Two resistors in series

Question

I know the summation equation for two or more resistors in parallel or series, and I know two parallel resistors will give more power.

But sometimes I saw some circuits that used two resistors in series, and I am wondering why that method was used and why they didn't use one resistor with a higher value (equal to total series resistors)?

Such as the following circuit diagram, two 33 kΩ resistors used in series. So why doesn't it use one 68K resistor?

Give it better results? I mean, noise filtering or something else?

Note: This circuit is an AC dimmer for a microcontroller.

Answer 1

It's the voltage rating on the resistors that is important here. They are powered from rectified 230 V AC and they need to have the correct voltage rating to suit their application. Two resistors in series having an individual rating of 200 V gives a total voltage rating of 400 volts (near enough if you ignore tolerances on values).

Take a look at the good old MRS16 and MRS25 range from Vishay: -

With 230 V AC present, the peak could be as high as 325 volts without even considering line transients. Clearly two resistors should be used. And, for SMT resistors this might be useful to consider: -

Answer 2

Reasons someone might put two resistors in series in a volume design:

1. A bit higher power was needed than what the commonly stocked parts can handle.

   Let's say a company standardizes on using 0805 resistors unless there is a good reason not to. They therefore end up with many 0805 values in stock, with only a few values of other packages. Now you need a 200 mW resistor. You could specify a 1206, but overall it is better for the company to use two 0805 resistors that they are already buying and stocking anyway.

   I have done exactly this a number of times.

2. To spread out the power dissipation. Two resistors spaced a little apart will cause a lower max temperature than a single resistor dissipating the same power.

3. To get higher voltage capability. This is most likely the reason in the particular example you asked about.

4. To get lower series capacitance. This can be a useful trick in high frequency applications.

5. To be able to tweak a value. In this case one of the resistors accounts for most of the value, like 90%, and the other the remaining 10%. For low volume hand-tweaked products, the smaller resistor can be changed out for calibration. A fixed ratio change of the smaller resistor results in a smaller ratio change in the overall resistor, so this method allows tweaking resistor values with higher resolution than the standard parts are available in.

   To be fair though, this sort of calibration adjustment is usually done with a parallel resistor, not a series one.

Answer 3

There are conditions where you must use resistors that don't have enough voltage rating (typically because they are small like SMD, etc.) So you use two of them in series to get the voltage rating to operate safely.

Answer 4

The reason may be the power or voltage capability of the resistor or even cost. The schematic you show has two 33k resistors being fed from 300V peak (rectified 230V mains). They dissipate a bit less than 1W worst case (the lamp off).

You could use a single 66K 1W resistor (it would get quite hot) but two 33K 1W resistors would be cooler (larger dissipation surface area and PCB area for each resistor). You could also potentially lower cost by using 33k 0.5W resistors, which might be cheaper than a 66K 1W resistor

You also see this done where extremely high voltages are used (ie you see this in high voltage multimeter probes) where the individual breakdown voltage of a resistor might become a problem.

Answer 5

66K resistors aren't easy to get hold of. 33K ones are.

You can get 68K very easily (it's one of the "basic" resistor values - E6), or 62K (which is part of the E24 range). The closest in a standard range is 66.5K, which is in the E96 range. Generally more expensive and harder to find, since they are used less often.

So to get 66K it's easiest to use two readily available 33K resistors.

You can read more about the standard resistor ranges here.

Answer 6

Another reason for using two resistors in series is safety. A resistor can fail short or open. If one resistor fails it can cause a catastrophic failure. With two resistors one failure does not have to bring the whole design down.

Answer 7

1. Power dissipation would be the most common reason (it seems like in this case). You have two resistors - only half of power gets dissipated on each.

2. But apart from that - there might be some other strange/weird reasons that are linked to the easability of reworking on the PCB, engineer's preferences or in extreme cases - engineering laziness.

   • easability of reworking on the PCB: if you know that resistance required is at least 33k - you put 33k and another one in series; you may want to tweak the other resistor at some stage to fine-tune the current you need. It might be pretth handy once you get first field returns of your product. It is pretty handy to have more resistors stuffed and sometimes 0-ohm resistors in case you want to disconnect some circuitry.
   • engineering preferences: in some cases it might be linked to BOM limitations or savings; If you use a lot of 33k on all different boards produced - why would get one 68K; This happens especially when you need precise 66k 1%. You can put two 33k 1% and save some costs.
   • finally engineering laziness - you have 33K symbol handy in your schematic editor and don't bother about creating another 68K component.

Please note that this answer has to do with low-frequency signals and resistors placed close to each other. It is a different story when you have a transmission line and resistors at both ends acting as termination.

Answer 8

By using two resistors you gain three things in this circuit. At 230 VAC you get twice the breakdown voltage for the resistors and you can handle twice as much power loss in the resistors. If you want to do 120 VAC you simply jumper one of the resistors.