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'24VDC to 5VDC Supply - Slightly [OT]'
1998\09\22@133505 by Ryan Bloom

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I need to regulate 24vdc down to 5vdc for a PIC project.
I've been using LM317's in TO-220 packages but they seem to go into thermal
runaway when I draw any current.
Does anyone have any good methods/schematics for this.

Thanks in advance,

-Ryan
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1998\09\22@135156 by Craig Lee

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Put a 12V or 15V zener to ground before the regulator.

Most regulators don't like to drop that much in one stage.

Also if you are using a bridge rectifier and a cap to change
a 24VAC transformer into DC, you will get about 30VDC,
exceeding the input spec for the regulator.

Craig

{Original Message removed}

1998\09\22@135610 by Harrison Cooper

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No matter what you do, you are going to get heat from power dissipation.
Several solutions.  DC to DC converter.  Power resistor to drop the
voltage (but this will be dependant on current draw), or several steps
of step down regulation, say to a 15 volt, or a 12 volt to the 5V (or
skip intermediate steps, again, depending on current draw).

Have you measured the current being pulled?  Do you have the 317 on a
heat sink?

{Quote hidden}

1998\09\22@143106 by evan

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When cost matters and efficiency does not, I use a 7805CT (TO-220) with a
$0.50  Thermalloy 6073 (or a Wakefield 273-AB) heatsink and a dab of thermal
grease, mounted with a metal screw & nut.  This gives 24Vdc to 5Vdc
regulation at over 200mA, in a small still-air enclosure with a 45 deg C
ambient.  The 4W dissipation makes things a tad warm of course --but no
thermal rruuunnaaaaawwaaaaaaay.

How much current do you need?

-Ed V.


> {Original Message removed}

1998\09\22@144534 by Dave VanHorn

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>I need to regulate 24vdc down to 5vdc for a PIC project.
>I've been using LM317's in TO-220 packages but they seem to go into
thermal
>runaway when I draw any current.
>Does anyone have any good methods/schematics for this.


You can't draw much current without a BIG heatsink.  It's not in
runaway, it's called thermal shutdown.
You're dropping 19V across the reg, so your current in amps, times 19,
gives you reg dissipation (aka wasted power) in watts.  OUCH!

Digikey has some switchers that are literally drop-in replacements for
those regs, that will run nice and cool. Your wallet may feel the
pinch though, at $12-$20  PT5101-ND will take 35V (max) in and deliver
1A out, with about .5-.6W dissipation. Compare that with 19W above.
http://www.digikey.com

1998\09\22@145127 by Dave VanHorn

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>Put a 12V or 15V zener to ground before the regulator.


At 1A, this would have to be a 12 or 15W zener respectively, and then
the zener needs heatsinking or it will self-desolder.  This is just
moving the problem.

>Most regulators don't like to drop that much in one stage.


ALL linear regulators suffer from the same problem. Current times
(Vin - Vout) = Heat.   There's no other option for linears. Sticking a
zener in front of it just moves the problem.  It's not a "preference",
it's physics.

http://www.digikey.com has some three-terminal regulators by Power Trends
that are 85% efficient, so for the same 5W output, you only have to
get rid of a fraction of a watt as heat.   PT5101N-ND as an example.

1998\09\22@161104 by Harold Hallikainen

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       Either get a bigger heat sink for the LM317 or go to a switcher.
National has their "Simple Switcher" line, and SOMEONE (who I don't
recall), has a switcher 7805 replacement (all in one piece.
       Finally, how much current are you trying to draw?  The typical
PIC application is pretty low power, so maybe the LM317 is oscillating
instead just dissipating too much power.  A 220nF to ground at both input
and output can help.  Also, any reason you're not using a 7805?  It
requires two less resistors than the LM317 (and the ideal design has zero
parts...) .
       I see someone else has suggested dropping voltage prior to the
regulator.  They suggested a zener.  The 7805 can take 35V input, so if
you are indeed driving the regulator with +24V, that should be ok.  The
LM317 can take an input-output voltage differential of 40 volts, so you
should be ok there.
       Adding the suggested zener in series with the input will move
some of the dissipation to the zener, reducing the regulator heat sink
size, but then requiring a heat sink on the zener.  I don't think
additional parts are the real solution here.  Also, if the load current
is relatively constant, you could use a resistor instead of the zener,
again moving dissipation to the resistor from the regulator.
       However, I'd first check to see if the regulator is oscillating
(look with a scope), then add a couple 220nF capacitors.  Also, you can
calculate the dissipation (about (Vin-Vout)*LoadCurrent), multiply by the
thermal resistance of the heat sink (degreesCtempRise/Watt), add to
ambient temperature and see where your case temperature lands.  If too
high, add heat sink or go to a switcher.


Harold



Harold Hallikainen
EraseMEharoldspam_OUTspamTakeThisOuThallikainen.com
Hallikainen & Friends, Inc.
See the FCC Rules at http://hallikainen.com/FccRules and comments filed
in LPFM proceeding at http://hallikainen.com/lpfm

On Tue, 22 Sep 1998 10:36:15 -0700 Ryan Bloom <RyanBspamspam_OUTMPL.NET> writes:
{Quote hidden}

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1998\09\22@163559 by Ryan Bloom

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Although my options sound limited, thanks for all the help.
I was hoping to avoid the distributed solution (BIG resistor or zener).
I will look into the switching type and see if it is cost prohibited.
BTW - I'm using a 317 because I need between 5 and 5.5vdc (I forgot to
mention that).

Thanks again,

-Ryan

-----Original Message-----
From: Harold Hallikainen [KILLspamharoldhallikainenKILLspamspamJUNO.COM]
Sent: Tuesday, September 22, 1998 1:08 PM
To: RemoveMEPICLISTTakeThisOuTspamMITVMA.MIT.EDU
Subject: Re: 24VDC to 5VDC Supply - Slightly [OT]


       Either get a bigger heat sink for the LM317 or go to a switcher.
National has their "Simple Switcher" line, and SOMEONE (who I don't
recall), has a switcher 7805 replacement (all in one piece.

<Snip>

1998\09\22@165441 by Peter L. Peres

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On Tue, 22 Sep 1998, Dave VanHorn wrote:

> http://www.digikey.com has some three-terminal regulators by Power Trends
> that are 85% efficient, so for the same 5W output, you only have to
> get rid of a fraction of a watt as heat.   PT5101N-ND as an example.

Isn't there a bunch of schematics to make a 7805 a switcher with one
transistor ? I've never used this, but it used to be in the National data
books. Has anyone tried one of these as a fix on an existing design ?

OTOH how about using a 555 as power oscillator at 1/4 duty cycle followed
by a D, L and C to feed a normal series regulator with 8-9 V ? This is one
that I've used when I had no choice (had to modify an existing design that
had a closed plastic case and fried regulators on a regular basis). I
don't know about passing FCC etc with it however.

Peter

1998\09\22@174901 by Roger Morella

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Check National Semiconductors simple switcher line.   8 pin dip, choke, diode,
and caps.  Should be able to get something running for under $6.  Also, check
Linear Tech for the same type of parts.  We use these swithers for dropping
anywhere from 6 to 35 volt inputs down to 5 volts with negligable heat.

Roger

Ryan Bloom wrote:

{Quote hidden}

1998\09\22@183645 by Dave VanHorn

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>> http://www.digikey.com has some three-terminal regulators by Power Trends
>> that are 85% efficient, so for the same 5W output, you only have to
>> get rid of a fraction of a watt as heat.   PT5101N-ND as an
example.
>
>Isn't there a bunch of schematics to make a 7805 a switcher with one
>transistor ? I've never used this, but it used to be in the National
data
>books. Has anyone tried one of these as a fix on an existing design ?


There's something in the databook about using an external follower,
basically, again, just moving the heat problem, not getting rid of it.


>OTOH how about using a 555 as power oscillator at 1/4 duty cycle
followed
>by a D, L and C to feed a normal series regulator with 8-9 V ? This
is one
>that I've used when I had no choice (had to modify an existing design
that
>had a closed plastic case and fried regulators on a regular basis). I
>don't know about passing FCC etc with it however.

I'm not clear on how much power he needs, I was assuming large-ish due
to his comments on heat.
Anything with much current demand would blow the 555 approach, plus
using transistors to switch large caps on and off isn't very efficient
either. An inductor is the right tool here.

1998\09\23@071524 by Keith H

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Ryan Bloom wrote:

> I need to regulate 24vdc down to 5vdc for a PIC project.

I had a project where I needed to run an 80C188 system
from an industrial 24V supply.
I used Micrel switchmode regulator chip 5758T.
5 pin TO220 package. Resistors set the output voltage
but some are fixed 5V or 3V outputs.

The Micrel parts seem to copy National's chip functions
or exceed them. The Micrel data books are very good,
I learnt a lot about switchmode PSUs, tech support from
distributor (Solid State Supplies) was excellent.

1998\09\23@125437 by Peter L. Peres

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On Tue, 22 Sep 1998, Dave VanHorn wrote:

> >Isn't there a bunch of schematics to make a 7805 a switcher with one
> >transistor ? I've never used this, but it used to be in the National
> data
> >books. Has anyone tried one of these as a fix on an existing design ?
>
> There's something in the databook about using an external follower,
> basically, again, just moving the heat problem, not getting rid of it.

Nonono. I mean the one with the follower, the coil, the recovery diode and
a resistive divider in the output to induce hysterezis. This is one of
those weird designs where the regulator stays linear for low Vi/Vo and
starts switching when current is demanded.

{Quote hidden}

I was suggesting the use of a 555 as switching regulator (series, open
loop) before the actual reg. If the 555 is used to give 100 mA it should
give about 300 mA available at the input of the linear regulator with the
given voltage drop.

Peter

1998\09\23@132700 by Dave VanHorn

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>Nonono. I mean the one with the follower, the coil, the recovery
diode and
>a resistive divider in the output to induce hysterezis. This is one
of
>those weird designs where the regulator stays linear for low Vi/Vo
and
>starts switching when current is demanded.


Eww.. I don't think I've seen that one. Deliberately destabilizing the
reg with added phase shift is what it sounds like. It could be
reasonable I guess. You'd get better efficiency at the low delta than
a conventional switcher.

>I was suggesting the use of a 555 as switching regulator (series,
open
>loop) before the actual reg. If the 555 is used to give 100 mA it
should
>give about 300 mA available at the input of the linear regulator with
the
>given voltage drop.


An external switch here? The 555 gets fairly warm at high temps.

1998\09\23@141852 by Peter L. Peres

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> 78XX switcher

Ok, I'll try a famous ASCII schematic from memory here:


         Q1: PNP
         e   c       Coil
Power O-*--V /----*---nnnnn----*------O Load +
       |  ===    |    A       |
       \   |     +-|<|-+      |
 R1    /   |      K    |      |
       \   |          ===     |
       /   |   +-----+        |
       |   | i |     | o      |
       +---*---* Reg.*--------*
               |     |        |
               +--*--+        \
                g |           /
                  |        R2 \
                  |           /
                  |           |   R3
                  +-----------*--/\/\/\--+
                                         |
                                        ===
                                        GND

 The circuit starts to oscillate when Ur1 > Ube Q1. R2 and R3 set the
hysterezis of the O/P voltage, which can be as small as 25 mV but 50 mV is
a good value in my experience. The output voltage is the nominal voltage
of the regulator and increases to that + 1/2 the hysterezis when
switching.  The switching frequency must be low enough to be within the
range of the 78XX. A 317 cannot be used here easily (as it has a constant
Ignd). The O/P smoothing cap is not drawn in this schematic. It influences
the switching frequency (together with the load current and Vi/Vo).

 In general, only regulators that sink the output transistor's base
current through the gnd terminal can be used in this scheme. 78XXs are
that.

 If R1 is choosen cunningly the SOA and thermal protections of the
regulator will be maintained and also apply to Q1 (mount on common
heatsink).

Peter

1998\09\24@214536 by Larry G. Nelson Sr.

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Try using a switch mode regulator. You can use a simple switcher from
National or look into Linear Technology. Linear Technology has a program
called switcher Cad that will help you design a switching power supply very
simply.


At 10:36 AM 9/22/98 -0700, you wrote:
{Quote hidden}

Larry G. Nelson Sr.
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http://www.ultranet.com/~nr

1998\09\25@021842 by Michael Hagberg

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will a switcher supply regulate DC in to DC out? or do you need AC in?

michael

You may leave the list at any time by writing "SIGNOFF PICLIST" in the
   body of a message to LISTSERVEraseMEspam.....MITVMA.MIT.EDU.

{Original Message removed}

1998\09\25@044016 by Brian Watson

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It's called a DC-DC Converter

-----Original Message-----
From:   pic microcontroller discussion list [EraseMEPICLISTspamMITVMA.MIT.EDU] On
Behalf Of Michael Hagberg
Sent:   25 September 1998 07:09
To:     RemoveMEPICLISTEraseMEspamEraseMEMITVMA.MIT.EDU
Subject:        Re: 24VDC to 5VDC Supply - Slightly [OT]

will a switcher supply regulate DC in to DC out? or do you need AC in?

michael

You may leave the list at any time by writing "SIGNOFF PICLIST" in the
   body of a message to RemoveMELISTSERVspam_OUTspamKILLspamMITVMA.MIT.EDU.

-----Original Message-----
From: Larry G. Nelson Sr. <RemoveMEnrTakeThisOuTspamspamMA.ULTRANET.COM>
To: EraseMEPICLISTspamspamspamBeGoneMITVMA.MIT.EDU <RemoveMEPICLISTKILLspamspamMITVMA.MIT.EDU>
Date: Thursday, September 24, 1998 8:53 PM
Subject: Re: 24VDC to 5VDC Supply - Slightly [OT]


>Try using a switch mode regulator. You can use a simple switcher from
>National or look into Linear Technology. Linear Technology has a program
>called switcher Cad that will help you design a switching power supply very
>simply.
>
>
>At 10:36 AM 9/22/98 -0700, you wrote:
>>I need to regulate 24vdc down to 5vdc for a PIC project.
>>I've been using LM317's in TO-220 packages but they seem to go into
thermal
{Quote hidden}

1998\09\25@104927 by Josˇ A. R. Beiral

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At 09:35 25/09/98 +0100, you wrote:
>It's called a DC-DC Converter
>
>-----Original Message-----
>From:   pic microcontroller discussion list
[KILLspamPICLISTspamBeGonespamMITVMA.MIT.EDU] On
{Quote hidden}

1998\09\25@105536 by Dave VanHorn

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>will a switcher supply regulate DC in to DC out? or do you need AC
in?


Yes.  It takes DC in, but you can put a simple bridge rect and cap in
front of it for using AC input.

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