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'[EE]:Circuit Question'
2002\04\29@153022 by kben

picon face
Hi,
I have this circuit charging a 8xAA battery pack.
http://www.members.dca.net/kben/misc/buck.html
I am also using a 500Hz PWM, not 100Khz.
The fet and coil specs are slightly different, but the closest
I could find from Digikey. My Vin is coming from a 20Vac 800ma
wall wart, going through a full bridge rectifier. I am able to
control the charging current OK with the PWM from my PIC.
The charger has 3 modes full 1000ma, soft 375ma,
and trickle 75ma.  The problem is on my first full test,
the wall wart got very hot after about 35 minutes of
FULL charging.  Can someone enlighten me as to why it is heating
up ?
Thanks,
Kevin

P.S. I am a programmer not an EE.

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2002\04\29@162014 by Tim McDonough

flavicon
face
> I could find from Digikey. My Vin is coming from a 20Vac 800ma
> wall wart, going through a full bridge rectifier. I am able to
> control the charging current OK with the PWM from my PIC.
> The charger has 3 modes full 1000ma, soft 375ma,
> and trickle 75ma.  The problem is on my first full test,
> the wall wart got very hot after about 35 minutes of
> FULL charging.  Can someone enlighten me as to why it is heating
> up ?

It's hard to say for sure not really knowing what "very hot" means (was the
plastic dripping off?) but since your system isn't 100% efficient I'm
suspecting an 800mA wall wart might be having trouble delivering 1A of
current at whatever voltage is needed to keep 1A going through the battery
pack. at different times during the charge cycle. As the cells are charged
a higher and higher voltage is required if you are truly maintaining a
constant current through the pack.

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2002\04\29@172053 by Bob Blick

face picon face
Hi Kevin,

Do you have a filter capacitor on the bridge rectfier? You need one.

Also, wallwarts are meant to run hot whenever close to ratings. My rule of
thumb, if you smell it when you enter the room, it's too hot, otherwise
it's OK :-)


Cheers,

Bob

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2002\04\29@174532 by kben

picon face
Hi Everyone,
Thanks for the responses. Well, it didn't melt,
so I guess I am OK there. It was hot to the touch,
where holding for more than a few seconds would not be
possible. As for the filter cap what value would you
recommend ?

{Quote hidden}

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2002\04\29@175511 by David Minkler

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face
Kevin,

Are you really using 500Hz PWM rather than 100kHz (this is more than two
orders of magnitude different)?  If so, the required inductor will be
more than "slightly" different.

Regards,
Dave

"Kevin A. Benedict" wrote:
> I am also using a 500Hz PWM, not 100Khz.
> The fet and coil specs are slightly different ...

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2002\04\29@180857 by Bob Blick

face picon face
> possible. As for the filter cap what value would you
> recommend ?

Rule of thumb is 1000uF per amp, minimum, for rectified AC. That will
typically give you 6 volts ripple peak to peak.

Cheers,

Bob

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2002\04\29@185832 by kben

picon face
Hi,
the slight difference I was refferring to was what
was listed on the web page and what I could actually
get from Digikey. I assumed it would be inefficent,
but as it was charging with the right current I wasn't
too concerned until it started heating up.
I would appreciate it, if someone could explain how
to calculate a proper value for the inductor for
my circuit.

{Quote hidden}

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2002\04\29@215043 by Tom Messenger

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Kevin,

I have two questions for you:

       a) What frequency is your circuit working at?
       b) How are you determining that the battery is charging at the correct
current?

Best regards,
Tom M.

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2002\04\29@224242 by kben

picon face
Hi Tom,
I included part of my original post below.
The circuit was designed for 100KHz.
I am using 500Hz.
I have my DMM inserted in the circuit to
measure the current. The circuit seems to be
functioning OK as the battery pack charges at
the right level, I am using a F872, and monitor
the temp of the pack also, to make sure it does not
get to hot. The problem was the wall wart heating up.

>        a) What frequency is your circuit working at?
>        b) How are you determining that the battery is charging at the correct

>current?
.
http://www.members.dca.net/kben/misc/buck.html
I am also using a 500Hz PWM, not 100Khz.
I am able to
control the charging current OK with the PWM from my PIC.
The charger has 3 modes full 1000ma, soft 375ma,
and trickle 75ma.

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2002\04\30@010144 by Tom Messenger

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When you made the change to 500 Hz, did you also change the size of the LC
filter network?

500 Hz is 200 times lower which will necessitate much different filter
components.  The filter network is designed to only store energy for the
period of the pwm. If you did not change the size of the network, it can
only handle a period of 10 usecs (100KHz). At 500 Hz, you have a period of
2 milliseconds (200 times longer) so you will need filter components much
larger. Your dmm may read the average current correctly but it is the peak
current that is causing the wall wart to get hot.

Tom M.

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2002\04\30@010950 by Rick C.

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I've noticed that commercial products with wall warts may run warm but not
hot. Somehow I don't think I'd leave the house very long with a cheap Chinese
wall wart running close to critical temperatures without good fire insurance.
Rick

Bob Blick wrote:

{Quote hidden}

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2002\04\30@070942 by Olin Lathrop

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> Your dmm may read the average current correctly but it is the peak
> current that is causing the wall wart to get hot.

I don't think so.  Short peaks should be covered by the filter capacitors.
I think the problem is that he broke his switching supply by running it 200
times slower than designed.  This causes it to function as a linear supply,
so 1A out requires at least 1A in.  Nobody should be surprised that a 800mA
wall wart gets hot at 1A.


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2002\04\30@135238 by Barry Gershenfeld

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One simple test you can do is get a resistor or some
lamps and load the power supply to 1000ma and see
how hot it gets.  That will answer that question
independent of anything to do with your circuit.

Barry

>The problem was the wall wart heating up.
>

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2002\04\30@163729 by kben

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OK, I think I have found my REAL problem,
besides drawing 1000ma from a 800ma wall wart.
When trying to get my SMPS to supply the 3 desired
currents, 70ma trickle, 375ma soft charge,
1000ma Full charge. I could not get the duty cycle
of the PWM to produce the proper charging currents.
So I reduced the FREQ of the PWM from 100Khz to 500HZ,
this allowed CCPR1L to be adjusted to produce the proper
current. However, I went back and reviewed my code and
the manual and realized I should have used CCP1CON
bits 5 & 4 for 10bit resoulution on the PWM.
Now I can run the PWM at 100Khz and get the 3 charging
currents I was after.
Oh, and I also order a 1500ma wall wart.

Thanks to all that offered suqqestions.
Kevin

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2002\04\30@220255 by Wagner

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> Hi Everyone,
> Thanks for the responses. Well, it didn't melt,
> so I guess I am OK there. It was hot to the touch,
> where holding for more than a few seconds would not be
> possible. As for the filter cap what value would you
> recommend ?

Woa, any wal wart that you CAN NOT hold in your hands for any time you want, is
very hot and it is asking for a firetruck to come by to cool it off.

That is totally out of control, probably the wire insulation is melting away,
what means a possible short circuit pretty soon. Now you know how SOME home
fires could starts.

Obviously that a 20V @ 800mA unit could generate some good heat if your circuit
is consuming the 16Watts the unit can deliver, but you are talking about 10V @
1000mA, thinking about your step down PWM working at 75%, it means 10W / 0.75 =
13.3W power consume, so it would be running at 83% of maximum power delivery,
what should run warm to warm-hot, but not hot enough to burn your hand after 10
seconds.

Of course the step-down requires a nice filter capacitor to have enough power to
be delivered to the PWM circuit.  The function of such capacitor is exactly
"convert" the 20V @ AnyCurrent to the PWM conversion. The PWM circuit will
extract as much current it needs from the CAPACITOR.  If the capacitor is not
present, the PWM circuit will try to extract the current it needs from the power
transformer, and THAT will generate heat, not even talking that the step down
conversion productivity will go south to less than 50% what means an enourmous
current hunger from the PWM.

The capacitor formula calculations is

C = 2.4 i/vr

where i = current in milliamperes and vr is the accepted ripple voltage.

Lets imagine the accepted ripple would be 500mV and the current 1000mA, so
capacitor value should be  2.4 x 1000 / 0.500 = 4800uF.

Probably the power transformer is working at its current limit, core magnetic
saturated, what reduces more and more its capacity to transport energy, heating
yet more.  That is an auto-destruction cycle.

Wagner Lipnharski
http://www.ustr.net
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