It is called the "brother in law effect". The design engineer has a brother in law that sells resistors and he (the design engineer) doesn't want his brother in law and sister to move in with him.

 

It is called the "brother in law effect". The design engineer has a brother in law that sells resistors and he (the design engineer) doesn't want his brother in law and sister to move in with him.

Sounds funny but in this age of socialogical bias i would not be surprised.

 

I wonder if there a use for the 3.3v and this is the easiest way to get a reference that’s dependent on VCC. Not sure why they used 0805 that takes a lot of room. They should be close enough from same batch/reel to give pretty close divisions.

 

Honestly I don't know the answer to why they used six resistors. My thought is that it could be they wanted a very specific resistance. Since resistors typically come in preset values, getting a resistance of a very specific value may require some series resistors, or series / parallel resistors or even just parallel resistors.

Pick any resistance you want and each of the tolerance series will have a standard value that is within the tolerance of that resistance (there may be a tiny gap here or there, I haven't checked all of the sequences for that). Using combinations to get a non-standard value is of limited utility.

As an example, let's say that you use the 5% series (the E24 values and you want 1.62 kΩ. There is a standard value that is 1.6 kΩ which means it's value can be anywhere from 1.62 kΩ to 1.68 kΩ. Using six 270 Ω resistors in series may get you a nominal value of 1.62 kΩ, but since each one can be between 256.5 Ω and 283.5 Ω, the total can be between 1.539 kΩ and 1.710 kΩ. Now, IF you use resistors that are from completely independent sources (not only from different lots, but from different machines altogether), then using lots of resistors will tend to average out the uncertainties. But six resistors isn't enough to have a huge benefit in that regard.

I will say this: I'd like to see a picture of the board. Better yet, a schematic. There could be inner layer connections that may be using the resistors as a divider network. But for the moment it's all guesswork without further information.

That was one of my first thoughts. But the TS piecemealed out quite a bit of pretty specific information so I'm doubting they would have missed that.[/QUOTE]

 

Without seeing a schematic:
1) max voltage if the capacitor failed. That may be the design goal.
2) power dissipation
3) maybe the company uses that resistor in other products.

Not likely
4) lower inductance. It is a reason to use multiple resistors instead of 1.

 

Without seeing a schematic:
1) max voltage if the capacitor failed. That may be the design goal.
2) power dissipation
3) maybe the company uses that resistor in other products.

Not likely
4) lower inductance. It is a reason to use multiple resistors instead of 1.

"4) lower inductance. It is a reason to use multiple resistors instead of 1."
Parallel reduces inductance series increases inductance.

 

Might depend on the value of the part too. There was some Tek made high frequency equipment that used 3 resistors in series rather than a single resistor. A single resistor would not work. Don't know if these were thru hole or SMT.

But for giggles read https://ims-resistors.com/wp-content/uploads/2017/03/whitepaper-PassiveSMT.pdf
Mounting an SMT resistor on it's edge can change it's parasitics.

 

@WBahn I did say "Honestly I don't know the answer". My expertise ends at "Passive Hobbiest".

 

Perhaps this series load bank protects something in a certain failure or condition? Just guessing since we need more information...

 

Other reason is power, if say 22 kOhm dissipates about 2...3 W then smd mainly have 0,0625W or 0,125W. If there were small voltage, one may have a choice betw parallel and serial connection, but in this case there are no choice, and seems that 6 pieces are satisfacting as voltage as power demands simultaneusly.

the voltage across each 270Ohm resistor is 3.3v as I've measured, you can calculate and find the result that shows 3 times of the needed wattage they are using.

 

Perhaps the multiple resistors enable tolerances to be 'averaged out', if a very specific resistance is required, rather than employ a more expensive close tolerance resistor.

No, the tolerance isn't that much important.

 

It is called the "brother in law effect". The design engineer has a brother in law that sells resistors and he (the design engineer) doesn't want his brother in law and sister to move in with him.

I think so

 

Thanks all for your help, I just want to explain somethings:
1- the application isn't that much of sensitivity, as they haven't put that amount of resistors to adjust the equivalent's value.
2- I support the idea of that they are using them to minimize the amount of used values in the company, and also to avoid the unexpected high voltages that may be caused because of the small size of the PCB, so they increase the length of the surface of the equivalent resistor.
3- The PCB is 2-layer and this network is just used as a load not for any other purpose of dividing or make some different references.
again, thanks for help <3

 

Might depend on the value of the part too. There was some Tek made high frequency equipment that used 3 resistors in series rather than a single resistor. A single resistor would not work. Don't know if these were thru hole or SMT.

But for giggles read https://ims-resistors.com/wp-content/uploads/2017/03/whitepaper-PassiveSMT.pdf
Mounting an SMT resistor on it's edge can change it's parasitics.

Yes interesting thanks.
So maybe they tried to reduce total capacitance. Caps in series reduces total capacitance.

The concensus now seems to be for reduced stock though.

 

Hi everyone,
I'm studying a product , when I opened it I found a set of 6 SMD resistors connected in series and want to know why to add 6 instead of 1 or 2.
the circuit is a delay-on timer, it uses 220VAC mains voltage, the circuit uses a capacitive divider to decrease the voltage to 20V, the circuit contains a relay; so to avoid the increasing of the voltage they put 6 resistors as a load when the relay is off so the same current flow and the same voltage will still.
when dividing the 20V by (6*270ohm) resistors we get 3.33V across each resistor and the dissipation in each resistor is about 0.04W, but here they uses 0.125W SMD resistors.
1- Sorry because I haven't the full schematic of the circuit.
2- why do they use 6 resistors instead of 1 or 2 equivalent ? (I know when dealing with high voltages they prefer to connect more than one resistor to avoid humidity of any thing that may destroy the circuit, but why 6 !!!)
3- why the resistors size is 3 times the required size? (SMD resistor with 3.2mm length, 1.6mm width).

Thanks.

Probably for safety. Less voltage and current through each, as well as 6 failure points instead of 1 or 2. Plus, it could be that the parts were cheaper, or that's what they had in stock or overstock of.

 

the voltage across each 270Ohm resistor is 3.3v as I've measured, you can calculate and find the result that shows 3 times of the needed wattage they are using.

A common design rule is to use parts rated for at least twice the power that they are anticipated to actually have to dissipate, so 3x isn't unreasonable at all. For low power applications the driving factor is often the footprint or stock/availability issues and you commonly see components used that are rated for orders of magnitude more power than they will ever actually dissipate because power simply wasn't a consideration in the component selection process.

 

Probably for safety. Less voltage and current through each, as well as 6 failure points instead of 1 or 2. Plus, it could be that the parts were cheaper, or that's what they had in stock or overstock of.

How does having six resistors in series result in less current through each?

How is having six potential points of failure better than having one or two? If ANY of the six resistors fails, that part of the circuit has failed. Unless the reduced voltage across each resistor is going to drastically reduce the probability of any one of them failing, that isn't going to be a good strategy.

Under normal operating conditions there is only 20 V total across the string. But since it uses a transformerless power supply it's possible that the six in series was done with the thought of dealing with the situation of the mains voltage getting applied directly to the string. But if the mains voltage is 240 Vrms, that would put something on the order of 5 W in each resistor, so it wasn't designed to tolerate it for long -- perhaps long enough for some other part of the circuit (fuse?) to react?

 

Reliability is based a lot on the number of connections. Six two-terminal components vs one results in 5 more connections therefore lower reliability.

However, the decrease may be very small if there are already a large number of connections.
For three examples all other factors equal:
5 connections to start, 5 new ones added, reliability goes down a lot.
500 connections to start, 5 new ones added, reliability goes down only a little bit more probably insignificant.
5000 connections to start, 5 new ones added, reliability goes down only a little bit more very insignificant.

 

Actually, just this past FridayI was modifying a board that had three phases (A, B & C). Each had four resistors in series. I don't recall the value (maybe 4701) and did not have access to a schematic, but being a multi-layer board I couldn't tell if there were inner layer connections or not. It was the first time I've ever seen such a thing. The board operated with a 240 VAC. I don't know what the board did or controlled, I only solder the things they want me to. Lots of double stacking 4990's and 2001's (parallel). Then again, it's not my place to know what exactly these boards do. I can guess. But that's all it is.

 

How does having six resistors in series result in less current through each?

How is having six potential points of failure better than having one or two? If ANY of the six resistors fails, that part of the circuit has failed. Unless the reduced voltage across each resistor is going to drastically reduce the probability of any one of them failing, that isn't going to be a good strategy.

Under normal operating conditions there is only 20 V total across the string. But since it uses a transformerless power supply it's possible that the six in series was done with the thought of dealing with the situation of the mains voltage getting applied directly to the string. But if the mains voltage is 240 Vrms, that would put something on the order of 5 W in each resistor, so it wasn't designed to tolerate it for long -- perhaps long enough for some other part of the circuit (fuse?) to react?

I beg your pardon- series, same current. As to why 6, anybody's guess. Without seeing the actual PCB and circuit, our speculation could be endless.