Art of Electronics SM: Capacitors Lab - "Mains Garbage Detector". Author Intent?

[rstofer]

An automotive tail light draws about 1A at 12V (the older lamps).  I don't know what it draws at 6V but it should provide a load for the transformer.

Actually, the brake element on this particular bulb draws a little over 2A at 12V and the tail light filament draws about 3/4A:
https://shop.advanceautoparts.com/p/sylvania-3157-longlife-mini-bulb-pack-of-2-3157llbp2/5081912-P

I didn't hunt for a bulb that might be easy to breadboard.  There are others...

[SpannersToSparks]

Cheers Ian,

I had assumed that bigger ohms = bigger load, looks like this is something for me to learn 

I measured the voltage, didn't blow up and read 245, so is a little on the high side here but not hugely.

[rstofer]

Ohm's Law:
http://www.physicsclassroom.com/class/circuits/Lesson-3/Ohm-s-Law

I rather like the delta-V idea versus what we usually use which if V (or E)

The famous equation is simply E=I*R.  For a given E, if R increases, I decreases.

[SpannersToSparks]

Ohm's Law:
http://www.physicsclassroom.com/class/circuits/Lesson-3/Ohm-s-Law

I rather like the delta-V idea versus what we usually use which if V (or E)

The famous equation is simply E=I*R.  For a given E, if R increases, I decreases.

Haha of course. I knew the law but somehow didn't make the connection .. ohms law .... I is inverse to R, so smaller R = more current. Great!

Is it a little more complex to work out for this circuit exactly, as its AC? Would we not need the impedance calc for it, or does it simplify? Please forgive the low level questions, this is all what I am working through right now

[SpannersToSparks]

An automotive tail light draws about 1A at 12V (the older lamps).  I don't know what it draws at 6V but it should provide a load for the transformer.

Actually, the brake element on this particular bulb draws a little over 2A at 12V and the tail light filament draws about 3/4A:
https://shop.advanceautoparts.com/p/sylvania-3157-longlife-mini-bulb-pack-of-2-3157llbp2/5081912-P

I didn't hunt for a bulb that might be easy to breadboard.  There are others...

I have a few bulbs in my kit, I will give this a go, thanks!

[Ian.M]

As the mains voltage can fluctuate with the time of day due to demand variations, if you want to characterise the transformer, you should measure the unloaded output voltage before and after measuring the mains voltage using the SAME DMM.   It would also be worth loading it resistively to about 1A and repeating the measurements, including the actual load current.

If you use a bulb (other than 6V to 6.5V) for a load, check the bulb current at 6.3V using your DC bench PSU before connecting it to the transformer to be reasonably sure you aren't going to overload it.   Beware of the burden voltage of your meter on its AC mA or Amps ranges - you really need to use two meters, with the voltmeter directly across the transformer output as inserting an ammeter will reduce the current sligtly, maybe enough to cause a detectable voltage rise.

[rstofer]

Haha of course. I knew the law but somehow didn't make the connection .. ohms law .... I is inverse to R, so smaller R = more current. Great!

Is it a little more complex to work out for this circuit exactly, as its AC? Would we not need the impedance calc for it, or does it simplify? Please forgive the low level questions, this is all what I am working through right now

In general, we are going to treat an AC source using its RMS voltage and internal impedance.  We want the Thevenin equivalent source.  We may not be able to get at the inductance value very easily but we can still look at voltage droop versus load current and come up with an equivalent internal resistance.

Yes, everything about AC is a lot more difficult.  Part of the reason is that impedance has a component that varies with frequency (reactance, inductive or capacitive) and we inevitably wind up with complex numbers when we want to draw the impedance on an X-Y (or R-X) plane (although sometimes we use magnitude and angle - not a vast improvement!).  Next up, we want to look at the transient (step) response and that tends toward differential equations (DEs)and then we get into Laplace Transforms because it is a markedly easier way to solve DEs.

Once you leave DC circuits, the math gets a lot deeper.  Even DC circuits can be pretty ugly and require matrix algebra for the solution.

I don't want to scare off the hobbyists, there are a lot of fun things to do that don't require much more than Ohm's Law.  Even op amp circuits can be analyzed with little more than Ohm's Law and Kirchoff's Current Law.



The Art of Electronics (and the companion lab manual) try to reduce the math and that's a very good thing.  OTOH, tools like Matlab and wxMaxima make the entire EE program much more approachable.  I was still using a slide rule when I graduated.  The HP35 had JUST been invented and it was well out of my price range.

[SpannersToSparks]

Thanks for the video link, I will give that a good watch.

Not to worry on recommending maths based material, though I am a beginner to electronics I have been a mechanical engineer for many years and so DE's, laplace etc are not entirely foreign. In fact it will be good to brush up on them!

I think AoE moves onto much of it anyway, especially the main text has a lot more than the manual. Looking forward to it

[rstofer]

Thanks for the video link, I will give that a good watch.

Not to worry on recommending maths based material, though I am a beginner to electronics I have been a mechanical engineer for many years and so DE's, laplace etc are not entirely foreign. In fact it will be good to brush up on them!

I think AoE moves onto much of it anyway, especially the main text has a lot more than the manual. Looking forward to it

Even if you have to brush up a bit, at least you have the math background when it comes up.

I have been working to recover whatever I might have known back in the '70s by helping my grandson with Calc I and, perhaps, Calc II and DEs.  We'll have to see...

I have also been playing around with wxMaxima and Matlab and they are fantastic tools.

Side issue:  If you like Control Systems, there is a really great video series



Given something like Matlab, the topic is a lot easier to manage.  No more Spirule for working root locus graphs!