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'[EE]: 5 Watt Resistor Drop Out from Lead'
2003\02\11@033733 by James

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I have the 5 Watt 20 ohm resistor drop out from the soldering lead due
to the heat generated. It is connected to input voltage of 18~19 Volt
from an DC adaptor of DC 12 V 1000mA non regulated.  My problem is the
lead of the resistor drop out from the soldering PCB after some time
(1-2 months). Of course the board have some minor burn mark.  How to
prevent this from happening ?

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2003\02\11@035023 by

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Lower the heat !?
Disconnect it from the power !?
Bolt it in place !?

Jan-Erik Söderholm.


James wrote :
>I have the 5 Watt 20 ohm resistor drop out from the soldering lead due
>to the heat generated. It is connected to input voltage of 18~19 Volt
>from an DC adaptor of DC 12 V 1000mA non regulated.  My problem is the
>lead of the resistor drop out from the soldering PCB after some time
>(1-2 months). Of course the board have some minor burn mark.  How to
>prevent this from happening ?

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2003\02\11@035433 by hael Rigby-Jones

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> -----Original Message-----
> From: James [SMTP:spam_OUTasiactTakeThisOuTspamTM.NET.MY]
> Sent: Wednesday, February 12, 2003 12:29 AM
> To:   .....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU
> Subject:      [EE]:  5 Watt Resistor Drop Out from Lead
>
> I have the 5 Watt 20 ohm resistor drop out from the soldering lead due
> to the heat generated. It is connected to input voltage of 18~19 Volt
> from an DC adaptor of DC 12 V 1000mA non regulated.  My problem is the
> lead of the resistor drop out from the soldering PCB after some time
> (1-2 months). Of course the board have some minor burn mark.  How to
> prevent this from happening ?
>
>
You have up to 19 volts across this resistor?  That gives a power
dissipation of 19^2/20=18 Watts which is just a little too much for a 5 Watt
resistor.  To stop the problem use a properly rated component and thermal
management e.g. a heastsink.

Regards

Mike

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2003\02\11@044245 by John Snider

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James <asiactspamKILLspamTM.NET.MY> wrote:
> I have the 5 Watt 20 ohm resistor drop out from the soldering lead due
> to the heat generated. It is connected to input voltage of 18~19 Volt
> from an DC adaptor of DC 12 V 1000mA non regulated.  My problem is the
> lead of the resistor drop out from the soldering PCB after some time
> (1-2 months). Of course the board have some minor burn mark.  How to
> prevent this from happening ?
-------------------------------------------------------------
I've noticed on commercial pc boards, that when a power resistor (or diode)
is soldered down - on thru-hole the lead is bent on the underside and
soldered along the trace to spread the heat -  On surface mount, I've seen
the lead actually sticking up on both sides, after being soldered for about
1/4" . Sort of a mini-heat sink....

John Snider

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2003\02\11@044912 by David Duffy

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James wrote:
>I have the 5 Watt 20 ohm resistor drop out from the soldering lead due
>to the heat generated. It is connected to input voltage of 18~19 Volt
>from an DC adaptor of DC 12 V 1000mA non regulated.  My problem is the
>lead of the resistor drop out from the soldering PCB after some time
>(1-2 months). Of course the board have some minor burn mark.  How to
>prevent this from happening ?

Try mounting a replacement one up off the PCB by at least 5 or 10 mm.
The longer leads will help with airflow and keep the solder joint cooler.
David...

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2003\02\11@061915 by Thys

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The way I read this a 5W resistor is a bit on the small side.


-----Original Message-----
From: pic microcontroller discussion list
[.....PICLISTKILLspamspam.....mitvma.mit.edu]On Behalf Of James
Sent: 12 February 2003 02:29
To: EraseMEPICLISTspam_OUTspamTakeThisOuTmitvma.mit.edu
Subject: [EE]: 5 Watt Resistor Drop Out from Lead


I have the 5 Watt 20 ohm resistor drop out from the soldering lead due
to the heat generated. It is connected to input voltage of 18~19 Volt
from an DC adaptor of DC 12 V 1000mA non regulated.  My problem is the
lead of the resistor drop out from the soldering PCB after some time
(1-2 months). Of course the board have some minor burn mark.  How to
prevent this from happening ?

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2003\02\11@082518 by Chris Loiacono

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I haven't followed this whole thread, but @ 19V, this is an 18W load as
someone noted. Most likely, your unregulated supply is closer to 12V when
loaded properly (somewhere near 1A). I expect it is not near an 18W live
load or it would have made some burn marks of it's own by now. In any case,
even if there's 12V when running, you are still talking about more than 7W.
If this is the case you want something like a 14 or 15W rated resistor. I
would check the voltage when the circuit is powered and loaded, then do a
little simple ohms law calc and size the supply and resistor properly before
you burn something bigger (like perhaps a structure fire).The 5W resistor
would do a good job as an igniter or a fuse the way it is now. You need to
check the temp ratings of all the nearby materials and the resistor. In
fact, what kind of resistor is this that has stayed intact through such
abuse?

The thought about mounting above the PCB plane is good practice also. At
about 10W or so, you'll start to see resistors encased in aluminum which are
made to be heat-sink mounted (better), or attached to a large plane of
copper on the board (genearally not as good). My personal rule is to limit
the on-board size to 10W and size power resistors at a minimum of 2x the
calculated power. You might think about adopting a similar 'rule'.

C

{Quote hidden}

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2003\02\11@082730 by Spehro Pefhany

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At 04:29 PM 2/11/2003 -0800, you wrote:
>I have the 5 Watt 20 ohm resistor drop out from the soldering lead due
>to the heat generated. It is connected to input voltage of 18~19 Volt
>from an DC adaptor of DC 12 V 1000mA non regulated.  My problem is the
>lead of the resistor drop out from the soldering PCB after some time
>(1-2 months). Of course the board have some minor burn mark.  How to
>prevent this from happening ?

Keep the resistor WELL within ratings for reliability. If you have
"minor burn mark", the resistor is running TOO HOT.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speffspamspam_OUTinterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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2003\02\12@005222 by James

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Since I need to use 20 ohm and more than 5 Watt. I need to increase the
wattage, but the increase of wattage will incur additional cost of
resistor and space in PCB.

If I use two resistors of 10 ohms 5 watt in series. Will the wattage
double to 10 W ?

James


{Original Message removed}

2003\02\12@011357 by Russell McMahon

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> Since I need to use 20 ohm and more than 5 Watt. I need to increase the
> wattage, but the increase of wattage will incur additional cost of
> resistor and space in PCB.
>
> If I use two resistors of 10 ohms 5 watt in series. Will the wattage
> double to 10 W ?

Yes, BUT you absolutely must understand what the circuit's demands are.
If you spell out in detail what you are trying to do, or even better provide
a circuit, and say WHY you are doing it this way, we may be able to help
with a better answer. For example, it is just possible that a simple SCR
preregulator will solve your problem or half wave rectification, or ....

Until we know the REAL application we can only guess at which of these or
other solutions are actually sensible or useful.


       Russell McMahon

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2003\02\12@033243 by James

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This is the supply to the dc motor driver IC. It need a current limiting
resistor to ensure no short circuit to the IC.

The IC is ROHM BA6219 or BA6222.


{Original Message removed}

2003\02\12@081240 by Olin Lathrop

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> If I use two resistors of 10 ohms 5 watt in series. Will the wattage
> double to 10 W ?

Yes.

This is a very basic electronics question.  If you're going to mess with
circuits, it's a good idea to learn something about them first.  There are
lots of ways to learn this, and I'm sure this list will not be short of
opinions and suggestions.


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Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

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