I was round his workshop today to help fix a bit of car trouble. Alas; turns out OBD and OBD2 are not the same creature.....nevermind.
What he did show me was some "conductive" ABS he had and wanted to do a project with it. I was sceptical - I had come across conductive plastic before and you either had to pump a couple of 1000V into it before it would conduct or it was only good enough for ESD/EMC purposes.
Basically "conductive" when used in terms of plastic means VERY high resistivity! To an electric source: its a resistor of MAHOOSIVE resistance!
A few Youtube hits yielded some interesting results - but one piece of information cropped up, if conductive ABS bends/deforms: it changes resistance.
This means it could have applications for:
- home made strain gauges
- bendable potentiometers
- movement sensors
- weighing heavy masses - e.g. a car
So, trying REALLY hard to remember our GCSE physics we did some thinking. (seriously there was smoke coming from the top of our heads!)
What we wanted to do was find out what the resistivity (ohm/metre) of this ABS was. Seen as all the bag had on it was "Conductive ABS" on it, this wasn't much help! Nor could he remember the supplier to bring up some datasheets.
The Experiment
Initially we thought: Right just bang an DMM in resistance mode across it!
Tried it and got diddly squwat! meter didn't budge.
So I came up with a better idea! Apologies for the lack of photos folks:
Jez had a bank of batteries on his desk he'd pulled out of something. I had a automotive DMM (in my tool box) with a micro-amp range on it. I was sceptical as to how precise and accurate it was, considering it cos about £30 - but give it a go.
Some Croc clips and about 4" of this conductive ABS and the circuit was constructed. Battery voltage was confirmed to be about 5.2V from a quick test with the DMM - DMM means Digital Multi-Meter by the way.
I first took a current reading at 40mm from positive point to negative point. Reading on ammeter said about 10uA. Same process again but at 50mm - current = 7uA. OK; this was enough data to just get a figure that should give us a sensible answer.
The Maths
OK so from experience - I found conductive plastic was in the hundreds of ohm-metre range.If I got 2 readings in the 100's of kilo-ohm-mm range then I knew we were in the right ballpark!
- roh( the p shaped symbol, which I'll use p for) = resistivity in OHMS-metre's
- R = resistance = Voltage / Current
- A = cross sectional area of ABS
- l = length of section that current is flowing through
Diameter of ABS filament = 3mm, therefore radius was 1.5mm. this gave a area of: 7.086mm^2
So for 40mm:
R = 5.2 / 10uA = 520,000 OHMS
Substituting R into the resistivity equation:
p = 520,000 (7.086 / 40) = 91891.585 OHMS-mm
(Remember: always keep units the same, don't mix cm with mm for example.)
OK: so roughly 91K - not bad, 10K shy off target!
Solving for data obtained at 50mm:
Solving for data obtained at 50mm:
R = 5.2 / 7uA = 742,857 OHMS
Substituting R:
p = 742,857 (7.086 / 50) = 105,277 OHMs-mm
OK: so about 5K out from target, looking good.
So you could sat that the resistivity of this sample of ABS is ~100kOHMS-mm or ~100OHMS-metre (as there are 1000mm in one metre)
Outcome and findings
So considering plastic is usually a good insulator, you'd think 100 OHMS-m is still resistive, and that conductive ABS is just a con - when actually this IS conductive for plastic!
Plastics can be in the order of 10's of Gigaohms-m (depending on plastics).
Also: this experiment cannot be trusted - turns out there are 2 measurements of measuring resistivity: Surface resistivity and Volume resistivity.
With the configuration of our experiment, we may have been measuring surface resistivity; so the current may have been flowing around the sample like a tube not through it.
As a result - the resistivity could actually be lower? Saying that: this was on fresh ABS: its properties could change after being extruded into a 3D object.
Another note: I wouldn't consider relying on conductive plastic to make your own resistors with a 3D printer - the tolerance on them will be shockingly bad - so I wouldn't expect 0.1% on values!
So: I'll leave you to pick holes in my Back-woods experiment - but it does demonstrate how you can use some bits lying on your desk to quickly determine some material properties!
Also that my micro-amps range on my £30 DMM probably works to a decent degree of accuracy!
Also that my micro-amps range on my £30 DMM probably works to a decent degree of accuracy!
