Resistors to Generate Heat

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DylanMargan

Joined Nov 17, 2018
8
Hi everyone and thank you for taking the time to read this. I know little to nothing about electronics and recently made a 5v 4-100ohm 1/4w resistor setup in parallel that put me at full capacity for my resistor wattage. My goal is to make a very small heating element the bottom of a hummingbird feeder that would prevent it from freezing, based upon a video I found (
). My operating temps would (only) be when it's near freezing or well below. My 100ohm setup works to generate some heat, but I feel like it could use a (bit) more and don't fully understand how to go about that. I would like to keep my supply to 5V if possible, so basically what I need to do is add more power (watts) to more capable resistors, right? something like 2 10ohm 5w resistors in parallel? (Example: R.jpg ). If I ran 5V through 2 of those in parallel, I get 1amp, 5W of power? More power, more heat, right? I just don't understand which type of capacitor is best given they are made out of different materials along with heat dissipation thickness/heat sinks.... So now comes the tricky part. What temps am I going to get with that setup? I don't want to fry the bottom of the feeder or start a fire... Just a bit more heat, but I can't seem to find or figure out that calculation. Sorry for my ignorance and I wouldn't be surprised if you rolled your eyes reading this, but any help would be appreciated!
 

dl324

Joined Mar 30, 2015
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What temps am I going to get with that setup? I don't want to fry the bottom of the feeder or start a fire...
We can't tell you because we don't have enough information and you can't provide it because ambient temperature is one variable.

Most of us would design a closed loop system where the heat generating element would only be used to maintain the desired temperature.
 

MrChips

Joined Oct 2, 2009
30,706
Each resistor will consumed power which you can calculate as:

Power = Volts * Volts / Resistance

Hence a 100Ω resistor at 5V will deliver 5 * 5 / 100 = 0.25W

You can keep adding resistors in parallel until you get the desired amount of heat or until your 5V supply cannot deliver any more amps.
You will not burn the feeder or start a fire.

Your next option is to increase the voltage of the power supply.
For example, if you double the voltage to 10V, the power goes up 4 times. Power = 10 x 10 /100 = 1W. Now you are likely to burn out the resistor unless the resistor is tightly coupled to the bottom of the feeder.

You cannot estimate the temperature rise because there are too many variables. Your best bet is to try it and see the results. Note that the rise in temperature when the air is at 0°C will be very different when the air is at 5°C or 10°C.
 

Thread Starter

DylanMargan

Joined Nov 17, 2018
8
So I should start by adding more than my (4) 1/4w rated 100 Ω resistors to my circuit? I guess I could add as many as I want until I get my desired temp? I suppose jumping to more volts or higher watt rated resistors with less Ω would be to eliminate my need for a large amount 1/4w, right? I know you can buy electric feeders, but at $32 for the base model, and with 4 feeders outside, well I figured it would go cheap and perhaps learn something while I'm at it. Thanks again for everyone's help!
 

dendad

Joined Feb 20, 2016
4,451
Without going into a lot of detail:

1. Those gold resistors can be had with mounting holes.
2. You would want to use a thin layer of Silicon grease (thermal grease) on the mating surface
3. You would want to insulate the other side.
4. You can get simple thermo-switch to turn it on below a given temperature.
5. USB is typically limited to 0.1 A without handshaking. That's only 0.5 W.
6. remember those 7W incandescent night lights - That gives you an idea of wattage.
7. if the video was the feeder, you might want to spread the resistors around the base.
 

Thread Starter

DylanMargan

Joined Nov 17, 2018
8
I'm going off the plug specifications that the USB is plugged into. It states 5v 1A. So how many 100Ω resistors can I add before I exhaust my supply? It's confusing to me because my ohm law calculation is .05amp. So 20 100Ω 5v makes 1amp? So I can add that many in theory? What would happen if I went beyond for arguments sake? Can I possibly add any more question marks??? :p
 

MrChips

Joined Oct 2, 2009
30,706
A 5V supply rated to deliver 1A is good for 5V * 1A = 5W.
Twenty 100Ω resistors (each taking 0.25W) in parallel will draw 20 x 0.25 = 5W.

Add more resistors and then you start drawing on the overhead of the supply. Most supplies will not suddenly die when you push it beyond its limits.
Keep adding more resistors and then you start straining the supply. It depends on the supply. Some supplies may shut down. A few may start to release some magic smoke.
 

MrChips

Joined Oct 2, 2009
30,706
btw, why are we discussing USB chargers? You ought to be looking at a 12VAC @ 1A wall adapter. That will give you 12W, a bit more than a 5V USB port.
 

Thread Starter

DylanMargan

Joined Nov 17, 2018
8
I just had a bunch of cell/USB chargers lying around. I probably have 12v units somewhere, but I not sure. I also bought a cheap soldering kit along with a 1/4watt resistor box. If I went 12v, I would exceed the wattage on the resistors unless I used 600ohms, right?
 

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