# How do I reverse-bias a resistor?

We all know that a photo diode can be used as a light detector and as a light emitter (although the way that it's built will usually make it specialized to prioritize one application over the other); an electric motor can become an electric generator and vice-versa.

How does one run a resistor in reverse bias (ie: turn heat into electricity, rather than electricity into heat)?

Something in my gut (but I had a hard time googling it) tells me that if I hold a resistor over a candle flame: it won't instantly become a little battery. So do I add an oscillating inductive coil around it; should I hold it over a candle but add a diode? I'm at a loss on this one.

Please note: I have like zero budget and no safe laboratory space for tinkering, so out of a general respect for my home: I have to avoid the funner flammable projects, so this is why I need to ask dumb questions, because I'm prevented from experimenting; otherwise I wouldn't have bothered you all.

I'm asking: what is a resistor's true [secondary] output (other than reduced electrical current) (is it 'heat', or is it just some subatomic phenomenon that manifests as 'heat'), and how do I feed that back into a 'dead' [as in: "no current is flowing through it"] resistor to get an electrical current flow? What form would this hypothetical flow take? Would each metal contact of the resistor become negatively charged [with each metal end acting like its own 'mini-ground' from the resistance-induced ionization]? (Internal friction can cause an electrical field, just like external friction can too right?)

I partially feel bad for asking [because it feels so elementary], but if I don't: I'll remain ignorant.

(I know this is April 1st in some parts, but I forgot, right after I typed out my whole question, so I'm gonna go out on a limb, and if I get accused of tomfoolery I'll just delete my question, and ask it on day that's not gonna net me suspicion (because I'm somewhat lazy and don't want to forget to ask on April 2nd))

• That's not what "reverse bias" means.... – DerStrom8 Apr 2 '18 at 1:59
• Resistors don't work that way, due to entropy. There's no bias on a resistor, heat is lost regardless of what direction the current flows. What you want is called a Thermo-Electric Generator (TEG) and can be accomplished using a Peltier junction and appropriate heatsink. A Peltier junction is essentially an array of thermocouples, which transduces heat into voltage or voltage into heat. They're not very efficient, but they do convert some heat into voltage. TANSTAAFL, so no building perpetual motion machines with this tech. – MarkU Apr 2 '18 at 2:08
• even if a resistor could produce electric current when subjected to heat, there is nothing that would determine the direction of the current, so there would not be any net current – jsotola Apr 2 '18 at 2:12
• First, repeal the Second Law of Thermodynamics. – Brian Drummond Apr 2 '18 at 10:01

The short answer is you can't because physics.

The long answer is you can't because voltage is a difference of potential.

What you're looking for does exist. Thermoelectric generators translate a difference in heat into a difference of potential through the reactions between different materials.

A resistor is generally a fairly uniform material exposed to a fairly uniform ambient temperature. There isn't anything inherent to its construction that would allow for a difference of potential by apply a single temperature.

The heat that is produced is a waste product. The process can't simply be reversed. The dual action in some components is possible due to specific electromagnetic phenomena inherent to their physical construction, so we can't arbitrarily assume all components can be used to generate a voltage.

And it's a good thing too. If that were the case we would have to be worried about voltage spikes as soon as the power is removed as the heat is absorbed like a magnetic field collapsing around an inductor. And because all components generate heat, there would be no way to control for it. Creating a stable circuit would be impossible.

• So, in response, I've known about TEGs and just assumed that they were like solid-state sterling engines and/or very IR sensitive solar panels; is that accurate? – user179283 Apr 2 '18 at 3:21
• Furthermore; if I could add a material gradient to a resistor, maybe, by slitting and adding an oriented heat-sink in some manner: could I get it to begin generating power through heat that way? – user179283 Apr 2 '18 at 3:23
• If I put an inductive coil around my resistor and passed an AC current through my inductive coil: would the resistor heat up faster or slower than an ordinary copper wire with the same inductor around (inductors transfer electrical current, that's a given, so now I'm just curious about how the inductive heating would shake-out), this method is kind of a "cheat" way of getting a resistor to generate power, but still – user179283 Apr 2 '18 at 3:25
• I upvoted you. I didn't touch the vote count on Tony Stewart's answer; I feel kinda like his troll comment was directed at me though😓. Not every "stupid" comment is from someone who believes in free energy, nor is every stupid person secretly a harvard proffessor pulling a dumb prank. Some people just legit don't know – user179283 Apr 2 '18 at 3:28
• @user179283 I don't think Tony's troll comment was directed at you, unless you were the one to initially down vote his answer without explanation. For whatever reason, I'm not seeing the up vote on mine, but I'm not all that concerned. Just hope it helped some. – Phil C Apr 2 '18 at 3:33

In general adding heat to a resistor will change it's resistive property. A negative temperature coefficient resistor will have it's resistance go down with increased temperature. On the other hand a positive temperature coefficient resistor will see an increase of resistance as temperature rises.

The change of resistance does not alter the fact that the resistor is still a dissipative device that essentially raises the net entropy of the universe.

A resistor produces an output voltage that rises with temperature. It's called "thermal noise" and is calculated as: -

$\sqrt{4\cdot k_B\cdot T\cdot R\cdot \Delta F}$

Where $k_B$ is Boltzmann's constant, T is the temperature in kelvin, R is the resistance in ohms and $\Delta F$ is the bandwidth of the noise you might be interested in.

See this wiki article for more information and see this calculator you can use to determine the values of RMS voltages produced.

For instance, with R = 10 giga ohm and at 500 degC (773 kelvin), over a 1 MHz bandwidth you could measure about 15 mV.

“Energy conversions” would be the correct term for your question.

e.g. Reciprocal energy Conversions include (thermo-peltier)(electro-chemical) (vibration/piezo-electric)!but resistors are not.

This is what Reverse Bias means by your example

simulate this circuit – Schematic created using CircuitLab

Photo Diode is reverse biased in both cases.

The high Resistor value is just a current sensor that converts photons into current then into voltage.

• I didn't downvote, but this answer still doesn't even remotely relate to the question. The question seems to be asking how to turn a resistor into a device that takes heat and turns it into current (the opposite of its normal operation - producing heat from the current flowing through it). He only used the photo diode as an example of a device being used backwards (he called it "reverse bias" but this is incorrect terminology) – DerStrom8 Apr 2 '18 at 2:07
• Yes I tried to explain the correct functions by examples and ignored the invalid assumptions. Resistors are never reciprocal energy devices, since they cannot store energy. ( unless they are negative resistors that amplify by resistance ratio (Semiconductors) drawing energy from supply. The question is absurd. so is this facebook.com/… – Tony Stewart Sunnyskyguy EE75 Apr 2 '18 at 2:10
• Thanks, I'll stop using "reverse bias" I was trying too hard to use proper terminology properly, and then embarrassed myself. @DerStrom8. I thought I knew how that word was being used and I didn't, my bad – user179283 Apr 2 '18 at 3:31
• @Tony Stewart, I didn't downvote you, I just now began checking in and reading. "Troll" is kinda a rude word to throw around. There's no need to add heat to a fire – user179283 Apr 2 '18 at 3:34
• My comments were not directed to you rather those idealists who are silent. I chose to explain reverse bias by example. “Energy conversions” would be the correct term for your question. Many areexplain reverse bias by example. “Energy conversions” would be the correct term for your question. Mreciprocal (thermo-peltier)(electro-chemical) (vibration/piezo-electric)!but resistors are not. – Tony Stewart Sunnyskyguy EE75 Apr 2 '18 at 4:06