Obtaining a specific resistance

Question

first of all, let me apologise for my inexperience with electronics.

Now that's over with, I'm trying to follow a guide that specifies the use of a specific temperature sensor with an output of 5v DC. The problem is that the sensor I currently have provides different values and that a resistance of 14.2k ohms is required. The problem I have is that this step provides no further information. The actual step is as follows: -

The outlined box above, which currently is set to 14.2, represents the resistance in to be added in parallel

I'd like to know how to obtain this specific resistance to place between the sensor and the controller that reads this value. I assumed I could build the resistance in a series.

I am under the impression that resistors can be placed in series, i.e. I could add a 10k ohm resistor and a 4k ohm resistor and then a 0.2k ohm resistor. Forgive my ignorance here, please, if this is completely idiotic. So I went to http://www.crownaudio.com/ohms-law.htm and entered 14200 and 5 into the respective fields and it specified that the watts was '0.00176'

Does this mean that when shopping for resistors, I can buy: -

http://www.maplin.co.uk/p/metal-film-06w-22k-ohm-resistor-m2k2

2.2k - 0.6w

http://www.maplin.co.uk/p/metal-film-06w-10k-ohm-resistor-m10k

10k - 0.6w

http://www.maplin.co.uk/p/metal-film-06w-2k-ohm-resistor-m2k

2k - 0.6w

I assume I need the same wattage for all but as above, adding all the resistances in series will create 14.2k ohms but they look so flimsy.

Thank you!

Answer 1

You're off to a good start. Yes, you can buy three resistors and wire them in series (not parallel!), although the first resistor you link to is a 22k, rather than a 2.2k, and if you connect the three resistors you've chosen, you'll get 34 k, rather than 14.2 k.

Despite the fact that they seem flimsy, the units you show will handle at least 100 times the power you expect to dissipate. After all, your calculator (correctly) showed that the desired resistor will need to handle .0018 watts, and each of your examples will handle 0.6 watts.

Finally, you really ought to be able to find a single 14k resistor to do the job, and you can forget building up combinations. Use this link http://www.daycounter.com/Calculators/Standard-Resistor-Value-Calculator2.phtml to determine that you can use a 14.3 k, 1% resistor and save yourself some effort.

And finally, finally, I'd recommend you stay away from Maplin. They have no search ability whatsoever, and this will make your attempts at finding appropriate parts a misery. As an alternative, try Digikey. I'm sure there are other suppliers in the UK as well, but that is one of the convenient go-to sources for parts in the US, and their website indicates they have facilities in the UK.

Answer 2

First of all, don't put the resistors in parallel, put them in series.

$$R_p = \frac{1}{\dfrac{1}{R_1} + \dfrac{1}{R_2} + \; ... + \dfrac{1}{R_n}}$$

Where \$R_p\$ is the equivalent resistance of the individual resistors in parallel, and \$R_1\$, \$R_2\$, ..., \$R_n\$ are individual resistors.

If you put them in series then they just add up:

$$R_s = R_1 + R_2 + \; ... + R_n$$

The latter is what you want.

Another thing, you can use two E12 resistors not three: 12K + 2K2 = 14.2K

You've already worked out that 5V across 14.2K results in trivial power consumption, so don't spend money on high-wattage parts. 1/4W resistors would be fine. Also, if you have to get resistors with different wattages then that would also be fine. There are circumstances where that wattage rating might matter, but this isn't one of them.

Answer 3

You can also put the resistors in parallel, but resistors in parallel have a lower total resistance than the individual resistors. The formula is \$1/R_t=1/R_1+1/R_2\$. A parallel combination of 15k and 270k will be 14.21k. The math is harder, but it works just as well. The power rating only needs to exceed the requirement, not match it, so 1/4 watt resistors will work fine (and be very cheap).

Those resistors you linked are all 1% tolerance, so you shouldn't try to get closer than that by just using values. You likely don't need to be that accurate, but if you need to you will have to buy precision resistors or measure the ones you get and adjust the series/parallel combination accordingly.