Circuit design conundrum - Page 1
Forum section: electronics / projects

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: March 09, 2017, 06:55:54 pm
content: Hi, I took apart my multimeter (DT830B) and it had several missing components from the board. On one part of the silk screen it said PTC and below it was a resistor. The PTC was not there. It appears the missing part and the resistor were in wired in paraell. Why does it work OK.My question is if I was to put in the PTC without changing anything else would it still work? Shouldn't the resistor be removed to use the function of the PTC?

author: w2aew
author_rank: Super Contributor
timestamp: March 09, 2017, 07:24:29 pm
content: Quote from: joseph nicholas on March 09, 2017, 06:55:54 pmSometimes manufacturers design options into the PCB so that it can be used with different end product models. Thus, it is common to see missing parts, or things that sometimes deviate from the silk screen. By doing this, they can save money by reusing boards for several products or product options.

author: DaJMasta
author_rank: Super Contributor
timestamp: March 09, 2017, 11:01:24 pm
content: There's also the chance that when the final board was made, it was determined that the more expensive part wasn't needed to meet the spec, so the dropped it from the BoM without respinning the already working board.I would not populate the part unless you know how it's working in the design, are certain it won't effect calibration (or if you can set that yourself), and you know it will actually improve some aspect of performance you deem important.

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: March 09, 2017, 11:15:41 pm
content: So it looks like there is no simple explaination. I've come across alot of circuits that look like they are designed to work with or without the extra parts. It sounds counter intutive to think that the extra parts can't be added without changing anything else. I have seen this in my multimeter circuit boards. For example, I see that the main fuse for voltage measurements have the fuse footprint in place, but these is no fuse present. I can't see anything added to close the circuit on the footprint, but the circuit still works.

author: lapm
author_rank: Frequent Contributor
timestamp: March 10, 2017, 03:42:59 pm
content: Quote from: joseph nicholas on March 09, 2017, 11:15:41 pmIts not that you cant add those missing components, but what components and values and how do they effect rest of circuit working?Its not that unheard of manufacturer design multimeter with all bells and whistles and then spin budject model by leaving few functions out and saving components costs... No need to respin pcb for that..Withour knowing exactly what components were left out, i would not recoment trying blindly...

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: March 10, 2017, 03:57:42 pm
content: If that was what is happening, then the manufacturer would have to put in some thing like a jumper wires to keep the circuit alive. So if you added a component like a gas discharge tube, you would need to cut the jumper for the new component to work otherwise power would just flow through the jumper and not the gas discharge tube, or say a mov or fuse.

author: cyberfish
author_rank: Regular Contributor
timestamp: March 10, 2017, 07:06:41 pm
content: Quote from: joseph nicholas on March 10, 2017, 03:57:42 pmOnly if the part is in series with something that needs to work. If it's in parallel (like in your case), it's perfectly reasonable to not short it.For another example, if you don't know how many capacitors you need in parallel, you can put in more in the PCB design, and not populate them all if fewer capacitors can meet your needs. You should leave them open and not shorted. There's no jumper wire to add.

author: Seekonk
author_rank: Super Contributor
timestamp: March 10, 2017, 07:34:49 pm
content: A PTC is usually included to prevent damage should you connect up the meter wrong. A PTC has a resistance in order for it to work. The resistor is to simulate that resistance. It would have to be removed if a PTC was installed. If this is in a current sense like ma, that resistor may be a zero ohm.

author: james_s
author_rank: Super Contributor
timestamp: March 10, 2017, 07:35:02 pm
content: Another situation which I've done myself is designing a board and including a few parts that I thought might be necessary but wasn't sure, then later determined they were not. Or I'd put them in the board layout in case we decided to add a feature in the future. There is considerable effort and expense in doing a respin on the boards so if you think you might need something like added input protection or decoupling, it's better to include it because you don't have to populate those components if you don't need them.

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: March 10, 2017, 08:21:48 pm
content: Hi, thanks for this. On the cheap multimeters which I like to play around with like the DT830 it comes in multiple flavors. The b model is basic, then there is one that will measure temperature, then one modle will output a simple sign wave. Some have say it has a buzzer, but it doesn't. The cob is probabily the same for all of them.How does the designer think when he lays out the pcb. Does he say "We want the b model to work by just putting in this piezo electric element, and not changing anything else, and design it to have a parallel trace or does he include a zero ohm resistor across the leads where the buzzer should connect? Is there a standard way to look at these thing? Or maybe a "best practices" way to circuit design for optional or alternative circuit board uses?

author: bson
author_rank: Unknown
timestamp: March 10, 2017, 09:13:14 pm
content: Quote from: joseph nicholas on March 09, 2017, 06:55:54 pmIt's probably designed to have either a fixed resistor OR a polyfuse installed during manufacturing. Maybe the fuse is for a special educational model.

author: james_s
author_rank: Super Contributor
timestamp: March 10, 2017, 10:11:15 pm
content: Quote from: joseph nicholas on March 10, 2017, 08:21:48 pmIt would make no sense to put a zero ohm resistor across the missing buzzer. What would that accomplish?The goal is to leave out any unused components possible. Sometimes you may have to install a cheap component like a resistor or jumper to bypass a more expensive active circuit that is not populated or to terminate the unused end of a transmission line but unless you have to do that to maintain circuit integrity the parts are simply left out.

author: Audioguru
author_rank: Super Contributor
timestamp: March 11, 2017, 02:30:41 am
content: They are mass produced and sometimes the assembly machine gets stuck and misses putting in a part. I have a AC to DC Wall Wart that has a missing main filter capacitor.

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: March 11, 2017, 09:40:58 am
content: Does anyone think there is a standard or best practices way to design a circuit board that will allow it to function without it being fully populated?Does it include alternative parts if the optional parts are missing which will allow it to work in its bare bones configuration?

author: timb
author_rank: Super Contributor
timestamp: March 11, 2017, 12:41:41 pm
content: Quote from: joseph nicholas on March 11, 2017, 09:40:58 amIt completely depends on the particular circuit, parts to be omitted, etc.Do you have a question about a particular component or design?

author: sentry7
author_rank: Regular Contributor
timestamp: March 11, 2017, 01:39:58 pm
content: This must be the doing of theMadman! A genius in disguise by the way.