Resistor heat calculation help please.

Hi,
I am using 3 x 100 ohm SMD resistors connected in parallel fed with a
12 volt supply to make a small heater for a device.

I have calculated that the resistors will give a total of 4.3 watts.

The surface area of the resistors is 61.44 mm2 (6.4mm x 9.6mm)

Is there any formula to calculate the temperature that the resistors
will reach?


Thanks.

 

[Lostgallifreyan]

tel1e@hotmail.com wrote in news:b8fa7d1a-5865-4b17-9b0c-
d41c2c07c0c8@w1g2000prm.googlegroups.com:

Hi,
I am using 3 x 100 ohm SMD resistors connected in parallel fed with a
12 volt supply to make a small heater for a device.

I have calculated that the resistors will give a total of 4.3 watts.

The surface area of the resistors is 61.44 mm2 (6.4mm x 9.6mm)

Is there any formula to calculate the temperature that the resistors
will reach?


Thanks.

No. You'd need a very complex model indeed to predict thermal coupling for
an SMT part in almost any context. This really IS one of those things best
done empirically. If anyone posts saying different to this, ask them how
they're going to quantify the thermal coupling when they don't know your
board, your solder fillet profile, ambient conditions, etc...

What you can do is look for a manufacturers figure for thermal coupling,
hopefully given in the °C/watt rating used for heatsinking. They might
specify some conditions you can use as a guide. None of which is a substitute
for the test you'll have to make anyway, by which time you'll know all you
need to know.

 

[Lostgallifreyan]

tel1e@hotmail.com wrote in news:c1343ba3-d66e-4e0e-9c3b-
5931f81d6330@r41g2000prr.googlegroups.com:

On 11 Jan, 19:18, Lostgallifreyan <no-...@nowhere.net> wrote:
te...@hotmail.com wrote in news:b8fa7d1a-5865-4b17-9b0c-
d41c2c07c...@w1g2000prm.googlegroups.com:

Hi,
I am using 3 x 100 ohm SMD resistors connected in parallel fed with a
12 volt supply to make a small heater for a device.

I have calculated that the resistors will give a total of 4.3 watts.

The surface area of the resistors is 61.44 mm2 (6.4mm x 9.6mm)

Is there any formula to calculate the temperature that the resistors
will reach?

Thanks.

No. You'd need a very complex model indeed to predict thermal coupling fo
r
an SMT part in almost any context. This really IS one of those things bes
t
done empirically. If anyone posts saying different to this, ask them how
they're going to quantify the thermal coupling when they don't know your
board, your solder fillet profile, ambient conditions, etc...

What you can do is look for a manufacturers figure for thermal coupling,
hopefully given in the °C/watt rating used for heatsinking. They might
specify some conditions you can use as a guide. None of which is a substi
tute
for the test you'll have to make anyway, by which time you'll know all yo
u
need to know.

Hi,
Thank you for your advice....I did think I was asking for the
impossible....trial & error it is then...

I rethought this a bit, there are some clues.. first, the 4.3W in that small
a volume might make a temperature rise big enough to make the subtleties
moot, it might melt the resistors off the board. second, a small change of
voltage, like just half a volt or so, might make the difference between the
parts staying put or falling off. If you're sure the 4 watts is enough to do
what you need, and it couples well to the load to be heated, then you'll
probably solve the parts-falling-off thing. If the contact is indirect, then
it's better to use more resistors, maybe 4 x 120 ohms, or even a 2x3 matrix,
i.e. parallel three series pairs with 200 ohms for each resistor.

I don't know what you're doing so I don't know if you need a controller, but
if you do, maybe the simplest way is to use an LM317 to control them. Adding
a thermistor to the control network can allow it to form a thermostat if
mounted with the heater resistors.

 

On 11 Jan, 19:18, Lostgallifreyan <no-...@nowhere.net> wrote:

te...@hotmail.com wrote in news:b8fa7d1a-5865-4b17-9b0c-
d41c2c07c...@w1g2000prm.googlegroups.com:

Hi,
I am using 3 x 100 ohm SMD resistors connected in parallel fed with a
12 volt supply to make a small heater for a device.

I have calculated that the resistors will give a total of 4.3 watts.

The surface area of the resistors is 61.44 mm2 (6.4mm x 9.6mm)

Is there any formula to calculate the temperature that the resistors
will reach?

Thanks.

No. You'd need a very complex model indeed to predict thermal coupling for
an SMT part in almost any context. This really IS one of those things best
done empirically. If anyone posts saying different to this, ask them how
they're going to quantify the thermal coupling when they don't know your
board, your solder fillet profile, ambient conditions, etc...

What you can do is look for a manufacturers figure for thermal coupling,
hopefully given in the °C/watt rating used for heatsinking. They might
specify some conditions you can use as a guide. None of which is a substitute
for the test you'll have to make anyway, by which time you'll know all you
need to know.

Hi,
Thank you for your advice....I did think I was asking for the
impossible....trial & error it is then...

Regards.

 

[Hal Murray]

In article <b8fa7d1a-5865-4b17-9b0c-d41c2c07c0c8@w1g2000prm.googlegroups.com>, tel1e@hotmail.com writes:

Hi,
I am using 3 x 100 ohm SMD resistors connected in parallel fed with a
12 volt supply to make a small heater for a device.

I have calculated that the resistors will give a total of 4.3 watts.

The surface area of the resistors is 61.44 mm2 (6.4mm x 9.6mm)

Is there any formula to calculate the temperature that the resistors
will reach?

I don't know of any formulas. It depends strongly on the details.
How big are the pads? How thick are the traces? How far to the
nearest via? How big are the vias?

You might get some ideas if you look at the right data sheet or
app note for power semi-conductors. I think the ones I'm think
of are for National regulators. The have numbers for various
patterns of copper that the heat tab gets soldered to.


What are you trying to do? Warm up a large area, or get a
small region very hot?

Is this a one-off project, or do you expect large production runs?

If I was trying to warm up a large area, I'd use more resistors.
If you have the typical 4 layer board, I'd use big pads with several
vias to the power/ground planes.

If I was trying to get something hot, I'd probably use an axial
leaded ceramic power resistor.

--
These are my opinions, not necessarily my employer's. I hate spam.

 

[Rafael Deliano]

warm a QFP 64 processor ... in my car.
A typical modern Freescale FLASH-controller

has the functional temperature range:
-40 ... +85´C C grade
-40 ... +105´C V
-40 ... +125´C M

If the ambient temperature is below approx 10 degrees C
the chip has this fault condition,
Maybe FLASH is solderjoints are failing.

MfG JRD

 

Hi,
Just to fill you in.....

I’m using the resistors as a heating pad to warm a QFP 64 processor. I
believe this chip is approx 14mm x 14mm.
This chip controls the dashboard gauges & warning lights in my car.
When the chip is cold the gauges don’t work properly…once the chip has
warmed up a little everything works fine.
If the ambient temperature is below approx 10 degrees C the chip has
this fault condition, if the temperature is more than 10 everything is
fine.
The problem is definitely with the processor & not anything else. I’m
very limited by space as obviously the processor is inside the
dashpod.

The resistor heating pad will be bonded to the processor & will have
trailing leads to come out of the dashpod.
The heater will only be in use for a few seconds (to warm the
processor enough to work), I will control this via a switch initially
so I can see how long the heater has to be on before the chip starts
to work, obviously this will vary depending on ambient temperature.
Eventually I could use a timing circuit to do this.
I suppose if I knew what I was doing I could build some form of
temperature control into the timer circuit so if the temperature was
over 10 degrees the heater wouldn’t operate.

Regards.

 

On 12 Jan, 15:06, Rafael Deliano <Rafael_DelianoENTFER...@t-online.de>
wrote:

 > warm a QFP 64 processor ... in my car.
A typical modern Freescale FLASH-controller
has the functional temperature range:
-40 ...  +85´C  C grade
-40 ... +105´C  V
-40 ... +125´C  M

If the ambient temperature is below approx 10 degrees C

 > the chip has this fault condition,
Maybe FLASH is solderjoints are failing.

MfG  JRD

Hi,
The processor is a Freescale MC68HC908AZ60 device.
Apparently the processor develops faults when it is cold due to poor
programming when manufactured.
A lot of vehicles from different manufactures suffer this problem &
obviously it shows up a lot more in the winter months!
The solder joints have been checked & re flowed & are fine....honestly
the problem is with the chip not liking to be cold....I know it seems
unlikely but it is the cause.

Regards.

 

[Franc Zabkar]

On Mon, 12 Jan 2009 06:30:35 -0800 (PST), tel1e@hotmail.com put finger
to keyboard and composed:

Hi,
Just to fill you in.....

I’m using the resistors as a heating pad to warm a QFP 64 processor. I
believe this chip is approx 14mm x 14mm.
This chip controls the dashboard gauges & warning lights in my car.
When the chip is cold the gauges don’t work properly…once the chip has
warmed up a little everything works fine.
If the ambient temperature is below approx 10 degrees C the chip has
this fault condition, if the temperature is more than 10 everything is
fine.
The problem is definitely with the processor & not anything else. I’m
very limited by space as obviously the processor is inside the
dashpod.

The resistor heating pad will be bonded to the processor & will have
trailing leads to come out of the dashpod.
The heater will only be in use for a few seconds (to warm the
processor enough to work), I will control this via a switch initially
so I can see how long the heater has to be on before the chip starts
to work, obviously this will vary depending on ambient temperature.
Eventually I could use a timing circuit to do this.
I suppose if I knew what I was doing I could build some form of
temperature control into the timer circuit so if the temperature was
over 10 degrees the heater wouldn’t operate.

Regards.

IIRC, you wrote that the bidirectional reset pin was cycling low when
an internal watchdog timer timed out. Maybe you could set up an
external timer for your heater which runs for 3 seconds, say, and use
the reset pulse to retrigger this timer. Once the uP is up and
running, the reset output will be inactive, and the heater timer will
time out.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

 

On 13 Jan, 08:47, Franc Zabkar <fzab...@iinternode.on.net> wrote:

On Mon, 12 Jan 2009 06:30:35 -0800 (PST), te...@hotmail.com put finger
to keyboard and composed:





Hi,
Just to fill you in.....

I’m using the resistors as a heating pad to warm a QFP 64 processor. I
believe this chip is approx 14mm x 14mm.
This chip controls the dashboard gauges & warning lights in my car.
When the chip is cold the gauges don’t work properly…once the chip has
warmed up a little everything works fine.
If the ambient temperature is below approx 10 degrees C the chip has
this fault condition, if the temperature is more than 10 everything is
fine.
The problem is definitely with the processor & not anything else. I’m
very limited by space as obviously the processor is inside the
dashpod.

The resistor  heating pad will be bonded to the processor & will have
trailing leads to come out of the dashpod.
The heater will only be in use for a few seconds (to warm the
processor enough to work), I will control this via a switch initially
so I can see how long the heater has to be on before the chip starts
to work, obviously this will vary depending on ambient temperature.
Eventually I could use a timing circuit to do this.
I suppose if I knew what I was doing I could build some form of
temperature control into the timer circuit so if the temperature was
over 10 degrees the heater wouldn’t operate.

Regards.

IIRC, you wrote that the bidirectional reset pin was cycling low when
an internal watchdog timer timed out. Maybe you could set up an
external timer for your heater which runs for 3 seconds, say, and use
the reset pulse to retrigger this timer. Once the uP is up and
running, the reset output will be inactive, and the heater timer will
time out.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.- Hide quoted text -

- Show quoted text -

Hi,
Your idea sounds excellent, but as I'm learning as I go with
electronics a manual switch will have to do until I figure out how to
build a simple timer circuit.

Thanks for the suggestion though.

Regards.

 

[Lostgallifreyan]

tel1e@hotmail.com wrote in news:c5429276-1410-4a45-990c-
e035d4f8ce51@x16g2000prn.googlegroups.com:

On 13 Jan, 08:47, Franc Zabkar <fzab...@iinternode.on.net> wrote:
On Mon, 12 Jan 2009 06:30:35 -0800 (PST), te...@hotmail.com put finger
to keyboard and composed:





Hi,
Just to fill you in.....

I’m using the resistors as a heating pad to warm a QFP 64 processor. I
believe this chip is approx 14mm x 14mm.
This chip controls the dashboard gauges & warning lights in my car.
When the chip is cold the gauges don’t work properly…once the chip h
as
warmed up a little everything works fine.
If the ambient temperature is below approx 10 degrees C the chip has
this fault condition, if the temperature is more than 10 everything is
fine.
The problem is definitely with the processor & not anything else. I’m
very limited by space as obviously the processor is inside the
dashpod.

The resistor  heating pad will be bonded to the processor & will have
trailing leads to come out of the dashpod.
The heater will only be in use for a few seconds (to warm the
processor enough to work), I will control this via a switch initially
so I can see how long the heater has to be on before the chip starts
to work, obviously this will vary depending on ambient temperature.
Eventually I could use a timing circuit to do this.
I suppose if I knew what I was doing I could build some form of
temperature control into the timer circuit so if the temperature was
over 10 degrees the heater wouldn’t operate.

Regards.

IIRC, you wrote that the bidirectional reset pin was cycling low when
an internal watchdog timer timed out. Maybe you could set up an
external timer for your heater which runs for 3 seconds, say, and use
the reset pulse to retrigger this timer. Once the uP is up and
running, the reset output will be inactive, and the heater timer will
time out.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.- Hide quote
d text -

- Show quoted text -

Hi,
Your idea sounds excellent, but as I'm learning as I go with
electronics a manual switch will have to do until I figure out how to
build a simple timer circuit.

Thanks for the suggestion though.

Regards.

Something's wrong here. Very wrong. This is to heat a CPU for critical
warning lights and gauges in a car? And in a situation that normally demands
a naojr manufacturer's recall at the behest of national government if they
don't do it voluntarilty, a person with limited knowledge of electronics,
below that needed to buld the simplest of timers is expected to do what for
once, really ought to be handled by formal authorites?

If this thing fails while you're driving, and the car crashes or hurts
another person, you're in for a vastly bigger and nastier shock than you're
going to suffer if you pay for it to be replaced properly. You specifically
mentioned warnign lights and gauges, so I don't think I'm over-reacting here.
This is critical stuff. Last time I heard of some guy who tinkered with his
vehicle and this resulted in deaths of people (in this case by breaking down
in frozen inaccessible conditions rather than a crash) the driver and owner
got seriously jailed.

If you have isolated as much of the problem as you have, good on you, take
that info to a places registeres ad qualified to do it under terms that won't
invalidate your vehicle insurance, etc. Use the info to make sure you get the
job done fast, as cheap as possible, as it saves diagnostic time. This isn't
one for DIY.

 

On 13 Jan, 17:36, Lostgallifreyan <no-...@nowhere.net> wrote:

te...@hotmail.com wrote in news:c5429276-1410-4a45-990c-
e035d4f8c...@x16g2000prn.googlegroups.com:
Something's wrong here. Very wrong. This is to heat a CPU for critical
warning lights and gauges in a car? And in a situation that normally demands
a naojr manufacturer's recall at the behest of national government if they
don't do it voluntarilty, a person with limited knowledge of electronics,
below that needed to buld the simplest of timers is expected to do what for
once, really ought to be handled by formal authorites?

If this thing fails while you're driving, and the car crashes or hurts
another person, you're in for a vastly bigger and nastier shock than you're
going to suffer if you pay for it to be replaced properly. You specifically
mentioned warnign lights and gauges, so I don't think I'm over-reacting here.
This is critical stuff. Last time I heard of some guy who tinkered with his
vehicle and this resulted in deaths of people (in this case by breaking down
in frozen inaccessible conditions rather than a crash) the driver and owner
got seriously jailed.

If you have isolated as much of the problem as you have, good on you, take
that info to a places registeres ad qualified to do it under terms that won't
invalidate your vehicle insurance, etc. Use the info to make sure you get the
job done fast, as cheap as possible, as it saves diagnostic time. This isn't
one for DIY.- Hide quoted text -

- Show quoted text -

Yes I agree with your point that the manufactures should be the fixing
this problem however like most car manufactures they seem to ba a law
unto themselves & if they say no well quite often that's the final
word.
Public pressure of the same (well very similar) fault on the Audi TT
did bring about free replacement dash pods regardless of the age of
the vehicle, however a recall wasn't the case...probably because that
would be admitting liability!

I do take your point, however I would still be in the same boat if I
had the dash pod fixed anywhere other than the manufacturer, who won't
fix the defective part but will fit a new dash pod for a exorbitant
amount of money.

The resistor pad heater I intend to fit is merely to save me having to
sit & wait until the processor has warmed itself up enough to spring
into life, approx 5 minutes in frosty conditions. I don't drive the
vechicle until the gauges have started to operate...not because of not
knowing haw fast I'm going or anything silly like that but because if
the car is moved when the gauges arn't working when they do come on
the mileage goes silly & clocks on a big chunk of miles!

Regards.

 

[legg]

On Sun, 11 Jan 2009 09:28:19 -0800 (PST), tel1e@hotmail.com wrote:

Hi,
I am using 3 x 100 ohm SMD resistors connected in parallel fed with a
12 volt supply to make a small heater for a device.

I have calculated that the resistors will give a total of 4.3 watts.

The surface area of the resistors is 61.44 mm2 (6.4mm x 9.6mm)

Is there any formula to calculate the temperature that the resistors
will reach?

The answer is in your question. This is a heater - therefore you would
logically be controlling the target temperature by removing power when
the required 'device' temperature is achieved.

You can reduce power consumption and improve thermal gradients between
heater and target by thermally isolating the bonded target and heater
from the surrounding ambient.

Limiting rise is only an issue if the power must be constant and is
determined by thermal impedance to the physical environment from the
source.

If the source of power is isothermal within a volume that has a
specific surface area, the surface temperature rise can be calculated
as a ball-park figure with better than 20% accuracy - but that's not
what you're asking for.

RL

 

[Lostgallifreyan]

legg <legg@nospam.magma.ca> wrote in
news:7ge1n45sousij3qkfktsvpmq4143ej3ar3@4ax.com:

On Sun, 11 Jan 2009 09:28:19 -0800 (PST), tel1e@hotmail.com wrote:

Hi,
I am using 3 x 100 ohm SMD resistors connected in parallel fed with a
12 volt supply to make a small heater for a device.

I have calculated that the resistors will give a total of 4.3 watts.

The surface area of the resistors is 61.44 mm2 (6.4mm x 9.6mm)

Is there any formula to calculate the temperature that the resistors
will reach?


The answer is in your question. This is a heater - therefore you would
logically be controlling the target temperature by removing power when
the required 'device' temperature is achieved.

You can reduce power consumption and improve thermal gradients between
heater and target by thermally isolating the bonded target and heater
from the surrounding ambient.

Limiting rise is only an issue if the power must be constant and is
determined by thermal impedance to the physical environment from the
source.

If the source of power is isothermal within a volume that has a
specific surface area, the surface temperature rise can be calculated
as a ball-park figure with better than 20% accuracy - but that's not
what you're asking for.

RL

Of course it's not what he's asking for! Did you reply late just to play the
smartarse with lectures on thermodynamics? It's a small processor on a board
in a car, how the hell is he supposed to thermally isolate it? You sound like
a guy with a head full of blurry ideas who didn't even read any of the posts
that were done before you came. By trying to make others look like fools,
answering the first post as if all the rest are stupid and beneath your
exalted threshold, you've made yourself look like a total fool yourself.

 

Hi,
just to let you all know that I made the heater & it works fine to
warm the processor to get it working.
I ended up using 6 x 33 ohm resistors for better surface area
coverage.
A 5 second blast of the heater is all that is required to get the
processor up & running fine.

Regards.