Resistor heat dissipation

Thread Starter

Rich2

Joined Mar 3, 2014
190
Hi, I've made a tiny heater with a 47 ohm 3w ceramic resistor cable tied onto a couple of old heatsinks. It's to go under a cut off plastic bottle to keep the frost off my new plants.
Power dissipated is just over 3w and it gets nice and hot (side of resistor 75°c, heatsink 65°c).
My thoughts are, what if I used a 100w resistor? It would still be dissipating 3.3w but would be cold and not make a good heater. But that doesn't make sense because they dissipate power AS heat. Where is my thought process going wrong?
 

Ian0

Joined Aug 7, 2020
3,186
Hi, I've made a tiny heater with a 47 ohm 3w ceramic resistor cable tied onto a couple of old heatsinks. It's to go under a cut off plastic bottle to keep the frost off my new plants.
Power dissipated is just over 3w and it gets nice and hot (side of resistor 75°c, heatsink 65°c).
My thoughts are, what if I used a 100w resistor? It would still be dissipating 3.3w but would be cold and not make a good heater. But that doesn't make sense because they dissipate power AS heat. Where is my thought process going wrong?
It would be just as good a heater.
Attach the 100W resistor to the same heatsink, and the heatsink will be at the same temperature.
 

Yaakov

Joined Jan 27, 2019
3,464
Hi, I've made a tiny heater with a 47 ohm 3w ceramic resistor cable tied onto a couple of old heatsinks. It's to go under a cut off plastic bottle to keep the frost off my new plants.
Power dissipated is just over 3w and it gets nice and hot (side of resistor 75°c, heatsink 65°c).
My thoughts are, what if I used a 100w resistor? It would still be dissipating 3.3w but would be cold and not make a good heater. But that doesn't make sense because they dissipate power AS heat. Where is my thought process going wrong?
You are wrong that it would be cold. It would be designed to dissipate more heat, so it would appear cooler on any portion of its surface but it would be radiating and convecting as much heat into the environment. In theory, a 100W resistor the size of a 3W resistor would be fine, but the materials would have to deal with much more thermal density. It would get hotter per unit volume, but not hotter in terms of total energy.

So, you are right that 3W of heat is the same no matter how much you spread it out, it just won't raise the temperature of a certain portion of the 100W resistor as high because if has more area.
 

BobTPH

Joined Jun 5, 2013
3,652
A resistor with a higher power rating achieves this in two ways.

1. Spreading the heat over a larger surface area.

2. Using materials that allow a higher operating temperature.

Number 1 is essentially the same as adding a heat sink.

In your case, replacing the 3W resistor with a 100W one would probably mean you could get by without a heat sink.

And yes, in an enclosed space, both would heat the air by the same amount.

Bob
 

ericgibbs

Joined Jan 29, 2010
13,586
But that doesn't make sense because they are capable of a higher dissipation power AS heat.

Hi Rich,
Modifying your assumption.

You could use a 12V 3W vehicle lamp as a heat source.

If the plastic bottles are in the garden, would also make a nice light show.;)

E
 

Thread Starter

Rich2

Joined Mar 3, 2014
190
Thanks for the replies :)
I've actually put a flickering 5mm yellow LED on it too so it looks like a candle at night :D
 

ericgibbs

Joined Jan 29, 2010
13,586
hi rich,
What are the plants you a growing in the bottles.?

Some growers use LED's that emit light that helps the plant to grow..

E
BTW: as you may know only about 2% of the power is emitted as light in a Tungsten filament, the rest is heat.
 

Thread Starter

Rich2

Joined Mar 3, 2014
190
I had a few different plants outside, tomatoes x 2 - now dead.
Cayenne chillies x 2 - still alive.
Jalepinõ chillies x 2 - now dead.
And a couple of flower plants for the hanging basket - one's dead the other one is hanging on.

All killed by April's relentless early morning frosts. Got 3 little heaters now for the survivors. Run off an old car battery, solar panel and a little digital thermostat.

Set some more tomato seeds last Monday, they're just coming up now :)
 

Ian0

Joined Aug 7, 2020
3,186
Our chillies were killed by the propagtor getting too hot! I'm currently designing a temperature controller for propagators, with a thermistor buried in the soil.
 

Thread Starter

Rich2

Joined Mar 3, 2014
190
Our chillies were killed by the propagtor getting too hot! I'm currently designing a temperature controller for propagators, with a thermistor buried in the soil.
I've got some flexible freezer room drain heaters 40w/m self limiting 3m long. Might do something with those, I've got a really naff polythene green house I could heat.
 
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