On Thursday, 20 November 2014 11:48:39 UTC, Theo Markettos wrote:
whisky-dave wrote:
On Tuesday, 18 November 2014 23:23:50 UTC, Terry Casey wrote:

The availability of accurate digital meters compounds these problems. It is
quite common to see posts in some forums on the lines of "I replaced R72
(100k) because it was reading high at 106.8k".

What's worse is when student comes to you asking for a 733.48K ;-)

I hope you start by taking an 80 ohm, 50 watt power resistor and telling
them to add some more resistors in series...

well I only supply 1/4W as standard from 10R to 10M I tell them to start with 10R and put them in series until you get the right value.
Or 2 1.5M in parellel might be good enough.
Sometimes I can get them to see the wood for the trees in under 10 mins,
while others I give up on and send them to their supervisors.


Of course, if the resistor had a tolerance of ±10%,

I've found it difficult top buy such a resistor with such a low tolernace
most are 5% and 2% are quite cheap when brought in bulk 100+

Depends. I've been buying carbon composition resistors recently - nice
performance ~1GHz but tolerance often 10-20%. Metal film resistors are
typically etched in a serpentine pattern that has too much inductance to be
good at high frequencies, likewise wirewound resistors.

Not really practical for what we're teaching. I could have brought some 10% 2w resistors for 88p each but then I'd have to explain why they are MORE expensive than the 5% or even 1% I keep.


I was asked to buy a set of E192 a cople of months ago.

I do have a 50R 0.01% that cost 15 quid !

At that sort of level it's temperature stability that matters as well as
value, I suspect (0.01% = 100ppm).

http://uk.rs-online.com/web/p/throug...2D393 8333226

think it's 3ppm

Perhaps you can insist the student relabels the cap by writing '0.01uF'
on it before putting it in the circuit? ;-)

no chance, I can't even get them to write it on their order forms.


Theo