Working with capacitors

Before I started all of this I knew what a capacitor was.  They’re like little batteries…. sort of….  So while capacitors are pretty easy to understand, their uses cases are a little different than I had expected.  So let’s take a quick look at how capacitors work.  The easiest way to see this is by trying something like this…

Note: Im still working out what size capacitor to use where.  It can be dangerous to use the wrong size capacitors in your circuits so make sure you understand the circuit before you decide how to apply a capacitor.  

Charge up a couple of small capacitors of different sizes on the power rail of your breadboard.  Then one by one plug them in to the LED…

Hey, light!  So the capacitor is storing some kind of charge that allows the LED to turn on when they are connected in a circuit.  The longer the capacitor is plugged in – the dimmer the LED will get as the light discharges.  If we want to see whats happening, we can charge up a capacitor and then check it’s voltage…

Here we can see that when we check the voltage of the capacitor after charging that we now have almost 5 volts in the capacitor.  So it seems that our capacitor will charge up to the supply voltage.  So what’s really happening here?  To understand this, we need to dive into the physics behind capacitors.  Don’t worry, this is relatively easy to grasp.

A capacitor is essentially two metal plates separated by what’s called a ‘dielectric’.  As you apply charge to a capacitor electrons are forced into one side of the capacitor.  As they enter, they repel the electrons on the other metal plate.  This leaves behind positively charged particles which are trying to attract the electrons through the dielectric on the first plate.  To summarize, the plate on the left develops a surplus amount of electrons. However – the capacitor can only hold so much charge.  This is what we saw above where the capacitor showed that it was able to hold the same amount of voltage as the power supply we charged it with.  When we connected the LED to the capacitor the excess electrons move back to the other plate through the LED releasing the charge.

So that actually makes pretty good sense.  What I didn’t anticipate initially was how capacitors would work when put in a circuit in both parallel and series.   For instance, consider this circuit…

This photo was taken right after we connected the power supply.  While it’s hard to see, the capacitor is in series with the LED.  Power coming from the power rail goes into the positive side of the capacitor and the negative side of the capacitor connects to the LED’s positive lead.  If we look at this circuit after about 10 seconds we see this…

Notice that our current reading has dropped down to almost nothing and that the LED is noticeably dimmer.  So what’s going on?  Well – the capacitor is in line with the current supplying power to the LED.  As it turns out, the capacitor can only take so much charge and once it’s fully charged current stops flowing through the capacitor.  This makes sense if we consider that only so many electrons can be displaced on the opposing plate inside the capacitor.  So while I can find nothing that clearly calls this out – my assumption is that this means you would never put a capacitor into a series circuit in this fashion.  Rather, you’d put them in parallel to the load you were trying to power…

Note: You’ll notice here that I don’t have a resistor inline with the LED.  This is a great way to burn out LEDs.  I quickly realized my mistake after burning out two here.  I just wasn’t thinking when I started doing these labs.  

Here we have the breadboard I have setup for parallel circuits.  On one branch I have the LED and 3 other branches I have capacitors.  With the power plugged in the LED stays lit brightly and the capacitors charge and then stop passing current.  I was measuring current in the example above but a more interesting example is to check the voltage of a capacitor with and without power…

So above you can see the circuit with the power supply connected.  One of our capacitors is showing almost 5 volts to match the power supply.  Now if we pull the power we’ll start to see the voltage slowly drop…

And continue to drop…

While I haven’t stated it yet, the more capacitors you have in parallel the more capacitance the circuit has.  Sort of like adding resistors in series the capacitance of the capacitors is added together.  Now – while we wouldn’t add the capacitor in series, that doesn’t mean we cant add them in series within a parallel branch.  What would happen if we did that?  Our capacitance actually gets much worse.  This is because all of the capacitors are sharing the same series circuit.  This is not a good design idea and I can’t find anywhere that recommends doing this.  What you do get out of series capacitors is an increased voltage rating as voltage rating becomes the total of all the capacitors.  However it would appear that you’re always further ahead just getting the right value capacitor.

What we havent’ talked about at this point is capacitor sizing.  From what I can tell, the big thing is to make sure that the capacitor is sized for your circuit voltage.  If it’s sized for a much larger voltage that you need that seems to be ok although the capacitor itself will be much larger in size.  The amount of capacitance is another factor but at this point Im still not exactly clear why you would pick a certain size.  Hopefully more to come on that soon as I find an application for capacitors.

 

Leave a Reply

Your email address will not be published. Required fields are marked *