# Choosing right Power resistor for 100W and 40 Celsius degree ambient temperature

#### Patrick19

Joined May 22, 2019
11
I have an application to heat an aluminium plate as heat sink - (thickness 1mm or 3 mm ) with surface area between 40, 000 mmp and 50, 000 mp to 40 Celsius degree - using Power resistor with an electric power of 100W, for instance
I found an article
https://www.digikey.com/en/articles...-in-power-resistors-and-the-use-of-heat-sinks
but does not help me to much and there are some differences related to sink calculator from link
Also , mention the righr power supply for power resistor ( voltage, current)

#### drc_567

Joined Dec 29, 2008
1,156
Some clarification if possible?...
The specific quantity that generates heat, in an electrical sense, is the magnitude of the DC current that can be sustained. More accurately,
$$Q=I^2R$$
Or, is 100 watts the maximum power that is to be used in heating the aluminum plate to 40 C?

Last edited:

#### SamR

Joined Mar 19, 2019
4,194
How much V and how much I in the circuit. Then throw on a safety net so to speak to keep the available W, I, and V above what is actually needed for the circuit. Note to self: learn what those safety factors are so you don't have to look them up each time.

#### drc_567

Joined Dec 29, 2008
1,156
As an initial calculation (yet to be checked) .... approximately 1915 Joules of heat are needed to raise a 60 cubic centimeters volume of Aluminum from 27C to 40C ... estimating a sheet thickness of 0.15 cm and area 20 cm x 20 cm.
So pick a realistic combination of I and R such that
$$I^2R=1915$$.
The estimation shown here does not include any convection heat loss due to ambient conditions.
... One relevant question would be to confirm the change in temperature that the Aluminum sheet is to experience?

Last edited:

#### SamR

Joined Mar 19, 2019
4,194
How about a circuit diagram so we know just what it is that you need in a resistor???

Joined Feb 20, 2016
4,135
Have you thought of using a 3D printer hot bed heater?

#### Papabravo

Joined Feb 24, 2006
18,424
How do you plan to transfer the heat from the resistor to the plate?
What units are represented by 40,000 mmp and 50,000 mp? I'm not familiar with those abbreviations.

#### oz93666

Joined Sep 7, 2010
737
I've no idea what this thread is about , the title doesn't make sense , nor does the opening post ...

Go slowly and explain what you are trying to achieve.

Joined Feb 20, 2016
4,135
I was wondering a bit about all too. maybe the "mmp" is mm squared???
We wait with bated breath for enlightenment!

#### Papabravo

Joined Feb 24, 2006
18,424
I was wondering a bit about all too. maybe the "mmp" is mm squared???
We wait with bated breath for enlightenment!
It must be a secret that he cannot reveal at this time, otherwise he might have to kill us. I am so sick and tired of confidentiality BS!

Joined Feb 20, 2016
4,135
Yeah! It is amazing how many think they are developing something new unique and special, only to re-invent the wheel.
If there is a special secret component to their device, asking on a public forum is not to wise. It does get pretty hard to help folk who are reluctant to divulge sufficient information.

#### Patrick19

Joined May 22, 2019
11
Some clarification if possible?...
The specific quantity that generates heat, in an electrical sense, is the magnitude of the DC current that can be sustained. More accurately,
$$Q=I^2R$$
Or, is 100 watts the maximum power that is to be used in heating the aluminum plate to 40 C?
-----------------------------------------------
Dear fellows
I asked even help to producer who mention few type of 100W resistor- in the link :
https://www.digikey.com/products/en?keywords=pf2472&stock=1
and a power supply of 200W :
https://www.digikey.com/short/ph0jcv
but nothing about thermal calculation, like in their example :
https://www.digikey.com/en/articles...-in-power-resistors-and-the-use-of-heat-sinks
Obiously it is need of some calculations, because max. temperature of resistor is just 155 Celsius degree

#### Patrick19

Joined May 22, 2019
11
I was wondering a bit about all too. maybe the "mmp" is mm squared???
We wait with bated breath for enlightenment!
-------------------
Yes, square mm

Joined Feb 20, 2016
4,135
So, have you looked at the 3D printer bed heaters? Off the shelf devices designed to heat just as you require.
They are available in various sizes and voltages.

#### Patrick19

Joined May 22, 2019
11
It must be a secret that he cannot reveal at this time, otherwise he might have to kill us. I am so sick and tired of confidentiality BS!
---------------------
Dear Papabravo you can find all necessary in the 2 links.
Your motto :Life is too short to sweat the small stuff. is all right, but sometimes we have to sweat at very small stuff. Life is not easy with some of us. Best regards. Patrick .

Joined Feb 20, 2016
4,135
There still remain many questions.
What form of control are you wanting to use? How accurate do you need the temperature?
What is the power supply?
Is the plate in free air, insulated, fan cooled, under water......

#### Patrick19

Joined May 22, 2019
11
How much V and how much I in the circuit. Then throw on a safety net so to speak to keep the available W, I, and V above what is actually needed for the circuit. Note to self: learn what those safety factors are so you don't have to look them up each time.
----------------
Dear fellows
I received a message from manufacturer who mention a 200W convertor : Enclosed AC DC Converter 1 Output 24V 8.8A 90 ~ 132 VAC, 180 ~ 264 VAC Input
So, we have 24 V direct current and 8,8A .
I mention bellow with copy / paste an example with power resistor from link
https://www.digikey.com/en/articles...-in-power-resistors-and-the-use-of-heat-sinks :

Example using thermal resistance to determine maximum power levels of TO-247

From the graph shown in Figure 2, the thermal resistance Rθ to air of an aluminum plate with a plate thickness of 2 mm and an expanded area of 40,000 mm2 is about 2.2°C/W. The thermal resistance of a TO247 resistor with a rated power of 100 W is 1.3°C/W and the flange contact thermal resistance is about 0.1°C/W. The upper limit on the operating temperature range of the resistance film of the power resistor is 155°C.

Figure 5: Temperature at each portion in heat flow from resistor to air.

Thus, if the thermal resistance of the heat sink and the ambient temperature are determined, the maximum power which can be applied to the resistor without exceeding its maximum operating temperature can be determined. In the case of Figure 5, when the ambient temperature is 50°C, the maximum permissible power P = (155°C - 50°C) / 3.6 = 29 W. Notice that, if a resistor flange is mounted on a heat sink which is maintained at 25°C, say by forced air or liquid cooling, the maximum power which can be applied is also the rated power of the device = (155°C – 25°C) / 1.3 = 100 W. Generally, maximum power levels quoted in product specifications assume that the back plate (and therefore the heat sink at the contact area) is maintained at 25°C.
---------------------------
So, we have to take into account thermal resistance and maximum temperature 155°C , and the size of aluminium sheet to produce 100W of heat. Thanks.

#### Papabravo

Joined Feb 24, 2006
18,424
---------------------
Dear Papabravo you can find all necessary in the 2 links.
Your motto :Life is too short to sweat the small stuff. is all right, but sometimes we have to sweat at very small stuff. Life is not easy with some of us. Best regards. Patrick .
I read both articles and they were less than illuminating as to your ultimate intentions and purpose. I'm sorry life is not easy for you, it has been easy for me since I learned how to work smarter instead of longer and harder. I'm sorry for your pain.

You did not mention or account for the three methods of heat flow from the plate to the ambient which are:
2. Conduction
3. Conduction
In order to maintain the 40°C temperature of the plate above ambient it will need to be inside an enclosure of some type that limits the ability of the ambient to take away the heat you have so laboriously created.

Last edited:

#### oz93666

Joined Sep 7, 2010
737
I still understand nothing .....

Your aim is to heat an Aluminium plate ???? With dimensions about 40 x 10 cm ?? How hot do you want it??? What are you going to do with this hot aluminium plate ???

#### Patrick19

Joined May 22, 2019
11
I read both articles and they were less than illuminating as to your ultimate intentions and purpose. I'm sorry life is not easy for you, it has been easy for me since I learned how to work smarter instead of longer and harder. I'm sorry for your pain.

You did not mention or account for the three methods of heat flow from the plate to the ambient which are: