DC blocking capacitor

Thread Starter

electronewb

Joined Apr 24, 2012
260
I red that a capacitor blocks DC but passes AC Why is it when I hook up a capacitor with a LED the LED lights up and pulses depending on the capacitor rating. If a capacitor blocks DC how come the LED lights up??
 

MrChips

Joined Oct 2, 2009
23,582
Imagine a 10V DC signal with a 4VAC signal superimposed on top of it.
So the signal fluctuates between 8V and 12V.
What would you call this, DC or AC?
Feed this to a load via a series capacitor. The 10VDC will get blocked but the 4VAC will still pass through. This is sufficient to turn on an LED.
 

MrChips

Joined Oct 2, 2009
23,582
I am using that as an example.

You can use a 9V battery supplying power to a 555 timer circuit as an example.
The output of the 555 timer consists of pulses going from 0V to 9V.
This is not just DC. This is pulsating DC.

Feed this to the LED via a blocking capacitor. The capacitor will block the DC component by reducing the AVERAGE voltage to ZERO. What will get through to the LED is the fluctuation, the AC component. This will still cause the LED to flash.
 

Thread Starter

electronewb

Joined Apr 24, 2012
260
OK so it's not DC that's actually powering up the LED but the effect of the DC on a capacitor by charging and discharging? So in theory if I measure after the capacitor with my voltmeter I should get zero Volt DC and would I get a reading in AC or on scope would the wave look a sine wave?
 

MrChips

Joined Oct 2, 2009
23,582
In theory, if the LED is replaced with a resistor, the average voltage would be zero.
You cannot use a voltmeter to measure this unless the voltmeter can measure average voltage. Yes you can observe this on a scope but it would only be a sine wave if the signal driving the capacitor is a DC plus sine wave.

With the LED in place you will not measure zero volts because of the rectifying effect of the diode. I would have to confirm this on a real circuit.
 

crutschow

Joined Mar 14, 2008
27,243
When you connect a DC voltage to a capacitor, it passes current until the capacitor is charged to the DC voltage. The larger the capacitor, the more current it takes to charge the capacitor. After it's charged, nothing passes through the cap unless the DC voltage changes. Thus a capacitor does indeed block steady-state DC.
 

Thread Starter

electronewb

Joined Apr 24, 2012
260
But what actually make the LED light up? Just like MrChips explained the effects of the capacitor and not the DC itself? So why do we find so many capacitor in electronics if it blocks DC I know in an audio circuit it would let the audio trough but most circuits have capacitors and the source is DC
 

crutschow

Joined Mar 14, 2008
27,243
But what actually make the LED light up? Just like MrChips explained the effects of the capacitor and not the DC itself? So why do we find so many capacitor in electronics if it blocks DC I know in an audio circuit it would let the audio trough but most circuits have capacitors and the source is DC
The LED lights from the charging current through the capacitor. Once the cap is charged the LED turns off since no further current flows. The point is that capacitors block DC only after they are charged to the steady-state DC value. Any change in DC level is coupled through.

We use capacitors in circuits so the the DC bias levels on each stage are unaffected by the bias levels on the next stage. The capacitors block the DC but let the AC signals pass through.

You cannot have capacitors in series with the DC power source since that would block the DC current required to power the circuit.
 

upand_at_them

Joined May 15, 2010
766
So there's no actual voltage going through the capacitor?
Voltage doesn't go through anything. It's a potential from one point to another. When you apply a voltage across a capacitor a current flows until it is charged to the voltage potential being applied. If you do nothing else it just sits there and the LED won't light. But if you allow it to charge in one direction, and then allow it to charge in the other direction, you have current flowing back and forth. And, hence, the LED will light up.

There absolutely is current flowing through the capacitor. Because charge builds up on one plate and is pulled off the other, so we just think of it as flowing right through the capacitor.

Also, there absolutely is a voltage across the capacitor. It just varies as the charge builds up and falls.
 

vpoko

Joined Jan 5, 2012
267
So why do we find so many capacitor in electronics if it blocks DC I know in an audio circuit it would let the audio trough but most circuits have capacitors and the source is DC
As crutschow said, if you connect a capacitor in series with a DC power source, you'll prevent current from flowing (once the capacitor is fully charged). But capacitors aren't always used in series, often times they're used in parallel. For example, a capacitor connected between the 5V and GND pins of a microcontroller does not allow DC current to flow through it, and that's a good thing (else you'd be shorting 5V to ground). But since it allows alternating current through, any quick changes (spikes or dips) in the voltage will be allowed through and away from your load (the uC, in this case).
 

Thread Starter

electronewb

Joined Apr 24, 2012
260
Voltage doesn't go through anything. It's a potential from one point to another. When you apply a voltage across a capacitor a current flows until it is charged to the voltage potential being applied
I think I'm starting to understand the DC blocking Let's go a few step back!!! So basically what do the work or make a LED light up or a motor to work is the amperage and not the
voltage???
 

upand_at_them

Joined May 15, 2010
766
I think I'm starting to understand the DC blocking Let's go a few step back!!! So basically what do the work or make a LED light up or a motor to work is the amperage and not the
voltage???
An LED only lights if a current goes through it. It needs a minimum voltage across it, sure, but its brightness is determined by how much current flows through it.

It is the flow of current that's doing anything anywhere.
 

Thread Starter

electronewb

Joined Apr 24, 2012
260
I just did another test!!! I charged a cap at 8V from a DC battery Why when I measure the voltage across the 2 leads I get exactly 8V DC?
 
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