Hello!
I'm in high school and learnt about capacitors recently. Just for fun, I took out a 3v button cell out of my wrist watch and charged a capacitor(100microfarad) using it. Then I connected the terminals to a LED and it blinked for a moment. Did I do everything rightly? Was something else expected to happen? Also, please suggest any more activities that can be done using capacitors, diodes, resistors and LED's. Thank You

 

Welcome to AAC!

Hello!
I'm in high school and learnt about capacitors recently. Just for fun, I took out a 3v button cell out of my wrist watch and charged a capacitor(100microfarad) using it. Then I connected the terminals to a LED and it blinked for a moment. Did I do everything rightly?

Did you learn that some capacitors are polarized and applying a reverse voltage of more than a volt or so with sufficient current can damage them (i.e. cause them to explode)? Did you learn that connecting two polarized caps back-to-back would create an unpolarized cap?

Was something else expected to happen?

When working with high voltage, high current, and/or high capacitance, bad things can happen when charging and discharging capacitors.

Also, please suggest any more activities that can be done using capacitors, diodes, resistors and LED's.

Not a lot of options with just those components and a power source. You could vary resistance and observe the effect on LED brightness; taking care not to drive more than 20mA through the LED. You can experiment with series/parallel connections. You can observe the effect of reverse biased diodes. You can observe the effect of inserting a capacitor in series with an LED.

If you have an AC power source, you can experiment with rectifiers.

If you add some transistors, you can experiment with logic gates, switches, amplifiers, etc.

 

Think of your capacitor as a cup of water. The bigger the cup the more it holds. Think of the LED as a straw. The bigger the straw the faster you can suck the water out of the cup. Larger cups with smaller straws will take longer to drain while smaller cups with larger straws will drain much faster.

The reason why your LED flashes is because it presents a basic short circuit across the capacitor. The energy stored in the cap is depleted nearly instantaneously (hence the flash).

LED's have a required amount of current to get them to light. So it would help to learn (or learn how to apply) ohms law. If you have an LED who's current requirements are 25 mA (0.025 amps) and you wish to power it through a 12 volt battery then you need to limit the current to 25 mA using a resistor. When you put that resistor in series with the LED and connect it to the battery (proper polarity) the LED will reliably light and stay lit until the battery is drained of its energy. (incidentally a capacitor can in some ways be though of as a battery, just lots smaller)

As mentioned, capacitors can be likened to a cup of water. Try this: Find a capacitor rated at 1000 µF and charge it from a double A battery. Observe polarity to be safe. The negative terminal of the cap goes to the negative terminal of the battery and the positive to the positive (pretty self explanatory). Touch the capacitor to the battery for just a second and it will be charged to nearly full capacity. Then take your LED and a 60 ohm resistor in series (one lead of the resistor attached to one lead of the LED) and touch the remaining leads to the capacitor (observe polarity). The LED will light and stay lit for a longer time than just a flash. Connect a higher value resistor, say 220Ω and touch it to a charged capacitor. The LED will be dimmer but stay lit longer.

I'm making up these values as I go. I don't have a lot of experience with LED's and resistances but I do know how to use ohms law to come up with the needed values.

As DL324 said - be careful with capacitors and high voltages. They CAN explode. When I was in high school I was trying to power a car radio from a 12 volt transformer. I had a diode in place to rectify the current but got a horrible hum (60 cycle). Someone said "Use a capacitor as a filter." So I tried the capacitor in several different spots. It was a 22000 µF cap rated at 16 volts. Tried it here - still had a hum. Tried it there - still had a hum. So I tried it on the mains. POW! Suddenly all I had was the remains of the can the capacitor once lived inside of. Fortunately it didn't hit me when it went flying. So - yes - you have to be careful when learning about electricity and electronics.

Eventually I figured out how to get rid of the hum all together. And yes, it was with the aid of a capacitor. But capacitors do a lot more than just filter out hum. Read up on them in AAC's library. Best to start at the beginning, even if it means going over materials you're already familiar with. Never hurts to have a solid base to work from.

And welcome to AAC.

 

Thanks for all that information and warnings. And yeah, I managed to get transistors, would you send me links for experimenting with logic gates, switches and the other cool stuff you just mentioned. Also, I have a little confusion, The capacitor had 25v mentioned on it and I used a 3v battery. What exactly happened here? What was the voltage in the circuit?
Thank You

 

Hello Tonyr1084, it was very interesting to read your paragraph and I would surely be careful with caps and stuff. Also, the cup and straw explanation was very good and I especially thank you for it. And I'll try the above mentioned activity. And I'm looking forward to learn more at AAC it's really cool in here

 

Happy to help.

As for the 25V on the cap - that's it's rating. In other words you can use it to store up to 25 volts. If you only charged it with 3 volts then that's all you got back when you tried your LED test. Charging it higher than 3 volts (up to but not over 25 v) will store more energy. Keep in mind that if you charge it to 3 volts and then short the leads you'll never see any kind of spark, not even in the darkest place on earth. But if you charge it up to 25 volts then short the leads - yeah, you'll see the light. Big spark.

Did you know that pencil lead is not made of lead but made of carbon? I once proved I could use a pencil as a dimmer on a low voltage light. Took a 12 volt wall wart (plug in module like what charges your cell phone - only a different voltage) and powered a small automotive light. By stripping bear the carbon rod and using paper clips I made the pencil act as a variable resistor. The pencil DID get warm but not hot enough to erupt in flames, and I have no idea what resistance or wattage the pencil can handle. Resistors come in various sizes as well as values. You can have a 100Ω resistor at 1/8 watt, 1/4 watt, 1/2 watt, 1 watt, 5 watts, 10 watts - - - and higher. My light bulb only drew 7 watts, so my pencil was at least able to handle that much power with only minor warming. You can literally draw a resistor on a piece of paper. Apply too much current and you can start the paper on fire. The drawn line has to be drawn fairly heavy.

Anyway, nuff about resistors. There are some really good articles in AAC about transistors and diodes. Take some time to read through them and learn more about how they work and how to use them. But the most basic understanding of electricity is power. How voltage, resistance, amperage and wattage are all inter-related. Devices are designed to handle so much power and so much voltage. Exceed their ratings and you can burn them out.

I'll look through my bag of goodies and see if I can come up with something interesting to try. Off hand I know you can use a 2N2222 transistor to make an LED flash. The LED will never extinguish but it does flash. I'll see if I can find that for you. OR you can search the internet for "LED Flasher Circuits".

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Here's a simple LED flasher using a 2N2222 transistor, a 470 µF electrolytic capacitor and a couple resistors. You can vary the flash rate by varying the 1KΩ resistor and/or the 470µF capacitor.

The way this works is that the 2N2222 has a breakdown voltage (don't ask me exactly what, you can find that on the data sheet). When the capacitor charges up to that level the transistor will go into breakdown and pass current to the LED. This will drain the capacitor to a level where the breakdown voltage no longer passes through the transistor. During breakdown the LED will be lit and off when the capacitor is below the breakdown voltage. Certainly not the greatest LED flasher you can build but it'll help you understand the functioning of a capacitor as well as how transistors break down.

A better circuit would use a 555 timer and some discrete components (resistors and capacitors), and you can build a much more controllable flasher out of that. But start with basics first and gain a firm understanding of them before moving on to gates and other logic circuits.

 

would you send me links for experimenting with logic gates, switches and the other cool stuff you just mentioned.

Try this http://www.circuitstoday.com/logic-gates I scanned the information and it seemed correct. It has an example of using diodes for an OR gate, but that has the disadvantage of decreasing signal levels that can affect down stream circuitry.

The capacitor had 25v mentioned on it and I used a 3v battery. What exactly happened here? What was the voltage in the circuit?

The capacitor charged to the battery voltage in 5 RC time constants. Since R was essentially 0Ω, it was instantaneous. Since you connected the LED across the cap with no current limiting resistor, only the forward resistance of the diode limited current. It's on the order of a few tens of ohms. The capacitor won't discharge completely because the LED stops conducting around 1V. Capacitor voltage would reach 1V in about 1 RC time constant.

 

Here's a more traditional oscillator circuit from wikipedia: https://en.wikipedia.org/wiki/Multivibrator

 

Thanks to everyone who's helping. All the information is very cool and I've saved it for reading with full concentration later. And the paper burning activity was awesome, I also tried to calculate resistance of pencil lead once using Ohm's law in our physics lab.