# Blocking capacitor

#### longhaircivy

Joined Sep 9, 2015
3
Placing blocking capacitor in series with load ,blocks DC, pass AC,
If you add a larger uF Capacitor, why does or how does AC voltage drop go down?

#### MrAl

Joined Jun 17, 2014
8,474
Because impedance goes down.

#### Audioguru again

Joined Oct 21, 2019
3,508
A larger capacitance value loads down the signal source causing its voltage to drop.
What is the output impedance of the signal source?
What is the impedance of the load?
What is the frequency?
The things above plus the capacitor value determine how much the voltage drops across the load.

#### BobTPH

Joined Jun 5, 2013
3,654
A larger capacitance value loads down the signal source causing its voltage to drop.
This would be true if the capacitor was in parallel with the load. But the OP is talking about a capacitor in series with the load. And he asked why the voltage drop goes down, not up.

The correct answer was in post #2.

Bob

#### crutschow

Joined Mar 14, 2008
27,701
The impedance of a capacitor is 1/(2pi*f*C) where f is the signal frequency and C is the capacitance, so the impedance is inversely proportional to capacitor size and frequency (impedance goes down as either of those are increased).
The frequency at which this impedance equals the equivalent resistance in series with the capacitor is called the corner frequency, where the response drops by 3dB.
Typically you want this frequency to be below the lowest frequency of interest for the circuit.

#### BobaMosfet

Joined Jul 1, 2009
1,848
Placing blocking capacitor in series with load ,blocks DC, pass AC,
If you add a larger uF Capacitor, why does or how does AC voltage drop go down?
Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3

#### MrChips

Joined Oct 2, 2009
24,160
Placing blocking capacitor in series with load ,blocks DC, pass AC,
If you add a larger uF Capacitor, why does or how does AC voltage drop go down?
That is a bit of a blind statement.
You cannot determine the effect of a capacitor until you see the entire circuit.
What is the output impedance of the driver?
What is the input impedance of the receiver?

#### MisterBill2

Joined Jan 23, 2018
9,237
Placing blocking capacitor in series with load ,blocks DC, pass AC,
If you add a larger uF Capacitor, why does or how does AC voltage drop go down?
It is not clear if you are asking about the voltage drop across the capacitor or across the load, whatever it is. Or ar you asking about the voltage at the source of the AC signal. And that matters a whole lot. So ket us know which voltage and if it is AC or AC + DC. Then you can get an answer that is not a guess.

#### Audioguru again

Joined Oct 21, 2019
3,508
An 8 ohm tweeter might have a 10uF coupling capacitor to cut frequencies below 2kHz but if you connect a capacitor that is 100uF and feed 200Hz then the current in the tweeter will be 10 times too high and the voltage across the tweeter will also be too high.

#### MisterBill2

Joined Jan 23, 2018
9,237
Placing blocking capacitor in series with load ,blocks DC, pass AC,
If you add a larger uF Capacitor, why does or how does AC voltage drop go down?
WHICH VOLTAGE!!!! There are three voltages here, the voltage of the source, whuich evidently also has some DC component, the voltage across the capacitor connected between the load and the source, and the voltage across the load, whatever that might be. So without more explanation of which the TS is asking about there is no reason for folks to be guessing.

#### longhaircivy

Joined Sep 9, 2015
3

#### longhaircivy

Joined Sep 9, 2015
3
WHICH VOLTAGE!!!! There are three voltages here, the voltage of the source, whuich evidently also has some DC component, the voltage across the capacitor connected between the load and the source, and the voltage across the load, whatever that might be. So without more explanation of which the TS is asking about there is no reason for folks to be guessing.
Voltage drop across capacitor

#### Audioguru again

Joined Oct 21, 2019
3,508
A larger capacitance passes an AC frequency better than a smaller capacitance.
At 100Hz, a 10uF capacitor impedance is 160 ohms. 1 divided by (2 x pi x f x C).
At 100Hz, a 100uF capacitor impedance is 16 ohms so of course its AC voltage drop is less.

#### vu2nan

Joined Sep 11, 2014
179
The TS is most probably referring to AC mains 'voltage dropping' capacitors as in the compact mains charging circuits of rechargeable battery-operated flashlights and shavers or as in LED 'mains on' indicators.

Regards,

Nandu.

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#### Audioguru again

Joined Oct 21, 2019
3,508
With no schematic we should stop guessing about his circuit.

#### MrAl

Joined Jun 17, 2014
8,474
Hello,

Quotes:
"Placing blocking capacitor in series with load ,blocks DC, pass AC,"
"If you add a larger uF Capacitor, why does or how does AC voltage drop go down?"

"Voltage drop across capacitor"

We all know what a DC blocking capacitor is and what it has to do and what main concern we have when selecting one and that main concern is how well it transfers a signal from one stage to the next. WE all know that the lower the cap value the better the lower frequency response is, and that is because the capacitor drops less voltage itself.

We could look at some source and load impedances, but we all know that for a DC coupled circuit if we dont upset the DC bias point too much a lower value resistor passes more current than a higher value resistor. We could look at some examples too.

#### MisterBill2

Joined Jan 23, 2018
9,237
The TS is most probably referring to AC mains 'voltage dropping' capacitors as in the compact mains charging circuits of rechargeable battery-operated flashlights and shavers or as in LED 'mains on' indicators.

Regards,

Nandu.
There is a very serious hazard associated with using a capacitor to drop the mains voltage, which is that if an alternative power source is used instead of the regular utility supplied power, there will probably be a lot more harmonics present, and hence more harmonic energy will pass through the capacitor. So one person tried to use one of those "Kill-O-Watt" analyzers on a portable generator output and immediately the "Kill-O-Watt" was destroyed. The current delivered by the capacitor was too much for the zener diode shunt regulator and so that device failed open.

#### MrChips

Joined Oct 2, 2009
24,160
If you are attempting to reduce AC mains voltage with a capacitor, we cannot go there.
This is a prohibited topic on AAC forums.

#### vu2nan

Joined Sep 11, 2014
179
There is a very serious hazard associated with using a capacitor to drop the mains voltage, which is that if an alternative power source is used instead of the regular utility supplied power, there will probably be a lot more harmonics present, and hence more harmonic energy will pass through the capacitor. So one person tried to use one of those "Kill-O-Watt" analyzers on a portable generator output and immediately the "Kill-O-Watt" was destroyed. The current delivered by the capacitor was too much for the zener diode shunt regulator and so that device failed open.
Hi MisterBill2,

I agree with you and also do not advocate the concept, which is outdated. But the thread starter's reference to 'capacitor' and 'AC voltage drop' brought back memories of those transformerless rechargeable flashlights and electric shavers of the bygone days of my youth. Robert J. Tolmie was the inventor and Sperry Corp was granted the patent (US3539898A) 'CHARGING MEANS FOR ELECTRICAL APPLIANCE' nearly 50 years ago.

I also came across a case of rectifier failure, caused by harmonics, 25 years ago.

An acquaintance, who had built a piece of equipment for export to Europe (designed for 3 x 380 V~50 Hz mains supply), could not wait for the delivery of an autotransformer he had ordered and went ahead with the testing (supply voltage over here is 3 x 415 V~50 Hz).

He planned to use a readily available 3 phase inverter instead, with its output voltage set to 380 V. I dissuaded him from doing that, anticipating an underloaded inverter generating harmonics. He went ahead in spite of that, and, after a couple of days, the electronic timers in the equipment failed.

By the time the timers were replaced the autotransformer had also arrived and the equipment was duly tested and cleared for dispatch.

The non-functional timers, which were sent to the manufacturer for repairs, were received later with the test report attributing the failure of their rectifiers to harmonics in the applied voltage.

Many thanks.

Regards,

Nandu.

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