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Electronic power and resistance rules

Electronic power and resistance rules

Anon_LG

Jun 24, 2014
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Lavaguava submitted a new resource:

Electronic equations and rules - Basic equations vital to electronics

CURRENTLY UNDER DEVELOPMENT

Introduction:

There are several equations needed for electronics circuits, used for calculating resistance, capacitance, inductance etc.

1. Ohms law, resistance, voltage, and amperage

This is probably the most important equation in electronics. It states that:

I=V/R

the equation is also re-arrangeable to give

V=I*R
and
R=V/I

The reason this equation is so important is that it can be used to calculate current (amps...

Read more about this resource...
 

(*steve*)

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Hi, I've approved this at the moment to make it easier to discuss.

Here are some comments:

There are several equations needed for electronics circuits, used for calculating resistance, capacitance, inductance etc.

I'd probably just stick to resistance. It's enough for its own resource. You can always do another one on capacitance, etc.

The reason this equation is so important is that it can be used to calculate current (amps or miliamps)

Be very careful. I wouldn't mention milliamps. It is something that can throw people. This is for beginners. Tell them amps, volts, and ohms.

, potential difference (volts)

I see what you're doing here, but it's better to say "potential difference (measured in volts)", otherwise people could think that potential difference and volts are the same thing (one is the phenomina, the other the unit of measurement). Same goes for current and resistance.

The hydraulic analogy is that the voltage between 2 points is the pressure difference (water flows from high pressure to low pressure areas,

I wouldn't put the analogy here. You've kinda assumed they know what this stuff is. If you want to explain it more basically, start another resource for that. Then you can put a link from this one to that one. It will help keep your resource simpler. Also note that other people have different analogies, and we don't want a fight about analogies :)

this was assumed to be the same with electric current, until it was discovered that electrons have negative charge and flow from areas of low potential difference to areas of high potential difference)

This needs rewording (and again, put it in another resource). Potential difference is something that exists between two points. High and low in this context refer to the amount of difference (the absolute magnitude of the voltage) not the sign of the charges.


Normally we'd put is as

P = V * I​

P for Power measured in Watts

And this is a perfect time to introduce V = I * R so you get P = I * R * I = I²R.

The other options can be left for later. I²R is a really important one though.

The difference between 2 points and the current flowing between these2 points gives the power in watts. Power and energy are different things but there is an equation that links them:

"The potential difference", or "the voltage between", but not just "The difference".


J = Ws (note lower case s for seconds)​

But here you've changed from specifying the thing to the units the thing is measured in.

P = E/t or E = P * t

E: Energy in joules (J)
P: Power in watts (W)
t: Time in seconds (s)​

Upper-case S is siemens or 1/Ω, a measurement of conductance.

There are two calculations needed for working out total resistance, one for resistors in series and one for resistors in parallel. It is important to know these as shops will not sell you a 258 ohm resistor or a 5 ohm resistor.

I understand what you mean, but I can point you to precision resistors that you can order as 258 ohms, should you desire one. Also, 258 is in fact an E192 value.

Here is a list of 258 ohm resistors available from one supplier :) (and here's a list of 5 ohm resistors)

The best reason is to say that you need a value that you don't have on hand, or which is difficult to obtain. If you want to get into preferred values, perhaps that's another resource too because it applies to many, many things.

1/(1/R1)+(1/R2)+(1/R3)+(1/R4)... (again carry on for how many resistors there are used) =Rtotal

the typical form of this equation is

1/Rtot = 1/R1 + 1/R2 + 1/R3 + ... + 1/Rn

There are good reasons for showing it that way.

Also note that if you use conductance (conductance is 1/resistance -- G = 1/R: G in S, R in Ω) then resistors in parallel can be simply calculated using

Gtot = G1 + G2 + G3 + ... + Gn

That might be a more advanced topic, but it does draw in the "S" that I previously talked about.

It's also useful to give a few heuristics:

  1. The total resistance in series is always larger that the largest individual resistance (trivial, but leads to...)
  2. The resistance in parallel is always less than the smallest individual resistance.
  3. Identical resistances in series can be multiplied (five 10Ω resistors in series have a resistance of 5 * 10Ω) -- again trivial, but leads to the more useful...
  4. Identical resistors in parallel can be "divided" -- five 10Ω resistors in parallel = (10/5)Ω

Resistors can be used in combinations of parallel and series to give any resistor value you could want (or if you had unlimited any value at all).

You need to remember resistor tolerance (and a plethora of other things that may be best left to a more advanced tutorial on resistors themselves).

4. Capacitance

5. How power equations can be used in motors

Take it from me, don't bite off too much. I've learned this with simple tutorials on LEDs, and now heatsinks. :D
 

Arouse1973

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Looking good Lavaguava.

Thanks Steve I have nothing to post now, only this :) Excellent comments BTW.
Adam
 

Anon_LG

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Thanks for the edits and all the advice, I will continue updating shortly.
 
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KrisBlueNZ

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I always refer to the circuit common rail as the 0V ("zero volt") rail, not "earth" or "ground"; these have other specific meanings. Using the name "0V rail" also makes it clear that it is the default reference rail for voltages measured "at" specific points in a circuit (rather than between two specific named points).

I think it is important to explain that voltages are always measured BETWEEN two points (the 0V rail being the implied second point, if none is mentioned), and current is measured THROUGH a path.
 

Arouse1973

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I always refer to the circuit common rail as the 0V ("zero volt") rail, not "earth" or "ground"; these have other specific meanings. Using the name "0V rail" also makes it clear that it is the default reference rail for voltages measured "at" specific points in a circuit (rather than between two specific named points).

I think it is important to explain that voltages are always measured BETWEEN two points (the 0V rail being the implied second point, if none is mentioned), and current is measured THROUGH a path.

Yes I second that Kris.
 

(*steve*)

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On a phone its hard to see the details, but it appears you show that E = W.

That's fine, but it is also superfluous. :)
 

Anon_LG

Jun 24, 2014
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This diagram however contains rules not discussed in this resource, such as inductance and capacitance. The diagrams are good but I would not add them, as Steve said, superfluous. Maybe suitable fro a future resource that explains these rules and equations in depth for a more advanced electronicist, this resource is for beginners.
 

(*steve*)

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They also don't lead directly to the more useful forms of I²R and V²/R. To add them as "triangles" is counterproductive.

The important thing about these so-called triangles is that they are mnemonic. The V = IR triangle allows you to remember ohms law equations and their various manipulations without having to consider actual math. Indeed, I have that one firmly in my head -- it was the form of one of the first equations I learned.

However to extend this to a plethora of other similar forms tends to destroy any mnemonic benefit if you see them all at once -- and especially if it's your introduction.
 
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