Here are some practical examples of Ohm’s law and power equations for makers. We’ll use Ohm’s law to choose a resistor for an LED, and double-check we have the right parts using the power formulae.
6 Likes
Hi Michael
Should be compulsory viewing. A good simple explanation on what seems to be a difficult to grasp subject for some. But very important. The basis for most everything I think. The old grey matter is slowing down a bit these days as I don’t do much so I still use the 2 triangle system as a quick reminder.
A point maybe worth a mention. If using the mA ranges on a DMM with dedicated mA ranges the voltage burden of the meter should be considered. As the meter you used seems to be on the 10A (9.999) range the meter resistance will not be much of a problem. That is why I suggest in my replies a meter used in series as you have done be used on the 10A range. You will still get 1mA resolution and for most scenarios this will be adequate.
This “voltage burden” can be a trap for the unwary as, if significant, can have quite an effect on any circuit and you find the measured situation bears no resemblance to the real world.
I can understand your desire to keep it simple at this stage but maybe this effect could be the subject of another video in the future. As an add on to this one possibly. Just so the inexperienced don’t get into too much confusion with a you beut lots of ranges DMM. For instance my DMM has µA, mA, A (10A max). I must say I have never found a use for the µA range yet but for marketing it looks good.
Cheers Bob
4 Likes
@Robert93820 I used to draw the triangle at the top of every page in an electronics exam!
On burden voltage:
I remember years ago watching Dave Jones’ videos on his uCurrent project - a test-equipment adapter for measuring currents with extremely low burden. If you haven’t seen this project before his video series on it is really neat. Dave even goes into design for manufacture and testing; something I’m very grateful for now 
3 Likes
Hi Michael
I vaguely remember that project. I seem to recall some constructors having trouble calibrating etc due to switch and other stray resistance. Switch resistance is mentioned in the article I notice.
Strays of any flavour can play a significant and disruptive part in most things. I remember when I was working we had a 40kW HF transmitter which flatly refused to tune above about 20MHz. This was a new unit and when it was built the fitters had wired a range change switch with the wrong size wire. It was too thin and so introduced so much stray inductance the coils that were meant to do this job had no effect and were completely swamped by strays. Same with bypass caps on the pre amp valve (valves???) sockets. The legs had been left full length instead of cutting short and the stray L disrupted the auto tuning process dramatically.
The text and background you linked should be on everyones reading list. If everyone did this it would save a whole heap of explanations.
Cheers Bob
3 Likes
