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'flexible input supply'
2003\02\11@151651 by Olin Lathrop

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> I have a simple Linear supply based on a 7805 with a CT'd transformer  a
> pair of diodes and 1000 uF input filter. The transformer primary
windings
> can be paralleled for 115V, or series connected for 230. It's a 2.5VA
> transformer and the circuit load is about 170 mA.
> It seems that at about 220V & higher I get the 3V headroom needed for
the
> 7805. Since 208V is so popular I have a problem with less than 3V, hence
the
> '05 can't regulate properly. The next *standard* transformer secondary
is
> something like 16V, which makes more heat than I'm comfortable with when
run
> on 240V . There's also no space for anything but the tiniest heat sink
for
> the reg.
>
> OK, so it's time to revise this supply since the product will at times
be
> used on 208V. I have thought of changing the bridge diodes to reduce
their
> drop and am also thinking about a different regulator (LDO type). I am
> hoping to come up with a change that will let me implement it on the
boards
> I have in stock while sliding it into the next revision.

If it most be a drop in replacement for existing boards, then the LDO is
probably the only option.  However, a long term answer is to redesign with
a buck regulator (these are the simplest type of switchers).  Then you can
even use the same transformer for 110-240V.  The extra voltage drop at
240V will just cause less power to be drawn instead of making heat.


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2003\02\11@165236 by Chris Loiacono

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> If it most be a drop in replacement for existing boards, then
> the LDO is
> probably the only option.  However, a long term answer is to
> redesign with
> a buck regulator (these are the simplest type of switchers).
> Then you can
> even use the same transformer for 110-240V.  The extra voltage drop at
> 240V will just cause less power to be drawn instead of making heat.



Any Suggestions?

What about Schottky's to reduce the drop in the rectification?

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2003\02\11@173330 by Dwayne Reid

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At 04:52 PM 2/11/03 -0500, Chris Loiacono wrote:

>What about Schottky's to reduce the drop in the rectification?

Are you fighting heat or are you still trying to eek out that last little
bit of headroom?

If its not a headroom issue and you are using a linear regulator, changing
to Schottky diodes just moves the heat from the diodes to the
regulator.  But if you need that tiny bit of headroom and / or you are
using a switcher, Schottky diodes will improve the headroom and reduce the
heat generated.

How much heat is produced by the 1n400x diodes at 170 mA?

dwayne

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Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
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2003\02\11@181306 by Chris Loiacono

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Hi Dwayne...

> >What about Schottky's to reduce the drop in the rectification?
>
> Are you fighting heat or are you still trying to eek out that
> last little
> bit of headroom?

Yes, I'm thinking about chipping out more crawl space. With conventional
silicon diodes I'm getting just under 7V to the regulator input, with a need
for 7.5V. (As Sid has reminded me)

> If its not a headroom issue and you are using a linear
> regulator, changing
> to Schottky diodes just moves the heat from the diodes to the
> regulator.  But if you need that tiny bit of headroom and / or you are
> using a switcher, Schottky diodes will improve the headroom
> and reduce the
> heat generated.
>
> How much heat is produced by the 1n400x diodes at 170 mA?
>
Good question - I don't know for sure, but heat isn't really the issue
unless I up the transformer secondary voltage. the 12.6V CT windings still
have the 7805 running pretty cool with 240 on the primary.

The reason this has taken so long us that today has benn nothing but
interruption after interruption..wah, wah...for sure.

Looking at it now, let's see....the secondary is rated 12.6V at 230 pri.
That gives me about 11.39V at 208 pri. Let's see 11.39*.636 = 7.25V Take
away the diode drop and only about 6 1/2 V remain. Herein lies the problem
since the 7805 requires 7.5V. It seems a diode swap to Schottky would only
get me another .5V....Hmmmm still not at 7.5...


OK, so while it may help improve regulation a bit, I am still thinking LDO.
I am also wondering about substituting LM340's for 7805's. Would the 340's
respond differently to low input V as compared to 7805's?



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2003\02\11@185219 by Dwayne Reid

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At 06:09 PM 2/11/03 -0500, Chris Loiacono wrote:

>OK, so while it may help improve regulation a bit, I am still thinking LDO.
>I am also wondering about substituting LM340's for 7805's. Would the 340's
>respond differently to low input V as compared to 7805's?

Naw - the '340 is almost identical to the 7805.  But, you know, the LM2931
is pretty inexpensive.

dwayne

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Dwayne Reid   <dwaynerspamKILLspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

Celebrating 18 years of Engineering Innovation (1984 - 2002)
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    `-'   `-'   `-'   `-'   `-'   `-'   `-'   `-'   `-'
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commercial email nor is intended to solicit commercial email.

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2003\02\11@191843 by Ray Gallant

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Will your load tolerate R & zener. {slewrate}

----- Original Message -----
From: "Chris Loiacono" <.....chrisKILLspamspam.....MAIL2ASI.COM>
To: <EraseMEPICLISTspam_OUTspamTakeThisOuTMITVMA.MIT.EDU>
Sent: Tuesday, February 11, 2003 5:52 PM
Subject: Re: flexible input supply


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2003\02\12@065835 by Roman Black

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Chris Loiacono wrote:

> Any Suggestions?
>
> What about Schottky's to reduce the drop in the rectification?


Am I missing something? I'm sure you said it was a
CT transformer with 2 diodes, just change to a full
bridge and double your output voltage provided the
current is suitable.
-Roman

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2003\02\12@221205 by Chris Loiacono

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I feel obligated to share my own results since you guys are always
forthcoming with support....Just in case anyone hasn't already figured this
one out...

The LM2940CT-5 LDO Reg. turned out to be a drop-in replacement for LM7805.
It's only rated 1 Amp, which in my ap is OK. Only change was to remove a .1
cap I had on the output and replace it with a 100uF 'lytic cap. The data
sheet calls for a minimum of 22uF, larger is OK.
Ran cool (120F) on 7.67V Input from my 240 (245 today)line & transformer. I
sent a test unit to another site so it could be tested on a 208V line with <
7V Input. Yes I realized that I could have simply done this with a bench
supply, but it's nice to make  non-EE show-me types happy from time to
time...

Thanks for the help 'yall.

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