# Beginning Electronics Rtotal

#### absofbeer

Apr 26, 2011
9
New to Electronics... Trying to find Total resistance of two 4700 ohm resistors in paraellel?

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#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,508
Diodes do not have a constant resistance. The question cannot be sensibly answered.

But resistors do.

For resistors in parallel you want the reciprocal of the sum of reciprocal values, so you want:

1/ (1/R1 + 1/R2))

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#### absofbeer

Apr 26, 2011
9
OK.. Thanks Steve.. Now I am really confused... I'm trying to learn about Electronics from this Forrest Mims book... Getting started in Electronics

On page 30 it tells me to get the Total resistance of two resistors. Multiply the two (4700 x 4700) = 22,090,000 then devide by the sum of the two (4700 + 4700 ) = 9400

22,090,000 devided by 9400 = 2350 ohms

What I'm after is which has more resistance resistors in series or resistors in parallel?

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Apr 26, 2011
9

#### absofbeer

Apr 26, 2011
9
And I meant RESISTORS... Not thinking right here...

#### davenn

Moderator
Sep 5, 2009
14,150
your maths above is correct for getting the value of 2 resistors in parallel

for resistors in series simply add them together

Capacitors are opposite...
when in parallel add the values, when 2 in series use the way you did with the parallel resistors
C1 x C2 / C1 + C2

Dave

#### absofbeer

Apr 26, 2011
9
OK Thanks... I haven't got to the Capacitor section yet... I think I read somewhere that they are like rechargeable batteries....

I guess I would like to know , Why is the resistance knocked down to 1/4 of the total sum of resistance of the two Resistors when they are wired in Parallel ? ...Two 4700 ohm resistors... wired Parallel = 2350 ohms resistance.... Wired in Series = 9400 ohms...

#### davenn

Moderator
Sep 5, 2009
14,150
OK Thanks... I haven't got to the Capacitor section yet... I think I read somewhere that they are like rechargeable batteries....

only in the very broadest of senses ... yes you can charge up a large value of capacitor and it will hold a charge for some time .... but not as long as a battery... internal leakage is greater for a capacitor than a battery, as a result it will self discharge quicker.

you will learn in your reading to come that capacitors have wide and varied uses including....

smoothing and bypassing of small AC voltages to ground on a DC line,
DC blocking when used in series in an AC circuit,
together with inductors to produce resonant circuits for filters and oscillators in audio and radio frequency (RF) circuits.

I guess I would like to know , Why is the resistance knocked down to 1/4 of the total sum of resistance of the two Resistors when they are wired in Parallel ? ...Two 4700 ohm resistors... wired Parallel = 2350 ohms resistance.... Wired in Series = 9400 ohms...

well for 2 resistors of the same value the resulting parallel value with be 1/2 of either of the resistors

do a few sums where the values of the 2 resistors are different and see the results you get when they are in parallel

have fun learning ask questions, there's lots of knowledgeable guys on here with wide and varied backgrounds in electronics. I'm an audio and RF technician.
My digital electronics knowledge is very limited

cheers
Dave

#### absofbeer

Apr 26, 2011
9
Oh.. I have know idea what I'm doing... I just started to try and pick this up so I can learn to use a Digital Multimeter and work on some pinball machines.. I can do basic repairs, but would like to learn how the MPU board and PCB etc work.. Also like to be able to bring a machine back from the "Dead" if you know what I mean.... I might be getting in over my head with all these signals.. waves... microfards... Boy oh Boy...

#### davenn

Moderator
Sep 5, 2009
14,150
Oh.. I have know idea what I'm doing... I just started to try and pick this up so I can learn to use a Digital Multimeter and work on some pinball machines.. I can do basic repairs, but would like to learn how the MPU board and PCB etc work.. Also like to be able to bring a machine back from the "Dead" if you know what I mean.... I might be getting in over my head with all these signals.. waves... microfards... Boy oh Boy...

well just keep in mind ... its not something you are going to learn "over night" it is going to take time and dedication. I have been playing with electronics since I was ~ 7 yrs old am now 50++
BUT you do really need to understand the principles and get a solid foundation in them.
Ohms law etc. I dont know what country you are in, maybe there are some nite tech courses you can do that would be a great introduction and would have practical hands on experiments.

cheers
Dave

#### absofbeer

Apr 26, 2011
9
Yea, I was hoping to just jump into it and pick it right up... It is definitely going to take a while.. I'm in the U.S. .... I'm 40... and have been a brick-layer my whole life.. This is all very new to me... I think a class would help me out also... I'm more of an On-The-Job kind of guy... I see what I can pick up, but might have to take a class..

Thanks, Bill

#### davenn

Moderator
Sep 5, 2009
14,150
hi Bill,

no probs, nice to meet you. I have been to the USA a couple of times 1 x 3 month stint and a 5 week session mainly storm chasing in the midwest. been through most of the states west of a line from Ohio sth to the gulf. missed Nth Dakota and Wisconsin and 1 or 2 of others

OK, failing the availability of a good tech teaching class at a college etc, you should at least purchase some basic kitsets. There are some that you follow a project book and can do some 20+ projects with the components supplied. really awesome practical practice

check out tandy or radio shack type hobby electronics stores

Dave

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#### alfa88

Dec 1, 2010
349
In Lieu of School

In lieu of going to school get one of those 101 project things at Radio Shack or EBay. Go through the course. Build some kits and if you're lucky they wont work and. You can cut your teeth on troubleshooting them. There are plenty of people here to hold your hands. I used to feel like such a failure when one of my projects wouldn't work but I came to realize that getting them to work is where your troubleshooting skills can get honed.

Apr 26, 2011
9
Davenn and Alfa88,, I did just that Radio Shack over 200 projects ... Electronics Learning Lab... $75.... Comes with two work books..Basic Electronics book 1 Digital Logic Projects book 2... Let's see where this gets me... I have the hardest time remembering all the equations of Ohm's law #### davenn Moderator Sep 5, 2009 14,150 Davenn and Alfa88,, I did just that Radio Shack over 200 projects ... Electronics Learning Lab...$75.... Comes with two work books..Basic Electronics book 1 Digital Logic Projects book 2... Let's see where this gets me... I have the hardest time remembering all the equations of Ohm's law

easiest way are these 2 triangles......

Volts on the top of one current and resistance below it
Watts ( Power) on the top of the 2nd one Volts and Current below it

hide the one you are looking for and do the math on the other 2 eg
hide V and you have I x R; hide I and you have V / R etc

Dave

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#### absofbeer

Apr 26, 2011
9
Thanks Dave,

I just read about the Ohm's triangle last night... I haven't seen the power one.. Thanks

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