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### Course: Electrical engineering>Unit 2

Lesson 1: Circuit elements

# Sign convention for passive components

The sign convention for passive components defines what we mean by positive and negative voltage and current. Created by Willy McAllister.

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• At Sal says that capacitors and inductors do not amplify a signal. That got me confused 'cause I was thinking that was exactly the function of a capacitor: give a short boost to voltage. Can someone explain, please?
• Hello Fábio,

Perhaps you heard this in regards to after-market car audio. Here we will often find large (1 Farad or larger) capacitors installed at the DC power input to the power amplifiers. Their purpose is to "boost" the input to the amplifier.

Here "boost" is an inappropriate word. A better word is to "maintain" voltage. To understand why consider the music. The lowest music tones (thumping base) requires significant power. This power in turn requires significant current flow. This high current flow can cause the voltage to sag. When the input voltage sags the amplifier's output power also drops. We could calculate the voltage drop using Ohm's law... The important point is that the voltage sag is related to the resistance of the circuit and to the ability of the car's electrical system to maintain the voltage.

This is where the capacitor shines. You see, the capacitor has what we call a low equivalent series resistance. By installing the capacitor close to the amplifier the capacitor can provide the immediate current demands of the amplifier for a "thump" of the music. The capacitor then recharges between thumps. The results is less sag and more power output.

Notice that the capacitor did not amplify the signal. It only provided the audio amplifier a more constant voltage which allowed the amplifier to output a higher power.

Regards,

APD
• So which directions do the electrons flow?
• Hello Steven,

Electrons flow from from negative to positive.

I made these videos to help answer questions about voltage and current. The second video may help you visualize the situation but you'll need to watch the first video first...

Regards,

APD
• At , I think the current direction is only correct when the capacitor is charging, when it is discharging, the current direction turns the opposite. Am I right?
• But when you mention current "goes in on the positive end" you are referring to negative charges as current right? Because conventional current as we all have learnt in school flow AWAY from the positive terminal because we take conventional current as positive charges flowing.
• Using "conventional" current direction, current flows OUT of the positive terminal of power sources (batteries) .

When we place voltage and current labels on passive components (resistors, capacitors, inductors) we put the positive sign of the voltage label on the end where current comes IN.
(1 vote)
• At how would you call a battery or voltage source a passive component if current flows OUT of the positive terminal and INTO the negative terminal, opposite of what Willy is saying for other passive components?
• The term passive usually applies to R, L, C. We use the term active for transistors and amplifiers, anything that can make a signal bigger. Power supplies (voltage or current sources) usually don't fall into either category. They don't seem "passive" because they are adding energy, and they don't seem 'active' because they don't make signals bigger. In this video on labeling 2-terminal elements with voltage and current indicators, I've kind of lumped power sources in with R L C as far as labeling techniques.

If you take a look at the following article with the same name, it talks about labeling power sources. https://www.khanacademy.org/science/electrical-engineering/ee-circuit-analysis-topic/circuit-elements/a/ee-sign-convention
• I thought Capacitors are used in alternating current, so how can there be only a single direction? The terminals must keep flipping their position. This happens with all the components in Alternating current...right?
(1 vote)
• Hello Abhishek,

Correct, capacitors are used in both AC and DC circuits. In the AC system the capacitor is charging, discharging, charging with opposite polarity, and then discharging.

Here is an interesting fact for you. Every physical capacitor is rated for either AC or DC operation. Depending on their chemistry some capacitors may be used for both. In general AC rated capacitors can be used in DC. DC rated capacitors will self destruct, sometime spectacularly, when used in AC.

I made another video related to your question. It is focused on the inductor but the same argument holds for the capacitor. See:

Regards,

APD
• so the essence of this lecture is ; the sign convention for passive circuit elements is that whenever we label the current through a passive component we should enter the current through the positive terminal of that component?
(1 vote)
• That's exactly right. You use the convention both ways. If you label voltage first, put the current arrow in the + terminal. If you label current first, put the + voltage polarity on the side the current is entering.
• what is meaning of the term ' to amplify ' ?
(1 vote)
• "Amplify" means to make larger. Suppose you have an electronic circuit that accepts a voltage as its input, and produces an output voltage that is a copy of the input, except larger by a factor of 10. We would say that circuit "amplifies" the input signal, and we would call that circuit an "amplifier" with a "gain" of 10.

A passive component, (resistor, capacitor, or inductor), cannot on its own cause a voltage to increase. On the other hand, silicon devices called 'transistors" are able to do that.
• Some Definitional Questions:

If a Chip is composed of Passives, then is that Chip must be a Passive?

Is a Transistor a Passive?

Are OpAmps Passives and why?
(1 vote)
• Hello Clayton,

We could use the word "active" to identify devices that contain amplifiers or digital logic. Using this definition a computer chips, transistors, and op-amps are all active devices.

Resistors, capacitors, and inductors would all be passive. Most would also say a diode is a passive device although things get complex when we talk about GTOs, SCRs, and Triacs...