" <krawczuk@dodo.com.au> wrote in message
news:4169231c@news.comindico.com.au...
hi, only a maximum of 1 amp.
thanks, mark k

**ONLY a maximum of 1 Amp! Yikes! Do the math. Far too much for a TO220 3
term reg. Here is what I would do, if I were you:

* Use a series pre-regulator, with a 2N3055 (with heatsinking) and a zener
(say: 12Volts).
* Use either a current booster with your 7815, or an LM338K (with
heatsinking).

The back of your Dick Smith catalogue should contain all you need. As does
the National Semiconductor data sheets.

Your requirement is a VERY inefficient use of materials. MUCH better to
lower the input Voltage to a more reasonable level.


--
Trevor Wilson
www.rageaudio.com.au
Click to expand...
 
"mark krawczuk" <krawczuk@dodo.com.au> wrote in message
news:4169b5f8$1@news.comindico.com.au...
no its not, LOOK at the data for the lm 78 XX regulators, they do handle
1
amp.........
Click to expand...

**They do handle 1 Amp, but they are limited in their ability to dissipate
power.

STOP TOP POSTING.


--
Trevor Wilson
www.rageaudio.com.au
 
hi, whats top posting ?
i accidently posted two posts exactly the same , cause my puter locked up
and wasnt sure the firsty one went thru.
mark k

--


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"Trevor Wilson" <trevor@SPAMBLOCKrageaudio.com.au> wrote in message
news:4169bbd9@news.comindico.com.au...
"mark krawczuk" <krawczuk@dodo.com.au> wrote in message
news:4169b5f8$1@news.comindico.com.au...
no its not, LOOK at the data for the lm 78 XX regulators, they do
handle
1
amp.........


**They do handle 1 Amp, but they are limited in their ability to dissipate
power.

STOP TOP POSTING.


--
Trevor Wilson
www.rageaudio.com.au
Click to expand...
 
**This is top posting.

"mark krawczuk" <krawczuk@dodo.com.au> wrote in message
news:4169c7f3@news.comindico.com.au...
hi, whats top posting ?
i accidently posted two posts exactly the same , cause my puter locked up
and wasnt sure the firsty one went thru.
mark k
Click to expand...
**And this is how humans post. AFTER the post made initially.


--
Trevor Wilson
www.rageaudio.com.au
 
"mark krawczuk" <krawczuk@dodo.com.au> wrote in message
news:4168ab07@news.comindico.com.au...
HI, i am in the process of building up a power supply using lm 7805
regs
etc.....
but the voltage before its had a capacitor across it , staright of the
rectifier is about 37 volts dc, which is too much for the reg, plus with
the
cap across it would increase it abit.

is there a simple way to reduce the voltage to the regualtor ?
thanks, mark k



You didn't say what the VA rating of the transformer is. e.g. how much
Click to expand...
current it is able to give, though I do get from subsequent posts that you
want 1amp from your 7805 reg.
However, if the tranny is able to provide 1amp, you may find it is
constructed so as you can remove turns from the secondary winding.
Somertimes the secondary is wound over the top of the primary, other times
they are side by side on the core. In either case, if the secondary is
traditional construction of layers of wire separated by layers of lacquered
paper you should be able to remove the top layer of paper and begin
laboriously unwinding the secondary without disassembling the interleaved
laminations of the iron core. It may be difficult until you get a layer or
two off which gives you more to room pull the wire back through the
"windows" of the core.
Unwind 10 or 20 turns, scrape the free end of the lacquered wire bright,
apply 240v to primary (safely insulated connections and earth wire connected
please)and measure with an AC voltmeter how much the AC voltage from the
secondary has dropped. This will give you a clue about the "turns per volt"
of the secondary and how many more turns you need to remove to reach a
target voltage of maybe 10 VAC. When you have got there, you can cover the
exposed winding with some of the salvaged paper or even ordinary brown paper
impregnated with varnish (not essential but looks nice and protects the
winding) and re-terminate the end of the winding.
Data sheet says that min input V for 7805 is 7.2VDC so you could even aim
for a lower target than 10VAC. Remember that the higher the inputV, the more
power the reg must dissipate to output 5V and the less current you will be
able to draw before the thing goes into thermal shutdown. Go lower than the
min 7.2 VDC input and it won't regulate. Diff between regulated output and
min input is termed dropout voltage and some reg types are made to have
small dropout voltages.
BTW, one amp from a 7805 is pushing it. May I suggest an LM317 in a TO3 can?
Data sheet available online from Nat Semi.
If you're not comfortable with the idea of butchering the tranny or it seems
like too much effort, a lower voltage transformer is the best answer.
However, altering the tranny does work. I've done it most recently with a 30
VAC transformer from an obsolete Telecom PABX power supply which is now part
of a trickle charger for a workshop 12v battery.

P
 
mark krawczuk wrote:
hi, only a maximum of 1 amp.
thanks, mark k

--



"MC" <MC@nonexistant.place> wrote in message
news:ckab14$4u3$1@news-02.connect.com.au...

mark krawczuk wrote:

HI, i am in the process of building up a power supply using lm 7805

regs

etc.....
but the voltage before its had a capacitor across it , staright of the
rectifier is about 37 volts dc, which is too much for the reg, plus with

the

cap across it would increase it abit.

is there a simple way to reduce the voltage to the regualtor ?
thanks, mark k

A different transformer (if practicable).
A high-power zener-diode (eg. 12V) in series with the regulator.

How much current do you need to use at 5V ?
Click to expand...
Get a different transformer, something around 6.3VAC or 9VAC.
With the transformer you seem to have now you will be wasting
(and dissipating) about 40Watts to get 5V at 1A.
 
mark krawczuk wrote:
HI, i am in the process of building up a power supply using lm 7805 regs
etc.....
but the voltage before its had a capacitor across it , staright of the
rectifier is about 37 volts dc, which is too much for the reg, plus with the
cap across it would increase it abit.

is there a simple way to reduce the voltage to the regualtor ?
thanks, mark k
Click to expand...

Hello Mark,
I realise you asked your question some time ago, but if
you are still reading this group you may find this article
http://michaelgellis.tripod.com/power4.htm
and the links interesting.
Regards,
John Crighton
Hornsby
 
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