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'[PICLIST] [EE] power supply heat'
2002\06\19@144955 by Chris Loiacono

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I have a circuit with a 7805 regulator that was set-up with a 12.6V CT
transformer and two diodes on the input side. This gave it an input voltage
of 8.9V (or, 6.3V x 1.414 = 8.9).

The 12.6V transformer output is rated with the primary at 230V. Now I am
considering using it on 277VAC. This should bring the input to the 7805 up
to about 120% of the 8.9V, 0r about 10.7V.

At 8.9V no heatsink was required with a .5A load. How can I pre-figure how
much power will be dissipated when on the 277V primary, so I can select a
heatsink, or perhaps decide that the whole idea is either OK or NG???

I checked the National datasheet for the 7805, and still came up short.
Is it as simple as saying that I will have 5.7V dropout at .5A, thus will
have 2.85W to deal with?

TIA,
Chris

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2002\06\19@150654 by Spehro Pefhany

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At 02:53 PM 6/19/02 -0400, you wrote:

Power dissipation is just:

P= (input voltage - output voltage) * (output current + regulator current)

Regulator current is typically around 5mA for the 78xx05.

You should work out heat sink size based on worst case input voltage,
output voltage tolerance, and ambient temperature.

Best regards,

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2002\06\19@182850 by Dwayne Reid

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At 02:53 PM 6/19/02 -0400, Chris Loiacono wrote:

>The 12.6V transformer output is rated with the primary at 230V. Now I am
>considering using it on 277VAC. This should bring the input to the 7805 up
>to about 120% of the 8.9V, 0r about 10.7V.

Don't do that!

I think that you will cook your transformer if you over-voltage the input
that much.  Core saturation.

How much power do you need?  We use several custom transformers for some of
our products - one of those has 120V, 277V, 347V primary taps and supplies
20 Vac @ 4A.  It mates with a little card that has a NS simple switcher
that you could set to whatever voltage you wanted within the range of 5Vdc
- 14Vdc.

dwayne

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2002\06\19@234626 by Bob Ammerman

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> >The 12.6V transformer output is rated with the primary at 230V. Now I am
> >considering using it on 277VAC. This should bring the input to the 7805
up
> >to about 120% of the 8.9V, 0r about 10.7V.
>
> Don't do that!
>
> I think that you will cook your transformer if you over-voltage the input
> that much.  Core saturation.

Doesn't core saturation depend on current, rather than voltage? I would
think that you'd have less chance of saturating at 277V than 230V.

Bob Ammerman
RAm Systems

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2002\06\20@005836 by Mike Singer

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{Quote hidden}

  But no-load current depends heavily on primary coil voltage.
How about having 1A no-load current with 277V? (Core saturation).

  Mike.

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2002\06\20@005841 by Dwayne Reid
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At 11:39 PM 6/19/02 -0400, Bob Ammerman wrote:
> >
> > I think that you will cook your transformer if you over-voltage the input
> > that much.  Core saturation.
>
>Doesn't core saturation depend on current, rather than voltage? I would
>think that you'd have less chance of saturating at 277V than 230V.

Try it and see!  Do it outside so that the stink has doesn't get to you ;)

I'm not sure how best to describe this.  A transformer core can handle only
a certain flux density before it saturates.  This is directly proportional
to applied voltage as follows:

Peak AC Flux (Gauss)   B(AC) = Vrms x 10^8 / 4.44 * N * f * A
B gauss  V volts  N #turns  f Hz  A cm^2

One of many web pages that describe this:
<http://www.coilws.com/magneticandhow.html>

I had a couple of our power supplies returned after failing because the
installer had inadvertently connected 347 Vac to the 277 Vac primary
tap.  They didn't smell nice :(

dwayne

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2002\06\20@010239 by Tom Messenger

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>> I think that you will cook your transformer if you over-voltage the input
>> that much.  Core saturation.
>
>Doesn't core saturation depend on current, rather than voltage? I would
>think that you'd have less chance of saturating at 277V than 230V.
>
>Bob Ammerman
>RAm Systems


Nope. The core must standoff the volt*second product. Thus, more volts
needs more core. Likewise, lower frequency means more seconds and also
means more core needed.

This is why higher freq operation is often desirable. If a transformer is
designed for one input voltage and you find you must increase the voltage,
it helps to offset this by raising the operating frequency also if possible.

The core in question *may* work at 277V but this can only be determined by
testing or consulting the manufacturer.  Since excess core usually means
excess expense, the cores are usually made to just barely do the job and
typically run warm at rated load. Raising the voltage will usually raise
the temperature also. Determination of suitability is left as an exercise
to the student.

Tom M.

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2002\06\20@090732 by Chris Loiacono

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This is why I contacted the manufacturer at the same time the original
message was posted here. I don't expec to hear from them any time
soon...Since changing the freq is out of the question, I am most curious
about Dwayne's solution...

CL

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2002\06\20@114713 by Alan B. Pearce

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What sort of wattage are you after?

RS Components have 50/100/200VA kits that comes with a pre wound 115/230
primary on it, and a piece of paper with tables for calculating a secondary.
The unit is not potted, and so it should not be too hard to either remove
the existing primary, and replace it, or else add a supplementary primary
winding before putting a secondary on it. If you did the latter then make
sure you put sufficient insulation in between the primary and secondary.

The kit has a plastic bobbin with primary windings, and a stack of
laminations to put in afterwards, along with the necessary metal pieces to
clamp it all up afterwards.

At rshttp://www.com/ look for catalogue # 182-9919 for 50VA, 182-9925 for
100VA and 182-9931 for 200VA

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2002\06\20@141404 by Peter L. Peres

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On Wed, 19 Jun 2002, Chris Loiacono wrote:

>I have a circuit with a 7805 regulator that was set-up with a 12.6V CT
>transformer and two diodes on the input side. This gave it an input voltage
>of 8.9V (or, 6.3V x 1.414 = 8.9).
>
>The 12.6V transformer output is rated with the primary at 230V. Now I am
>considering using it on 277VAC. This should bring the input to the 7805 up
>to about 120% of the 8.9V, 0r about 10.7V.
>
>At 8.9V no heatsink was required with a .5A load. How can I pre-figure how
>much power will be dissipated when on the 277V primary, so I can select a
>heatsink, or perhaps decide that the whole idea is either OK or NG???
>
>I checked the National datasheet for the 7805, and still came up short.
>Is it as simple as saying that I will have 5.7V dropout at .5A, thus will
>have 2.85W to deal with?

Maybe you can remove 1 diode and have more ripple. This will reduce the
power on the regulator. Simulate your circuit with spice. I am amazed that
you ran 9V into it (dissipating 2W) and did not need a heatsink. You will
need a small heatsink for 3W but you can remove the diode as I said or you
can add a resistor to spread the load. 3.3ohms/2W in series will go a long
way I think. You can put it before the filter cap too but then you will
need another value.

Peter

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2002\06\21@124240 by Doug Butler

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If you want to run off of 277VAC and you only have 230V transformers
available, get a 230V to 48V transformer and wire it as a stepdown
autotransformer.  230V + 48V = 277V aprox. so you can input 277 to both
windings in series (correct phasing) and get 230 out, everything will be in
spec.  Ideally you would get a transformer with both 48V and 12.6V windings
so you don't need two chunks of iron.

To test phasing try putting a light bulb in series with the primary on your
first test.  If the phasing is wrong the core will saturate and the light
will light.  If the lamp stays dark you have the phasing right.  This test
is done with no load on the output.

Doug Butler
Sherpa Engineering


> {Original Message removed}

2002\06\21@125929 by mike

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On Fri, 21 Jun 2002 12:41:21 -0500, you wrote:

>If you want to run off of 277VAC and you only have 230V transformers
>available, get a 230V to 48V transformer and wire it as a stepdown
>autotransformer.  230V + 48V = 277V aprox. so you can input 277 to both
>windings in series (correct phasing) and get 230 out, everything will be in
>spec.  Ideally you would get a transformer with both 48V and 12.6V windings
>so you don't need two chunks of iron.
,,and you need to have the full mains spec isolation between the 48
and 12.6v windings, which will often not be the case for standard
transformers, which only have full isolation between primary and
secondary, not between secondaries..

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