Good day all! There is little talk today regarding negative resistance passive components, Mostly because they where used mostly in the old days with the early radar detector technologies, "Tunnel Diode" Proved to be interesting in the day as they could be used as oscillators and amplifiers as well. And worked perfect on the microwave bands. But then semiconductors and transistors quickly took over.
But negative resistance is still a rather interesting effect to study and experiment with. (Rumor has it. That negative resistance devices may even have anomalies dealing with the ohms law.) It can amplify part of the negative AC current cycle where negative resistance takes place within this repetitive cycle. But in the case of a diode, needs some sort of DC bias voltage to work as an AC amplifier. It does not have to be very much at all!
So where can you find negative resistance?
-Tunnel and Microwave/Radar Didoes
-Home made Cat Whisker Copper Oxide Didoes
-High Voltage Spark Gap ( Tesla took advantage of the negative resistance of his spark gap so he could amplify the AC currents generated and received from his wireless from a distance, Given he had a great ground connection. )
As you know, All the above mentioned have several or more interesting oscillator and amplifier circuits if you want to search for them. For example, In the case of oscillation, Usually called "Relaxation Oscillator" As a result to the negative resistance properties of the circuit.
So today I'm going to show you how to built the Words most simple and safe low voltage resistance amplifier that can amplify AC with the help of a half charged 1.5 volt battery (DC bias) and light up an LED! Sounds cool right! So here goes.
1. Two or more identical LEDs
2. 1 or more diodes (Ge type preferred)
3. 1.5 Volt Battery
4. Two or more clip jumper connection cables
5. FRS Radio or Similar (For providing nearby source of low power RF also known as a form of AC)
Step 1: Cooking a LED
Building our simple negative resistance diode
First thing we need to do is slightly cook a LED until its light starts to change color by getting darker and tinted but not completely cooked and burnt out.This only takes a couple of seconds. I find it works best with a 6 volts battery pack. I connect the battery pack to the LED light for about 5 seconds as I watch the color change then quickly disconnect to not let the LED completely burn out and go dim. It may take a few tries to get it right so it is a good idea to have some spare LEDs on hand. This yellow LED turns dark orange after a few seconds of 6 volts!!
Congratulations, We just built our negative resistance device!
Step 2: Putting It Together.
The easy part!
Take your partially cooked LED and connect the anode long side to the cathode "line" side of a regular diode and together with the help of the wire clips. Then connect the two remaining ends contacts together with another wire clip (the loose length instead of just connecting the two diodes together, The wire length between the two diodes act as a sort of crude loop antenna) Now take your 1.5 volt battery and connect the + side of the battery to the plus side of the circuit( the longer LED pin) and do the same with the negative side on opposite end. You will notice you do not have enough power to turn on the LED light. This is normal. The DC battery will be our amplifier DC bias power source.
Step 3: Testing the Circuit
Experienced users may skip this part. If you have a regular LED and have never played with simple RF diode detector circuits. May I recommend that you take the extra step to first experiment with that. Simply connect a regular LED to the circuit instead of the cooked one. Place your FRS antenna 1 inch away from the LED. Press the transmit button and you will see the LED faintly light up. This is because LEDs are also diodes and this only diode circuit acts as a crude AC to DC power supply (rectifier circuit) energized by the nearby RF as a form of AC provided by the FRS radio or other nearby transmitter. Cool!
Now run the circuit as intended with the cooked LED and you will notice much more brightness! As it behaves like an AC amplifier. read bellow.
Now running the circuit.
Make sure everything is connected as simple as a circuit this is. Connections may get loose. Now push on the talk button of your FRS radio or similar transmitter ( The 6 inch *give or take* connecting wires act as a good resonant antenna at the UHF frequencies) You will notice your cooked LED lights up nice and bright at normal color and you can pull the radio several inches away before it dims out once it is lit and starts amplifying! That is the LED is acting as a negative resistance diode amplifier and doing the job amplifying the RF/AC signals with the help of small DC bias supply and making it's own LED emit a bright light as a side effect. Cool!
Step 4: Experiments to Try
Some interesting experiments to consider
Try adjusting and vary the low voltage DC bias to find the "sweet" spot where AC amplification (LED brightness) is at its best. Perhaps a variable resistor.
Try replacing the battery with a small capacitor. Then the circuit becomes in part a DC power supply provided there is nearby RF/AC to energize it. What I'm getting at is you may be able to use the circuit's rectification properties and DC stored in capacitor as the source of DC bias and still get a very nice bright LED without actually needing the 1.5 Volt DC bias battery at all!! No it's not over unity I'm sorry but still very interesting stuff!
And many more negative resistance experiments? Usually these kinds of experiments require much higher voltages in order to drive neon and spark gaps etc and can be dangerous and intimidating. This is a great introductory way to get into it without getting hurt and learning about negative resistance and or RF.
Still not convinced?
Food for thought. Florescence tubes need a ballast, That is an Induction coil acting as a current limiting filter to counter the effects of negative resistance within the tube. It takes more power to activate the tube then is needed to keep it bright. Without a protection ballast. The negative resistance would cause the AC currents inside To amplify to a point it would damage the tube. Even sudden fluctuations in input voltage could instantly destroy the florescence tube. With that logic let's experiment some more with our modified LED.
Bring your radio antenna about once inch from your LED. turn and keep pressing the talk TX button, After a moment or two. The LED glows brightly, Slowly as your still holding the TX button, Bring your radio a few more inches away. Perhaps 6 inches. You will notice the LED remains very bright and can hold this brightness a short distance away from the RF source without flicking off. As we are then observing the negative amplifier properties of the LED quick in, Needing much less power to stay light up then it needs to activate. ( Acting very much like a Florence tube)
Try again. Repeat with a regular LED and you will notice the effect to be greatly diminished or not noticeable at all! (Your LEDs can always react and flicker close by to RF without any modification such as turning it into a negative resistor as I describe in this article.) The amplification properties of a negative resistance device is what is really interesting!