Resistors substitutions -how to apply Ohm's law in a PCB?

[Gledison]

Hello mates!
im struggling to find the right values for my resistors. in theory I know how to apply the ohm's law to get the right values (difficult math )
the problem is: how to connect two resistors in series or parallel in a given PCB? I mean, Im sure is possible, but it mess up a little bit the populated board.
I would like to ask you experts, what are the limits for the substitutions?
for example, if I need a 360k, can I use a 390 instead? I know we can play with the 5% accurancy, but that will give me a 378k, will 390k than be too much?
if I need a 20k, a 22k would be ok?
knowing this will save me some nerves...
thanks a lot in advance for any help!

[slacker775]

Just teepee two resistors to get the right value.   If you need 20k but only have 10ks, just connect two 10ks in series by putting one lead of eac in either of the holes and standing them up diagonally and connecting their second leads to each other.  I don't have a pic handy but it's pretty simple and works every time.  Works real well for obscure values.

[Thomas_H]

Depending on the use it is for you can vary a lot
For example:
resistors that bring vref to an opamp, They can be from 470k-2M2
If you are looking at a filter you need to be as close to the value written as you can or you will have a different cutoff frequency.

Serial resistors just add up, paralled can be calced here: http://www.sengpielaudio.com/calculator-paralresist.htm

[Gledison]

Just teepee two resistors to get the right value.   If you need 20k but only have 10ks, just connect two 10ks in series by putting one lead of eac in either of the holes and standing them up diagonally and connecting their second leads to each other.  I don't have a pic handy but it's pretty simple and works every time.  Works real well for obscure values.

thanks for your suggestion. I was thinking that this is the way to go, but never saw in any board. yeah a picture would be great. but I would try it anyways...thanks a lot. 

[Gledison]

Depending on the use it is for you can vary a lot
For example:
resistors that bring vref to an opamp, They can be from 470k-2M2
If you are looking at a filter you need to be as close to the value written as you can or you will have a different cutoff frequency.

Serial resistors just add up, paralled can be calced here: http://www.sengpielaudio.com/calculator-paralresist.htm

Hi, i can do the parallel and series calcs
when u say, "to be as close to the value written as you can  " .. example: i need a 360k, can i use a 390 instead? or a 20k, can i use a 22k ?
cheers

[RobA]

[...]
when u say, "to be as close to the value written as you can  " .. example: i need a 360k, can i use a 390 instead? or a 20k, can i use a 22k ?
cheers

It really depends on the situation and the circuit. Some things need to be pretty much exactly as specified and some can be hugely different and still work.

[Gledison]

[...]
when u say, "to be as close to the value written as you can  " .. example: i need a 360k, can i use a 390 instead? or a 20k, can i use a 22k ?
cheers

It really depends on the situation and the circuit. Some things need to be pretty much exactly as specified and some can be hugely different and still work.

basically im talking about a R3(360k) and R11(20k) for the sea urchin that are connected to the TL072P and PT2399 respectively.The other is a 33ohm before a voltage regulator 7805...
for R3 I was thinking to try a 390k and for R11,  a 22k...
I will build in a breadboard first ...

[Thomas_H]

R7- 33R can be 47R as well. You can also leave it off(jumper), its a low frequency filter with C2.
R3(360k)  sets the amplification of OpAmp1A, It will be a little louder if you put in a 390k. Change R2 to 200k as well to compensate for it. any combination where R3=2xR2 will do as long as R2>100k
R11(20k) is part of a filter and you will have a slight change in frequency if you change it to 22k. You may not notice it.

[Gledison]

R7- 33R can be 47R as well. You can also leave it off(jumper), its a low frequency filter with C2.
R3(360k)  sets the amplification of OpAmp1A, It will be a little louder if you put in a 390k. Change R2 to 200k as well to compensate for it. any combination where R3=2xR2 will do as long as R2>100k
R11(20k) is part of a filter and you will have a slight change in frequency if you change it to 22k. You may not notice it.

this is valuable info, thanks a lot!
looking to the schematis.., It would not be a prob to change the R2 to 200k if its directly contencte to the IC1?
cheers

[Thomas_H]

this is valuable info, thanks a lot!
looking to the schematis.., It would not be a prob to change the R2 to 200k if its directly contencte to the IC1?
cheers

Why should it? As written R3 and R2 belong together and define the amplification factor of OpAmp1.

[Gledison]

this is valuable info, thanks a lot!
looking to the schematis.., It would not be a prob to change the R2 to 200k if its directly contencte to the IC1?
cheers

Why should it? As written R3 and R2 belong together and define the amplification factor of OpAmp1.

you'r right. will try with those values.
cheers

[RobA]

One thing to note is that R2 and C1 also act as an HP filter and the corner frequency is 1/(2 pi R2 C1). In the case of moving R2 to 200k, this will be fine as it'll just move the frequency down a little bit. If you were to lower R2 though (and R3 respectively), you'd need to increase C1 to get back to the same corner frequency. One other thing about R2 is that it sets the input impedance for the effect. So, going up in value is good in this regard too, although higher values for R2 and R3 will introduce more noise as well, so going too high is also bad. But 200k, should be fine in all these considerations.

[ch1naski]

So, R2 is serving double duty in this circuit, partly helping to set the amplification factor of the Opamp, and also as the resistor in an RC FILTER?

Sent from my SPH-L710 using Tapatalk 4

[RobA]

Yeah. The input structure including the op amp is an active HP filter with gain in the pass band frequencies.