# 24V to 5V

J

#### John Popelish

Jan 1, 1970
0
I need to convert a 24V source to 5V / 1.0 A max output. I tried using
a 7805 but it generates alot of heat. Is there a way of gettinh 5V
without the excessive heat generation? Thx!
The methods used in switching power supplies is to turn the
full 24 volts on for 5/24ths of the time at some high rate,
and low pass filter the resulting pulse stream with a low
loss LC filter so that the 5 volt average appears at the output.

http://www.interq.or.jp/japan/se-inoue/e_ckt22_2.htm

D

#### [email protected]

Jan 1, 1970
0
I need to convert a 24V source to 5V / 1.0 A max output. I tried using
a 7805 but it generates alot of heat. Is there a way of gettinh 5V
without the excessive heat generation? Thx!

M

#### Meat Plow

Jan 1, 1970
0
I need to convert a 24V source to 5V / 1.0 A max output. I tried using
a 7805 but it generates alot of heat. Is there a way of gettinh 5V
without the excessive heat generation? Thx!

The drop in voltage is going to generate heat no matter what. Maybe you
could experiment and spread the drop across 2 devices 24-12-5 and use a
larger heatsink or two. Are you afraid the heat is going to shorten the
life of the 7805 or is it just undesirable in a certain environment?

--
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J

#### Jon Slaughter

Jan 1, 1970
0
I need to convert a 24V source to 5V / 1.0 A max output. I tried using
a 7805 but it generates alot of heat. Is there a way of gettinh 5V
without the excessive heat generation? Thx!

Add a fan and heat sink?

Use additional regulators to sucessively drop the voltage down(say a 20V to
15 to 12 to 8 to 5). This essentially just spreads out the heat over the
different regulators). This difference here is that you can use the outputs
for power sources too(Although you have to be careful about the total power
used).

Your best bet is simply to get a transformer that actually drops down to the
regulator for no reason. 16W is not much if you get a better regulator but I
think its way to much for the 7805.

I would suggest you run down to radio sluts and get a 78M05 or even the
LM317. These are better regulators that are essentially drop in replacements
for what you want. You could also use an external transistor to handle the
current(and can achive much larger currents if you wish).

Either get a new regulator, use some method do distribute the heat across
different elements, use a heat sink and/or fan, or get a transformer or
wall-wart that gets you around 10V instead of 24V.

Jon

H

#### Homer J Simpson

Jan 1, 1970
0
I need to convert a 24V source to 5V / 1.0 A max output. I tried using
a 7805 but it generates alot of heat. Is there a way of gettinh 5V
without the excessive heat generation? Thx!

Switch mode PS?

J

#### John Fields

Jan 1, 1970
0
The drop in voltage is going to generate heat no matter what. Maybe you
could experiment and spread the drop across 2 devices 24-12-5 and use a
larger heatsink or two. Are you afraid the heat is going to shorten the
life of the 7805 or is it just undesirable in a certain environment?

---
Sorry, no cigar! Not even a lollipop.

The correct answer is "Use a buck regulator".

The reason is that, doing it that way, power will be transferred
from the source to the load reactively, with a very small penalty
paid for the transfer.

If he needs 5 watts out, max, and a 90% efficient regulator, the
regulator will have to dissipate 1/2 watt, which is decent.

The other way, if he goes totally linear, no matter how he splits
things up he'll have to dissipate:

Pd = (Ein - Eout) * Iout = (24V - 5V) * 1A = 19 watts

in the regulator for 5 watts into the load.

Totally indecent.

J

#### John Fields

Jan 1, 1970
0
Add a fan and heat sink?

Use additional regulators to sucessively drop the voltage down(say a 20V to
15 to 12 to 8 to 5). This essentially just spreads out the heat over the
different regulators). This difference here is that you can use the outputs
for power sources too(Although you have to be careful about the total power
used).

Your best bet is simply to get a transformer that actually drops down to the
regulator for no reason.

---
neglecting the quiescent current in the regulator, for 5V across a 5

Pd = (Ein - Eout) * Iout = (24V - 5V) * 1A = 19 watts
---
16W is not much if you get a better regulator but I
think its way to much for the 7805.

I would suggest you run down to radio sluts and get a 78M05 or even the
LM317. These are better regulators that are essentially drop in replacements
for what you want.

---
Not true. The LM317 requires a two resistor voltage divider to set
the output voltage.
---
You could also use an external transistor to handle the
current(and can achive much larger currents if you wish).

M

#### Meat Plow

Jan 1, 1970
0
---
Sorry, no cigar! Not even a lollipop.

The correct answer is "Use a buck regulator".

The reason is that, doing it that way, power will be transferred
from the source to the load reactively, with a very small penalty
paid for the transfer.

If he needs 5 watts out, max, and a 90% efficient regulator, the
regulator will have to dissipate 1/2 watt, which is decent.

The other way, if he goes totally linear, no matter how he splits
things up he'll have to dissipate:

Pd = (Ein - Eout) * Iout = (24V - 5V) * 1A = 19 watts

in the regulator for 5 watts into the load.

Totally indecent.

Well the point is moot since a self contained smps is the way to go.
I'm more of an experimenter and trial and error learner than book.
And I have much more experience with linear devices than reactive learning
in an era when everything was filled with tubes. Built my first 50 watt
guitar amp from scratch 33 years ago and not from a kit but from a xerox
copy of a Traynor amp.

--
#1 Offishul Ruiner of Usenet, March 2007
#1 Usenet Asshole, March 2007
#1 Bartlo Pset, March 13-24 2007
#10 Most hated Usenetizen of all time
Pierre Salinger Memorial Hook, Line & Sinker, June 2004
COOSN-266-06-25794

J

#### jasen

Jan 1, 1970
0
I need to convert a 24V source to 5V / 1.0 A max output. I tried using
a 7805 but it generates alot of heat. Is there a way of gettinh 5V
without the excessive heat generation? Thx!

switched mode power conversion - a buck converter.

M

#### Meat Plow

Jan 1, 1970
0
switched mode power conversion - a buck converter.

They cost more than a buck.

--
#1 Offishul Ruiner of Usenet, March 2007
#1 Usenet Asshole, March 2007
#1 Bartlo Pset, March 13-24 2007
#10 Most hated Usenetizen of all time
Pierre Salinger Memorial Hook, Line & Sinker, June 2004
COOSN-266-06-25794

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